2.4 Growth responses of crop plants in saline-sodic soil . ... 2.4.1 Two-phase process of growth inhibition by salinity . ..... of EC is Siemens per metre (S m-1).
Limitations to yield in saline-sodic soils: Quantification of the osmotic and ionic regulations that affect the growth of crops under salinity stress
Ehsan Tavakkoli BSc. Hon (Agricultural Engineering) MSc (Agricultural Sciences)
School of Agriculture Food and Wine Faculty of Science The University of Adelaide, Australia Thesis by publication submitted to the University of Adelaide for the degree of Doctor of Philosophy
January 2011
DEDICATION
I would like to dedicate this thesis to a number of people without whom I could not stand where I am today. This thesis is dedicated to my late grandfather, who passed away at the age of 99 in the course of my Masters degree at UNE. His soul will live on within me for his encouragement and support enabling me to continue my studies in Australia. Also this thesis is dedicated to my parents who were my first teachers and taught me the best kind of knowledge and for their love, endless support and encouragement. Finally, this thesis is dedicated to Professor Acram Taji who supported me all the way since the beginning of my studies in Australia, and who has been a great source of motivation and inspiration.
i
Table of Contents ABSTRACT ............................................................................................................................. v DECLARATION ................................................................................................................... vii Acknowledgments................................................................................................................. viii 1. INTRODUCTION ................................................................................................................... 1 1.1Thesis outline ...................................................................................................................... 3 2. LITERATURE REVIEW ...................................................................................................... 5 2.1 Saline, sodic and saline-sodic soils: definitions .............................................................. 5 2.1.1 Salinity .................................................................................................................... 5 2.1.2 Sodicity .................................................................................................................. 7 2.1.3 Saline-sodic soils .................................................................................................... 9 2.2 The physical and chemical properties of saline-sodic soils ......................................... 10 2.2.1 Clay dispersion in sodic soils ................................................................................ 10 2.2.2 The chemical properties of saline-sodic soils........................................................ 15 2.3 Causes of salinity ............................................................................................................ 17 2.4 Growth responses of crop plants in saline-sodic soil ................................................... 19 2.4.1 Two-phase process of growth inhibition by salinity ............................................ 19 2.4.2 Effects of salinity on plant available water: Osmotic stress ................................. 23 2.4.3 Effects of specific ion toxicity in crops ................................................................ 27 2.4.3.1 Mechanisms of Na+ toxicity in plants ................................................... 28 Selectivity of potassium uptake at the plasma membrane ................. 31 Binding of calcium to the plasma membrane ................................... 32 2.4.4 Plant photosynthesis ............................................................................................. 34 2.5 Mechanisms of salt tolerance in crop plants ................................................................ 37 2.5.1 Osmotic adjustment .............................................................................................. 39 2.5.2 Reduced uptake and translocation ........................................................................ 40 2.5.3 Transport and compartmentation .......................................................................... 42
ii
2.6 Chloride dynamics in crop plants in relation to salinity responses ........................... 44 2.7 Salinity tolerance in barley and faba bean ................................................................... 50 2.7.1 Barley (Hordeum vulagare) .................................................................................. 50 2.7.2 Faba bean (Vicia faba) .......................................................................................... 51 2.8 Breeding for improving salt tolerance in crop plants ................................................. 52 2.8.1 Screening Methods ............................................................................................... 54 2.8.1.1 Germination ......................................................................................... 55 2.8.1.2 Photosynthesis and other physiological indicators .............................. 56 2.8.1.3 Field vs. controlled conditions ............................................................. 57 2.9 Salinity research ............................................................................................................. 59 2.9.1 Soil and nutrient culture systems ......................................................................... 59 2.9.1.1 Soil culture systems .............................................................................. 59 2.9.1.2 Nutrient solution systems ..................................................................... 60 2.9.1.3 Field studies ......................................................................................... 61 2.9.1.4 Growth conditions and salinity treatments .......................................... 62 2.9.2 Separating the osmotic stress from ionic toxicity ................................................ 63 2.10 Conclusions and further research ............................................................................... 65 List of articles presented for this thesis .............................................................................. 68 List of peer-reviewed conference papers presented for this thesis .................................. 69 3. Chapter 3 The response of barley to salinity stress differs between hydroponics and soil systems 4. Chapter 4 Growth of faba bean in saline-sodic soils: Monitoring of leaf development and water use dynamics enables the quantification of osmotic and ionic regulation at whole-plant level 5. Chapter 5 Additive effects of Na+ and Cl- ions on barley growth under salinity stress 6. Chapter 6 High concentrations of Na+ and Cl- ions in soil solution have simultaneous detrimental effects on growth of faba bean under salinity stress 7. Chapter 6 Effective screening methods for salinity tolerance: pot experiments but not hydroponics are plausible models of salt tolerance in barley 8. Genotypic variations of faba bean in response to transient salinity at whole-plant level
iii
9. GENERAL DISCUSSION AND CONCLUSIONS ........................................................... 70 9.1 Introduction ..................................................................................................................... 70 9.2 Relative importance of ion (Na+ and/or Cl-) toxicity and osmotic effect to growth and yield reduction under different levels of salinity ............................................................... 74 9.3 Relative importance of Na+ and Cl- toxicity in growth reduction of barley and faba bean ............................................................................................................................... 76 9.3.1 Barley ................................................................................................................... 77 9.3.2 Faba bean ............................................................................................................. 78 9.4 Evaluation of crop salt tolerance in solution and soil cultures under controlled environmental conditions: Are they good surrogates for evaluating whole-plant response to salinity under field conditions? ...................................................................... 80 9.4.1 Barley ................................................................................................................... 81 9.4.2 Faba bean ............................................................................................................. 83 9.5 Conclusions ...................................................................................................................... 84 9.6 Recommended future research ....................................................................................... 86 References for literature review and general discussion ....................................................... 89
iv
ABSTRACT Salinity reduces yields of agricultural crops in many arid and semi-arid areas of the world where rainfall is insufficient to leach salts from the root zone. Salinity reduces plant growth and yield by two mechanisms, osmotic stress and ion cytotoxicity. Munns et al. (1995) proposed a two-phase model of salt injury where growth is initially reduced by osmotic stress and then by Na+ toxicity. However, some uncertainty exists regarding the relative importance of the two mechanisms. This is due to the difficulty in separating the osmotic effect from specific ion effects because of the overlap in the development of the two type stresses during the development of salinity stress. There has also been some recent debate about the importance of soil Cl-, and by implication plant Cl- uptake, as predictors of damage and yield loss, rather than electrical conductivity. Where NaCl is high, increased uptake of Na+ ions will be associated with high uptake of Cl- ions. Reliable and effective salt tolerance screening techniques to predict field performances are important for breeding programmes. Thus, in comparisons between results from laboratory and/or glasshouse soil and solution culture screening techniques and field evaluations of salt tolerance, it is important to verify whether or not the laboratory conditions can predict responses to field stresses. The main objectives of this research were to:
determine which of the two ions most frequently implicated in salinity, Na+ and Cl-, is most toxic to barley and faba bean
quantify the relative importance of ion (Na+ and/or Cl-) toxicity and osmotic stress on growth and yield reduction under different levels of salinity
v
investigate whether hydroponics and pot experiments under controlled environmental conditions are useful surrogates for evaluating whole-plant response to salinity under field conditions.
High concentration of Na+, Cl- and NaCl separately reduced growth, however the reductions in growth and photosynthesis were greatest under NaCl stress and were mainly additive of the effects of Na+ and Cl- stress (Chapter 5 and 6). The results demonstrated that Na+ and Cl- exclusion among genotypes are independent mechanisms and different genotypes expressed different combinations of the two mechanisms. The results also suggested the two-phase model of salt stress may not be appropriate at all levels of salt stress. Osmotic stress was the predominant cause of reduced growth at high levels of salinity, while specific-ion toxicity was more important under mild salinity stress (Chapters 3 and 4). In barley, the effects of salinity differed between the hydroponic and soil systems. Differences between barley cultivars in growth, tissue moisture content and ionic composition were not apparent in hydroponics, whereas significant differences occurred in soil. Reductions in growth were greater under hydroponics than in soil at similar EC values and the uptake of Na+ and Cl- was also greater (Chapters 3 and 7). Early assessment of salinity tolerance at seedling stage was found to be unsuitable. This work has also established sound screening procedures that significantly correlated with field evaluation of grain yield in genotypes of barley and faba bean (Chapters 7 and 8). Salt exclusion coupled with a synthesis of organic solutes were shown to be an important component of salt tolerance in the tolerant genotypes and further field tests of these plants under stress conditions will help to verify their potential utility in crop improvement programs.
vi
Declaration This work contains no material which has been accepted for the award of any other degree or diploma in any university or other tertiary institution to Ehsan Tavakkoli and, to the best of my knowledge and belief, contains no material previously published or written by another person, except where due reference has been made in the text. I give consent to this copy of my thesis when deposited in the University Library, being made available for loan and photocopying, subject to the provisions of the Copyright Act 1968. The author acknowledges that copyright of published works contained within this thesis (as listed below) resides with the copyright holder(s) of those works. I also give permission for the digital version of my thesis to be made available on the web, via the University’s digital research repository, the Library catalogue, the Australasian Digital Theses Program (ADTP) and also through web search engines, unless permission has been granted by the University to restrict access for a period of time. Tavakkoli E., Rengasamy P., McDonald, GK (2010) The response of barley to salinity stress differs between hydroponics and soil systems. Functional Plant Biology 37, 621-633. Tavakkoli E., Rengasamy P., McDonald GK (2010) High concentrations of Na+ and Cl- ions in soil solution have simultaneous detrimental effects on growth of faba bean under salinity stress. Journal of Experimental Botany 61, 4449–4459. Tavakkoli E., Fatehi F., Coventry S., Rengasamy P., McDonald GK (2011) Additive effects of Na+ and Cl- ions on barley growth under salinity stress. Journal of Experimental Botany. 62, 2189-2203.
Date: 24/1/2011 Ehsan Tavakkoli
vii
Acknowledgements Completing this thesis has been a long road, and without the help of many people along the way it would not have been possible. I would like to gratefully acknowledge the input and support of my principal supervisor, Associate Professor Glenn McDonald and my co-supervisor Dr Pichu Rengasamy, who have guided my research throughout this project and for their patience and the many discussions during all stages of the research. As I prepared this thesis they have spent many hours revising written material, and their availability, insights and assistance have been much appreciated but words alone cannot express the thanks I owe to them. Several other individuals have provided me with significant support over the course of this project. In particular, I am very grateful to my best friend, Dr Graham Lyons, for his invaluable advice, continuous support and friendship. His gentle reassurances and encouragement at various times in the course of my study are very much appreciated. I am very grateful to Waite Analytical Services (Teresa Fowles, Lyndon Palmer, Matthew Wheal and Deidre Cox) for accurate and timely analysis of soil and plant samples. I am also very grateful to my colleagues and friends in the Plant Nutrition Group, in particular Dr G. Lyons, Prof. R. Graham, Dr Y. Genc, Dr W. Bovill and Mr D. Keetch and in SARDI, Dr G. Sweeney, Dr. J. Emms and Mr. H. Drum for their support, frequent advice, technical assistance and friendship. My thanks go to Mr S. Coventry (National Barley Breeding Program, UA) and Dr J. Paull (National Faba Bean Breeding Program, UA) for their expert management of field trials and providing the seeds of barley and faba bean genotypes for this study. I am thankful also to Prof M. Tester for making the Lemna Tec imaging system accessible during this research and the Australian Centre for Plant Functional Genomics for enormous help in letting me to use the lab equipments for these studies. I am very grateful to Prof S. Tyerman, Dr R. Munns, Dr C. Grant, Dr A. McNeil, Dr D. Chittleborough, and Prof. D. Suarez for stimulating discussion on different aspects of this research project. Many people at the School of Agriculture, Food and Wine (UA) and South Australian Research and Development Institute have also assisted me in this research. I would like to thank Mrs. A. Marchuk, Mr. C. Rivers, Mrs. W. Sullivan and Dr. Y. Shavrukov for their assistance in the laboratory. Many thanks also to Mr. P. Ingram for his help with glasshouse and growth chambers arrangement. I wish to sincerely thank the Grains Research and Development Corporation for funding and the University of Adelaide, School of Agriculture, Food and Wine for funding and hosting this PhD study. Finally, and most importantly, I would like to thank my family: mum and dad, my sister Dr M. Tavakkoli and my brother in law Dr H. Fadavi for their positive source of encouragement, love and support throughout this study.
viii
Chapter 1 1. Introduction With the world's population expected to grow from 6.8 billion now to 9.1 billion by 2050 (Nature 2010), a there is concern about our ability to produce sufficient food to meet the burgeoning population. The world's population more than doubled from 3 billion between 1961 and 2007. According to the World Bank and the United Nations, between 1 and 2 billion humans are now malnourished, due to a combination of insufficient food, low incomes, and inadequate distribution of food. As the world population increases, the food problem will become increasingly severe, conceivably with the numbers of malnourished reaching 3 billion. For example, the per capita availability of world grains, which make up 80 % of the world's food, has been declining for the past 15 years (Kendall and Pimentel 1994). Certainly with a quarter million people being added to the world population each day, the need for grains and all other food will reach unprecedented levels. More than 99 % of the world's food supply comes from the land, while less than 1 % is from oceans and other aquatic habitats (Pimentel et al. 1973; Pimentel et al. 1994) and so maintaining soil fertility and increasing crop production is of worldwide significance. Hunger and poverty must be overcome by expanded food production by applying sustainable and viable soil fertility and plant nutrition management.
Salinity is an important environmental factor that reduces crop productivity in many agricultural areas, mainly in arid and semi-arid regions (Rengasamy 2010b). In Australia, of the 7.6×106 km2 of agricultural land about 33% (2.5×106 km2) has sodic soils that have a potential to develop transient salinity. Of the area of land devoted to 1
cropping approximately 80% can be affected by some form of salinity - 16% from water table-induced salinity and 67% from transient salinity - costing the farming economy about $A1.330 billion per annum, in lost opportunity (Rengasamy 2006).
Salt in the soil solution inhibits plant growth for two reasons. First, the presence of salt in the soil solution reduces the ability of the plant to take up water, and this leads to slower growth. This is the osmotic or water-deficit effect of salinity (Munns 1993). Second, excessive amounts of salt enter the transpiring leaves and this may further reduce growth. This is the salt-specific or ion-excess effect of salinity (Munns 1993). The physiology of plant responses to salinity and their relations to salinity resistance have been much researched and frequently reviewed in recent years (Cheeseman 1988; Yeo 1998; Flowers 2004; Chinnusamy et al. 2005; Munns and Tester 2008). However, it has been difficult to assess with any confidence the relative importance of ion toxicity and water deficit to reduction in growth (Rajendran et al. 2009). Despite the fact that the osmotic effect on growth of more salt tolerant species such as wheat and barley is often much greater than the salt-specific effect (Ueda et al. 2004; Munns and Tester 2008), the relative importance of the mechanisms that regulate the growth rate are not wellunderstood. Additionally, this may be further be complicated in soil-grown plants where the effect of soil physical properties may interact with the soil solution to determine soil water potential and water uptake by plants. Therefore many interpretations have been proposed regarding the physiology and causes of growth and yield reduction in the field, the question whether the cause of the reduced growth is water deficit or ion excess is still not resolved.
In annual crop and pasture plants, research on salt tolerance has focussed on the effects of Na+. However, there has been some recent debate about the importance of soil Cl, 2
and by implication plant Cl uptake, as predictors of damage and yield loss, rather than electrical conductivity (Dang et al. 2006b; Dang et al. 2008). Chloride toxicity is known to be important in some species, especially perennial plants, but there is little information on its impact on annual broadacre crops. Little is known concerning the primary acquisition mechanisms of Cl by plants, and knowledge about its subcellular distribution and flux dynamics is scarce (Britto et al. 2004).
1.1 Thesis outline Chapter 2 develops a conceptual framework of constraints caused by soil salinity and sodicity in cropping systems, and the current literatures on the impacts of salinity on soil, water and plant relationships in soil systems are discussed.
Chapter 3, is based on a paper published in the journal of Functional Plant Biology. In this chapter the relative importance of different mechanisms of salinity tolerance in hydroponics and in soil in two varieties of barley that are known to differ in their salt tolerance and ability to exclude Na+ are compared. We also tested the hypothesis that the responses to salinity in soil-based systems are different to those observed in hydroponics.
Chapter 4 aims to quantify the relative importance of ion toxicity and osmotic stress to growth reduction at different levels of salinity among two faba bean genotypes differing in their salt tolerance. To describe the different phases of salt stress, we provide an example of a non-destructive, real-time method to assess the growth of faba bean plants during a greenhouse experiment using commercially available image capture and analysis equipment (LemnaTec ‘Scanalyser 3D’). Daily measurements of plant growth
3
in parallel to plant water use allowed the continuous monitoring of responses to salinity stress.
Chapters 5 and 6 are based on papers that have been published independently in the Journal of Experimental Botany. In Chapter 5 the extent to which the Na+ and Cl contribute to ion toxicity in barley is critically assessed on selected barley varieties with different mechanisms of salt tolerance in both hydroponic and soil systems. In Chapter 6, the relative importance of toxicity of Na+ versus Cl in faba bean was assessed using two genotypes of faba bean differing in their ion exclusion mechanisms and salt tolerance in a soil-based experiment.
Chapter 7, is based on a paper that has been submitted to the Journal of Experimental Botany. The aims of work in this study were to evaluate the genotypic variation for salinity tolerance among 60 varieties of barley in a supported hydroponic system and to investigate possible physiological traits that could be used as screening criteria in selected genotypes in a soil-based experiment and in the field.
Chapter 8, is based on a paper submitted to the journal of Field Crops Research. This study reports the results of two experiments conducted to compare the responses to salinity in hydroponics and in the field in a diverse range of faba bean cultivars and to assess the value of tissue Na+, Cl- and K+ concentrations as a criterion for salt tolerance and assess the importance of different mechanism of salinity tolerance in two systems.
In chapter 9 the conclusions are drawn and a perspective into future research is given.
4
Chapter 2 2.0 Literature review This literature review develops a conceptual framework of constraints caused by soil salinity and sodicity in a soil-based cropping system. The impacts of salinity on soil, water and plant relationships in soil systems are discussed. The review attempts to address the constraints to plant growth and nutrition in saline-sodic soils. Then the mechanisms of osmotic stress and Na+ toxicity to plants are explained along with Cldynamics in crop plants in relation to salinity responses. In addition, the mechanisms of salinity tolerance and the relative importance of these various processes are discussed. The different experimental techniques in salinity research and the advantages and limitations of these experimental methods as well as some aspects of growing plants for salinity experiments, comparisons between experiments and the relevance of experiments to field situations are summarized. The review concludes with a summary of the research hypotheses.
2.1 Saline, sodic and saline-sodic soils: definitions 2.1.1
Salinity
Salinity is the concentration of dissolved mineral salts in soil solution as a unit of volume or weight basis (Ghassemi et al. 1995). The major ions present in a soil solution are the anions chloride (Cl-), sulphate (SO4 2-), bicarbonate (HCO3-), carbonate (CO3 2-) and nitrate (NO3 -), and the cations sodium (Na+), calcium (Ca2+), magnesium (Mg2+), and potassium (K+). In hypersaline soils (originated from the evaporation of sea water), other constituents can be present, such as barium (Ba), strontium (Sr), lithium (Li), 5
silicon dioxide (SiO2), rubidium (Rb), iron (Fe), molybdenum (Mo), manganese (Mn), and aluminium (Al3+) (Tanji 1990), but such soils are agriculturally non-productive and are not considered further. The formation of CO3
2-
and HCO-3 is affected by pH and
these ions are only present in soils of pH 9.5 or greater (Rengasamy 2010b). Saline soils are classified as those with ECe > 4 dS m-1 and an exchangeable Na+ percentage (ESP) < 15, which equates to an approximate NaCl concentration of 40 mM (Rengasamy 2010b). The ratios of the ionic constituents in soil-water depend on the chemical reactions that take place in soil-water-plant systems under different conditions. Chemical analyses provide full details of salinity (pure water or soil-water extract) and specific ion concentration. However, as a general predictor, salinity usually is described as total salts irrespective of its constituents.
Electrical conductivity (EC) is used as a fast method to evaluate soil salinity and is based on the fact that the electrical current transmitted between two electrodes under standardised conditions changes with a change in soluble ionic salts. The basic SI unit of EC is Siemens per metre (S m-1). In agriculture, EC is often low; thus deciSiemens per metre (dS m-1) is widely used. The unit (mmhos cm-1) used in the past is numerically equal to dS m-1. Electrical conductivity can be related to electrolyte concentration for different solution conditions. log Co = à +ϖ log EC where Co is the salt concentration expressed in mmol L-1, ϖ and à are empirical parameters which vary with different mixed solutions, and have values of about one. Soil-water salinity depends on the water content at which the salinity needs to be determined. Separating the soil solution from the soil sample is difficult (Dyer et al. 2008) and the quantity of extracted water within the normal plant available range is 6
usually insufficient to conduct chemical analyses. Therefore, an extra known volume of water can be added to the soil sample before extracting the soil solution. The extraction process can be performed after mixing a given weight of soil with a certain volume of water. Different soil:water extract ratios have been used to predict soil salinity such as 1:5, 1: 2.5 and 1:1 extract. A good approximation of soil-water salinity is that measured in a saturated soil paste extract (ECe). Saturated soil paste extract can be prepared in which a given weight of soil is saturated and then the soil solution extracted. Since the water content at saturation is nearly twice that of field capacity, the EC of the saturated extract is approximately half that of the soil at field capacity (Rhoades et al. 1989). Soil salinity in the field can be monitored using various instruments. Examples of these instruments are a salinity sensor based on electronic conductivity, time domain reflectometery (TDR), and inductive electromagnetic meter (Dudley 1994).
2.1.2 Sodicity Sodic soils are generally defined by the parameter of exchangeable sodium percentage -1
(ESP) which can be calculated as follows (concentrations in cmolc kg );
ESP = (100 × Exchangeable Na+) / Cation Exchange Capacity
The sodicity of irrigation water and soil solutions is defined using the parameter of Na+ adsorption ratio (SAR) and can be calculated as follows (where the concentrations Na+, Ca2+ and Mg2+ are measured in mmol L–1.concentrations in mM);
SAR = [Na+] / [Ca2+ + Mg2+]1/2
7
No universally accepted critical ESP for sodic soils exists. The current definitions for sodic soils are based upon the critical ESP at which soil dispersion occurs, with the critical value ranging between 5% (McIntyre 1979) and 15% (USSL 1954). In Australia, three classes of sodicity, non-sodic (ESP < 6%), sodic (ESP 6%-15%) and strongly sodic (ESP >15%) are assigned (Rengasamy 2006). These conflicting sodicity classifications demonstrate that the behaviour of soils in the presence of exchangeable Na+ differs according to numerous factors. These factors include the electrical conductivity (EC) of the soil solution and leaching water, soil texture, clay mineralogy, organic matter content, position within the profile, pH and the suite of accompanying cations (Rengasamy 2010b). Thus, the sodicity classification of a particular soil needs to be determined according to its behaviour in the environment and corresponding limitations to productivity, rather than using a critical ESP value.
Using the measured ESP as an indicator of the level of Na+ on soil exchange surfaces may have errors due to the difficulties in determining the CEC. Qadir and Schubert (2002) concluded the reasons for incorrect estimation of ESP was because: (a) the extraction of exchangeable Ca2+ and Mg2+ during the chemical analysis process might cause some CaCO3 and MgCO3 to dissolve, erroneously leading to an increase of CEC, especially in calcareous soils; (b) the CEC in variable charge soils depends on pH, solute concentration and buffering capacity of soil-water extract; (c) the removal of Na+ by extraction from a source that does not contain a true form of exchangeable Na+, such as Na+ zeolites. Furthermore, determining the CEC is time consuming. In contrast, SAR is thermodynamically more appropriate because it approximates the activities of various cations in solution. In addition, SAR requires fewer parameters (the concentration of Na+, Ca2+ and Mg2+), and can be determined from the same soil water extract used to 8
evaluate the EC in soil solution. SAR, however, does not take into account the change of Ca2+ concentration in soil solution as a result of change of solubility of the Ca2+ (Qadir and Schubert 2002). Sodium remains soluble and in equilibrium with exchangeable soil Na+ all the time. Conversely, Ca2+ does not remain completely soluble and might be raised in soil solution because of dissolution of soil minerals and usually precipitates in the presence of carbonates, bicarbonates and/or sulphates in solution.
Moreover, in Australia many saline-sodic soils, particularly subsoils, have higher exchangeable Mg2+ than Ca2+. Rengasamy et al. (1986) concluded that the enhanced clay dispersion in high magnesic saline-sodic soils is due to the lower flocculating effect of Mg2+ compared to Ca2+. Therefore, there is a need to derive and define a new ratio of these cations in place of SAR, which will indicate the effects of Na+, K+, Mg2+ and Ca2+ on soil structural stability. This may be achieved by using a formula analogous to the SAR but which selectively incorporates the dispersive effects of Na+ and K+ on the one hand with the flocculating effects of Ca2+ and Mg2+ on the other. The concept cation ratio of soil structural stability (CROSS) proposed by Rengasamy and Marchuk (2010) is of such instances.
2.1.3 Saline-sodic soils A saline soil dominant in Na+ ions is saline–sodic and becomes sodic when salts are leached. Similarly a sodic soil becomes saline–sodic when Na+ salts accumulate in soil layers (Rengasamy 2010b). Generally saline–sodic soils have a spectrum of disorders and the soil solutions have a range of values of SAR and EC. Saline sodic soils are 9
those with an ECe > 4 dS m-1 and an ESP > 15. Further, as the pH of the soil increases above 8, it becomes alkaline and carbonates dominate the anions. Thus, salts affect plants through adverse soil properties of alkalinity and sodicity, properties imposed on the soil by mobile salts. Different categories of salt-affected soils that are generally found in different parts of the world, with criteria mainly based on SAR and EC of the saturation extracts of the soil and pH measured in 1:5 soil–water suspensions are shown in Figure 1 (Rengasamy 2010b).
The increase of pH could cause a significant increase of the ESP. It has been shown that there is a linear relationship between the ESP and the pH of the soil saturated paste. A small increase in the pH could result in a large increase in ESP values. This suggests the increase in the pH enhances the preference of Na+ to be adsorbed on clay colloids (Ezlit et al. 2010). It also indicates the increase of ESP with pH is the main factor determining the clay deflocculation at given sodicity and salinity levels. Thus, the negative effect of pH on soil deflocculation may be due to the increase of the ESP.
2.2
The physical and chemical properties of saline-sodic soils 2.2.1. Clay dispersion in sodic soils
Saline-sodic soils are subject to severe structural degradation and restrict plant performance through poor soil-water and soil-air relations (Rengasamy and Olsson 1991). Large proportions (86 - 2%) of saline-sodic soils in Australia have dense subsoils with an alkaline pH (8-9-5) trend, and their subsoil clay is highly dispersible due to adsorbed Na+.
10
Figure 1. Categories of salt affected soils based on Na+ adsorption ratio (SARe) and electrical conductivity (ECe) measured in soil saturation extract and pH1:5 measured in soil water suspension and possible mechanisms of impact on plants. Toxicity, deficiency or ion-imbalance due to other elements (e.g. B, K, N, P) will depend on the ionic composition of the soil solution. The diagram also shows the cyclic changes of the categories as influenced by the climatic factors and land management. (Note: In Australia, 1:5 soil : water suspension is commonly used for measurements of EC (and also for SAR) because of the easiness of measurements. Preparation of soil saturation extract is laborious and costly. Saturation extract is prevalently used in USA and other parts of the world and therefore to compare the research data, particularly salt tolerance thresholds for crops based on ECe, conversion of EC1:5 to ECe (Kelly and Rengasamy 2006) has become a necessity (Rengasamy 2010b).
11
Clays are those mineral particles in the soil with a diameter of less than 0.002 mm. As such, they make up a large proportion of the internal surface area of the soil and contribute significantly to soil physical and chemical properties. The stability of the aggregates in a soil depends upon the relative strength of the forces that exist between the clay fraction of the soil and the soil solution (Sumner 1993). The clay particles in dry aggregates are linked together by strong attractive forces and the distance between adjacent clay particles is generally less than 1 nm (Rengasamy and Olsson 1991). When a dry soil aggregate is hydrated however, interactive forces lower the potential energy of the water molecules, with the resultant release of energy being used partially for the structural transformation of the clay aggregate and partially in the release of heat (Rengasamy and Olsson 1991). The structural transformation of the aggregates that occurs upon their hydration may include swelling, slaking and dispersion. Dispersion involves the breakdown of a soil into particles of < 2 μm, which then diffuse through the dispersing solution (Churchman et al. 1993). The dominant soil factor contributing to dispersion is exchangeable Na+ but non-soil factors, such as the application of external stresses, also contribute (So and Cook 1993).
The diffuse double layer (DDL) is the interface between the surface of a clay mineral and the soil solution and consists of the negative charge of the clay surface and the cations in the soil solution. The thickness of the DDL is smaller when dominated by 2+
3+
+
divalent (e.g. Ca ) or trivalent (e.g. Al ) ions, but larger where monovalent (e.g. Na ) ions predominate. The thickness of the DDL is also reduced by solutions with high electrolyte concentrations (Rengasamy and Sumner 1998). When a soil has a high ESP and the electrolyte concentration of the soil is sufficiently low, the distance between clay particles upon hydration increases to such an extent that the particles begin to 12
separate, resulting in accentuated swelling. When sodic soils disperse and then dry, the result is the formation of a massive structure, without any hierarchical arrangement of clay particles into micro and macro-aggregates. Dispersion has numerous adverse effects on the physical properties of saline-sodic soils including reduced hydraulic conductivity, increased susceptibility to surface crusting and hard-setting, reduced water infiltration, increased runoff and soil erosion, reduced soil aeration and poor soil drainage (Quirk and Schofield 1955).
The hydraulic conductivity (HC) of a soil is a measure of its ability to transmit water when exposed to a hydraulic gradient. The maintenance of stable soil aggregates is important in sustaining soil HC, as HC is largely dependant on the structure of the soil matrix. Macropores are primarily responsible for the transmission of water through a profile and the return of aerobic conditions after a watering event, while micropores are largely responsible for the storage of water within the soil profile (Quirk 1978). Shainberg and Caiserman (1971) established that there is a significant negative correlation between soil ESP and HC, even at low sodicity levels and that a constant reduction in HC is achieved at higher sodicity levels. The nature of this relationship is also highly dependent on the EC of the percolating solution and is influenced by all of the aforementioned factors; hence soil ESP alone does not predict the HC of soils (Quirk and Schofield 1955). The primary mechanism responsible for the decreased permeability of saline-sodic soils at low EC values is the swelling of clay domains (Quirk and Schofield 1955). It remains unclear to what extent the blocking of soil pores upon clay dispersion contributes to reductions in the HC of sodic soils (Quirk 2001).
13
Infiltration is the movement of water into the soil. The primary soil constraint to infiltration in many soils is the formation of a thin surface layer with higher strength, smaller pores and lowers HC than the underlying soil (Bradford et al. 1987). The poor aggregate stability exhibited by saline-sodic soils upon wetting contributes to this seal or crust formation and consequent reductions in infiltration rate (Kazman et al. 1983).
Crops produced on saline-sodic soils frequently suffer aeration stress after irrigation or rainfall (Jayawardane and Chan 1994) as restricted infiltration of water results in waterlogging in the surface soil layers and restricted internal drainage results in waterlogging in sodic subsoils (McIntyre 1979). When a soil becomes waterlogged, the pore space in the soil structure that usually allows the exchange of gas between the soil and atmosphere is filled with water and diffusion of oxygen is severely reduced. The consequences of waterlogging for the plant may include reduced or ceased growth, the death of root apices and changes to the patterns of nutrient accumulation (BarrettLennard 2003). Given that Na+ uptake can be increased under hypoxia (Wetson and Flowers 2010) and anoxia (Drew and Lauchli 1985), this can exacerbate salinity stress in saline-sodic soils.
Plant Available Water Capacity (PAWC) is a phrase used to describe the amount of water present in a soil between field capacity and permanent wilting point. A negative correlation has been established between soil EC, ESP and PAWC (Dang et al. 2006b; Hochman et al. 2007). This has been attributed to the loss of porosity in the PAWCrange of saline-sodic soils due to the processes of swelling and dispersion (McCown et al. 1976). The resultant reduction in water storage can lead to the crop suffering premature water stress (Rengasamy 2002; 2010b). Similarly, restricted root growth in
14
saline-sodic soils may result in lower plant rooting depth and a lower effective profile of available water (Nuttall et al. 2003; Dang et al. 2006a; Dang et al. 2006b).
Productivity of crops grown under dryland conditions depends on efficient use of rainfall and available soil moisture accumulated in the period preceding sowing. However, single or multiple factors of physical subsoil constraints (e.g. reduced hydraulic conductivity, reduced water infiltration, reduced soil aeration and poor soil drainage) are present in many saline-sodic cropping soils that restrict ability of crop roots to access this stored water and nutrients. Planning for sustainable cropping systems requires identification of the most limiting constraint and understanding its interaction with other biophysical factors.
2.2.2 The chemical properties of saline-sodic soils
Saline-sodic soils (ECe > 4 dS m-1 and an ESP > 6) are often associated with a number of chemical properties that affect the availability of nutrients to plants. These include elevated pH, high soil solution Na+ and Cl- concentrations, altered exchange equilibrium and changes in redox potential. While NaCl and other highly soluble salts can be found in high concentrations in the soil solution, soils may contain sparingly soluble salts such as gypsum in amounts much greater than can be held in the soil solution. These salts precipitate out of the soil solution and may form layers of visible salt crystals within the soil profile (Bernstein 1975).
The calcareous saline-sodic soils commonly used for cereal production in Australia tend to have alkaline pH values because they are often found in weakly leached 15
environments (i.e. dry climates) (Isbell 1996). Soil alkalinity occurs as a result of the cumulative effects of long-term inputs of bases and outputs of acids. Alkaline anions such as bicarbonate, carbonate or hydroxide occur in the soils mainly as a result of removal of hydrogen ions from the soil, through the weathering of silicate minerals. If there is sufficient flushing of the profile with water containing dissolved CO2, then alkaline bicarbonate salt weathering products are leached out of the soil profile and it will not become alkaline. The contribution of the carbonate precipitation to the rise in pH of saline-sodic soils can be determined using data collected by Cruz-Romero and Coleman (1975). Their data indicated that at ESP values between 0-100, the pH of montmorillonite clay without CaCO3 present increased from 7.2 to 7.7 with increased ESP, and the pH of montmorillonite clay with CaCO3 present increased from 8.2 to 10.0 as ESP increased. Hence the presence of CaCO3 is necessary for large rises in pH, which in turn explains the presence of some neutral and acid sodic soils.
High soil solution Na+ affects the availability of nutrients to plants due to changes in the ion exchange equilibria and solubility of some compounds. The exchangeable cations found in irrigated Vertosols consist largely of Ca2+ and Mg2+, with a small proportion of K+ and variable proportion of Na+. Irrigation water may also contain appreciable quantities of Na salts (USSL 1954). The equilibrium that exists between the cations in the soil solution and the cations on the exchange sites is constantly changing according to the moisture content of the soil.
The chemical properties of saline-sodic soils that can limit crop growth and water extraction include high concentrations of Na+, Cl-, and high levels of extractable boron. These tend to be co-correlated, and spatially variable, in the alkaline soils that dominate
16
large sections of the regions of the grains industry in Australia. Despite the apparent significance of subsoil properties to crop production, few quantitative data are available to define the relationship. There is a need to quantify the relative importance of a range of subsoil constraints in explaining variation in crop yield to give a better understanding of the specific contribution of each constraint.
2.3
Causes of salinity
There are three major types of salinity based on soil and groundwater processes: groundwater associated salinity, transient salinity and irrigation salinity (Rengasamy 2006). Groundwater associated salinity, commonly known as dryland salinity, occurs in discharge areas of the landscape where water exits from groundwater to the soil surface bringing salts dissolved with it. In landscapes where the watertable is deep and drainage is poor, salts, which are introduced by rain, weathering and aeolian deposits are stored within the soil profile. The concentration of salts often fluctuates with season and rainfall and salt accumulation in soil layers is a common feature in sodic soils regions. This type of salinity is termed ‘transient salinity’ (Rengasamy 2002) and is also known as dry saline land or magnesia patches in South Australia.
The term transient salinity was first used by Hutson (1990) to model salt accumulation in the root zone under irrigation practices. Transient salinity is a term to denote the temporal and spatial variation of salt accumulation in the root zone that is not influenced by groundwater processes and a rising saline watertable. Sodic soils can be particularly susceptible to the development of transient salinity. Transient root zone salinity is caused by two major factors: water and solute flux and hydraulic conductivity of the root zone layers (Hutson 1990), which are affected by sodicity. Water infiltration 17
is very slow if the subsoils are sodic and water does not move down below that layer (Rengasamy 2002). This can cause temporary waterlogging in the subsoil and a saturated zone. Salts, derived from rainfall and soil weathering reactions, accumulate in the saturated zones in the soil profile. After the wet season, when the water evaporates quickly, salt accumulation in the sodic subsoil layers is exacerbated. The development of transient salinity can be strong in low rainfall environments because of the low rates of leaching in sodic clays, low rainfall in dryland areas and high transpiration by vegetation and high evaporation during summer (Rengasamy 2002). The amount of salt accumulating is not large, but can be detrimental to crops (Rengasamy 2002). This zone of high salinity fluctuates with depth and also changes with season and rainfall.
Secondary salinity is salinisation associated with human activity, mainly as a consequence of improper methods of irrigation. Poor quality water is often used for irrigation, so that eventually salt builds up in the soil unless the management of the irrigation systems is such that salts are leached from the soil profile. According to Flowers and Yeo (1995) too few attempts have been made recently to assess the degree of human-induced secondary salinization and this makes it difficult to evaluate the importance of salinity to future agricultural productivity. Anthropic salinization occurs in arid and semi-arid areas due to waterlogging brought by improper irrigation (Bresler et al. 1982). Secondary salt-affected soils can be caused by human activities other than irrigation and include deforestation, accumulation of air-borne or water-borne salts in soils, salinization caused by contamination with chemicals and overgrazing (Pessarakli 1991; Fitzpatrick et al. 1994; Szabolcs 1994; Bond 1998).
18
2.4
Growth responses of crop plants in saline-sodic soil
The deleterious effects of salinity on plant growth are associated with (1) low water potential of the root medium which causes a water deficit within the plant; (2) toxic effects of ions mainly Na+ and Cl−; and (3) nutritional imbalance caused by reduced nutrient uptake and/or transport to the shoot (Munns and Termaat 1986; Hasegawa et al. 2000; Ashraf 2004). This section will discuss these responses and the consequences for crop growth under salt stress.
2.4.1
Two-phase process of growth inhibition by salinity
The general response of plants to salinity is reduction in growth (Ghoulam et al. 2002) that occurs in two phases (Figure 2), a model which was proposed by Munns and Termatt (1986) and developed in a number of further papers (Munns et al. 1995; Munns and Tester 2008). Phase I is the reduction in growth caused by osmotic stress, and phase II is the growth reduction caused by ion toxicity. The two phases occur consecutively, and the transition between phases I and II may occur after days or weeks, depending on a range of factors including salt concentration, environmental conditions and plant physiology (Figure 2) (Munns et al. 1995).
Phase I affects the rate of expansion of new leaves. The rate of cell division is reduced, as is the size, but not depth of the cells, resulting in smaller, more succulent leaves (Munns and Tester 2008). The duration of phase I will therefore be affected by a range of variables. Increasing external salt concentration or temperature would reduce the duration of phase I, as would a faster initial growth rate of the plant (Munns et al. 1995).
19
NOTE: These figures are included on page 20 of the print copy of the thesis held in the University of Adelaide Library.
Figure 2. The growth response to salinity stress occurs in two phases: a rapid response to the increase in external osmotic pressure (the osmotic phase), and a slower response due to the accumulation of Na+ in leaves (the ionic phase). The solid line represents the change in the growth rate after the addition of NaCl. (a) The broken line represents the hypothetical response of a plant with an increased tolerance to the osmotic component of salinity stress. (b) The broken line represents the response of a plant with an increased tolerance to the ionic component of salinity stress based on (Munns et al. 1995). (c) The dashed line represents the response of a plant with increased tolerance to both the osmotic and ionic components of salinity stress (Munns and Tester 2008). Phase I would also be shorter for plants that accumulate salts faster, as the growth reductions caused by osmotic stress would be overtaken by the toxic effects of salt accumulation in the plant (Munns and Tester 2008). Phase II is the toxic phase of Na + and/or Cl- accumulation in the shoot tissue. This phase is characterized by an increase in senescence of older leaf tissue (Munns and Tester 2008). The effect of salt on growth during this stage is determined by the rate of salt accumulation in the tissues, and the degree to which the plant can tolerate high salt concentrations. Once salt concentrations build up to toxic levels, leaf tissues die. Senescence occurs in older leaves due to the longer time for salt accumulation, although plants may preferentially divert salt into older tissues in order to protect new leaves (Munns et al. 1995). Phase II is salt specific, and growth reductions depend on sensitivity to particular ions present. This point would also coincide with the onset of visible symptoms of salt damage in terms of necrosis and
20
senescence of salt affected tissue. Growth would then be reduced by both decreased rate of new leaf production, and increased death of older leaves (Munns and Tester 2008).
While a two phase model is a useful description of the way in which salinity acts to reduce plant growth, it must also be considered that it is a simplistic representation of a complex process. The effect of salt on plants may initially be only osmotic, at the moment in which the plant comes into contact with the salt, but when plant starts to take up salt and accumulate to high concentrations, the toxic phase of salinity stress commences. After the initial osmotic adjustment, ion uptake may appear to have limited impact in terms of visible symptoms of salt stress, but the metabolic demands of maintaining osmotic adjustment and water uptake, preventing excessive ion uptake, compartmentalizing ions, and protecting cellular processes, will result in a lower growth for the plant. This effect is compounded when the accumulation of salts in plant tissue reaches concentrations that result in death of that tissue, so that the plant loses photosynthetically active leaf area to supply growing tissues with assimilates, further reducing the overall growth rate of the plant.
The transition between phases observed by Munns et al. (1995) is the point at which the toxic effects of salinity increase from interrupting cellular processes, to causing tissue death, accentuating the difference in growth rate. However, not all responses to salinity are consistent with this model. Using Krichauff (a variety with a relatively good Na+ exclusion) wheat plants and several electrolyte solutions to impose different levels of salinity, Rengasamy (2010a) clearly indicated the continuous operation of an osmotic effect as the EC of the soil solution increases (Figure 3).
21
NOTE: This figure is included on page 22 of the print copy of the thesis held in the University of Adelaide Library.
Figure 3. Dry matter production of wheat in relation to EC of the pot soil solution comparing NaCl, CaCl2, Na2SO4, and Hoagland nutrient solution treatments (Rengasamy 2010a). The osmotic effect was predominant and severely restricting of plant growth above a certain value of soil solution EC, which in that study was 25 dS m-1, corresponding to an osmotic pressure of 900 kPa. It was shown that below this EC value, ionic effects due to Na+, Ca2+, SO4
2–
, and Cl– were significant at low EC values. Further
investigations are necessary to find out whether the individual ionic effects are due to toxic effects or ion imbalance effects. The clear understanding of the mechanisms will help plant scientists to develop strategies in selection and breeding of salt-tolerant plants (Rengasamy 2010a). In summary, while the two-phase model is useful in explaining how a plant responds to salt stress, its ability to explain differences among genotypes that exhibit differences in salt tolerance have been less successful. Some uncertainty exists regarding the relative importance of the two mechanisms. This is due to the difficulty in separating the osmotic effect from specific ion effects because of the
22
overlap in the development of the two type stresses during the development of salinity stress which was not covered by two-phase model.
2.4.2 Effects of salinity on plant available water: Osmotic stress
One of the important effects of soil salinity on plant growth is to induce plant water deficits. This occurs because of the decline in the osmotic potential of the soil solution as salt concentrations increase and a consequent reduction in water uptake by roots. Water availability is also affected by soil texture and the physical structure of the soil. This is in contrast to the hydroponic systems that are used commonly in studies on salinity, where water availability and uptake are determined only by the osmotic potential of the nutrient solution. Understanding the differences between soil and hydroponic systems is important to the development of experimental systems that can replicate field responses and produce more reliable screening methods.
Water potential (Ψ) is a thermodynamic concept that helps to explain the movement of water through the soil-plant system. It is defined as the potential energy per unit mass, volume, or weight of water. Water uptake by plants, and their growth rate, is affected by the water potential of the growth medium. Soil water is subject to a number of force fields, which causes its potential to differ from that of pure, free water. Such force fields resulted from the attraction of the solid matrix for water, as well as from the presence of solutes and the action of external gas pressure and gravitation (Iwata et al. 1994). The total soil water potential (Ψt) is the sum of several component potentials: Ψt = Ψm + Ψo + Ψp + Ψg
23
where Ψm, Ψo, Ψp, and Ψg are the matric, osmotic, pressure, and gravitational potential components (Campbell 1988). The total potential of water in soil, referenced to the chemical potential of pure liquid, is equivalent to the chemical potential of the soil water at a chosen temperature and pressure. Matric potential is due to the adhesion/cohesion and surface tension forces between water and soil particles. It is the main component of total soil water potential in non-saline soils. The osmotic potential of the soil is due to the concentration of soluble salts in the soil solution, and makes a significant contribution to total soil water potential in saline soils (Groenevelt et al. 2004). The effects of gravity (gravitational potential) are not generally considered in calculations of total soil water potential (Cresswell et al. 2008). Under saline conditions, the low osmotic potential associated with the high concentration of salts in the soil solution will decrease total soil water potential and affect both the rate of water use, and the final soil water content to which the plant can extract water. The importance of this will change with soil water content due to the changes in the concentrations of salt.
The degree to which plant growth is reduced during stress largely depends on the severity of the stress. Mild osmotic stress leads rapidly to growth inhibition of leaves and stems, whereas roots may continue to elongate (Westgate and Boyer 1985; Nonami and Boyer 1990). The degree of growth inhibition due to osmotic stress depends on the time scale of the response, the particular tissue and species in question, and how the stress treatment was given. Arrested growth can be considered as a way to preserve carbohydrates for sustained metabolism, prolonged energy supply, and for better recovery after stress relief (Bartels and Sunkar 2005). Continued root growth under salt stress may provide additional surfaces for sequestration of toxic ions, leading to lower salt concentration. For example, salt tolerance of barley was correlated with the better 24
root growth rates coupled with fast development and early flowering (Munns et al. 2000b).
Increase of salt in the root medium can lead to a decrease in leaf water potential and, hence, may affect many plant processes (Sohan et al. 1999). At very low soil water potentials, this condition interferes with the plants’ ability to extract water from the soil and maintain turgor (Sohan et al. 1999). Thus, in some aspects salt stress may resemble drought stress. However, at low or moderate salt concentrations (higher soil water potential), plants can adjust osmotically (accumulate solutes) and maintain a potential gradient to continue the influx of water. Under such conditions growth may be moderated, but unlike drought stress, the plant is not water deficient. Several authors have found that water potential and osmotic potential of plants declined with an increase in salinity, whereas turgor pressure increased.
The effect of salinity on permanent wilting point (PWP) was modelled by Groenevelt et al. (2004), using the soil water retention curve as a base. From this the two extremes of soil water extraction were shown, from matric potential only, to the full effect of both matric and osmotic potentials. Depending on a plant’s ability to osmotically adjust, with no adjustment, the plant will experience either the full osmotic stress, or if the plant is able to fully osmotically adjust it will overcome the osmotic potential component and experience only the matric potential. Most plants fall in the range between these two extremes. As the soil dries, the relationship between matric and osmotic potential, in terms of their contribution to total soil water potential, changes (Groenevelt et al. 2004). While osmotic potential decreases in a linear fashion, due to a simple concentration/dilution effect with soil water content, matric potential decreases in a 25
manner proportional to the logarithm of soil water content, the exact relationship determined by an individual soil’s water retention curve (Groenevelt et al. 2004). A plant in drying soil is exposed to increasing levels of both water stress and osmotic stress, because the matric potential and the osmotic potential decrease simultaneously with decreasing soil moisture (Shalhevet 1993; Glen and Brown 1998). This is common in arid soils, in which salts often concentrate near the surface as the soil dries between rains, and in irrigated soils which can accumulate damaging levels of salts between irrigations (McCree and Richardson 1987; Shalhevet 1993). Both low soil osmotic potentials and low soil matric potentials (associated with reduced water content) cause low water potentials in plants resulting in reduced leaf expansion rates, lower photosynthetic rates per unit leaf area and reduced growth (Rawson and Munns 1984).
Studies in which plants were grown in drying soils at different salinities show a more complicated response, in which soil salts actually mitigate some of the negative effects of water stress. For example, plants in drying soils usually survive longer in saline than in non-saline soils, because salt-stressed plants grow less and, therefore, deplete soil moisture more slowly than non-stressed plants (McCree and Richardson 1987; Shalhevet 1993). Studies of the combined effects of salt and water stresses on growth of maize (Stark and Jarrell, 1980) and sorghum (Richardson and McCree, 1985) showed that although salinity reduced the rates of leaf expansion under well-irrigated conditions, it also allowed leaf expansion to continue down to lower leaf water potentials under water stress. Furthermore, salt stress can increase instantaneous leaf water use efficiency by reducing stomatal conductance to a greater extent than photosynthesis, thereby allowing plants under salt stress to produce more dry matter than plants in nonsaline soil on the same quantity of water (Richards, 1992). Finally, 26
salt stress can precondition plants to low soil water potential by allowing them to osmotically adjust, enhancing their ability to survive as the soil dries (Shalhevet, 1993). Thus the combined effects of salinity and water stress may be less detrimental to plant growth than the sum of the separate effects.
In several plants, salt tolerance and drought tolerance are linked through a common mechanism of salt uptake for osmotic adjustment (Flowers and Yeo 1995). Physiological studies have often dealt separately with salt and water stresses, but in the field, salt stress is usually accompanied by water stress. Despite their importance, relatively few studies have considered the combined effects of water and salt stress on plants.
2.4.3
Effects of specific ion toxicity in crops
The ion-specific phase of the response to salinity starts when salt accumulates to toxic concentrations. If the rate at which they die is greater than the rate at which new leaves are produced, the photosynthetic capacity of the plant will no longer be able to supply the carbohydrate requirement of the young leaves, which further reduces their growth rate. While the osmotic stress has an immediate effect on growth, the ionic stress impacts on growth much later, and with less effect than the osmotic stress, especially at low to moderate salinity levels. Only at high salinity levels, or in sensitive species that lack the ability to control Na+ and/or Cl transport, does the ionic effect dominate the osmotic effect. In the following sections the mechanisms of ion toxicity are discussed.
27
2.4.3.1 Mechanisms of Na+ toxicity in plants
With basic soil and solution culture experiments highlighting that Na+ (and Cl–) has an effect on growth and nutrient accumulation in a variety of plant species, experiments have also been conducted in order to determine the mechanisms of these effects. This issue has largely been addressed in the context of the effects of NaCl salinity on nutrient uptake. The effects of NaCl on plant growth are apparent at a number of different levels. At the molecular level, NaCl stress is manifested in reduced binding of Ca2+ to the plant plasma membranes (Yermiyahu et al. 1994). At the whole plant level NaCl stress is manifested in reduced root (Kent and Lauchli 1985; Kurth et al. 1986) and shoot growth (Cramer et al. 1989; Yeo et al. 1991; Cramer 1992) and changes in ionic composition of the plant (Ben-Hayyim et al. 1987; Reid and Smith 2000).
In shoots, high concentrations of Na+ cause a range of osmotic and metabolic problems for plants. Leaves are more susceptible to salt stress than roots because Na+ (and Cl–) accumulates to higher concentrations in shoots than in roots (Munns and Tester 2008). Roots tend to maintain fairly constant levels of NaCl over time, and can regulate NaCl levels by export to the soil or to the shoot. Na+ is transported to shoots in the rapidly moving transpiration stream in the xylem, but can only be returned to roots via the phloem. There is limited evidence of extensive recirculation of shoot Na+ to roots, suggesting that Na+ transport is largely unidirectional and results in progressive accumulation of Na+ as leaves age (Apse and Blumwald 2007).
Sodium has the ability to compete with K+ for binding sites essential for cellular function. More than 50 enzymes are activated by K+, and Na+ cannot substitute in this 28
role (Bhandal and Malik 1988). Thus, high levels of Na+, or high Na+/K+ ratios can disrupt various enzymatic processes in the cytoplasm. Moreover, protein synthesis requires high concentrations of K+, owing to the K+ requirement for the binding of tRNA to ribosomes (Blaha et al. 2000) and probably other aspects of ribosome function. The disruption of protein synthesis by elevated concentrations of Na+ appears to be an important cause of damage by Na+.
Osmotic damage could occur as a result of the build up of high concentrations (possibly several hundred mM) of Na+ in the leaf apoplast, since Na+ enters leaves in the xylem stream and is left behind as water evaporates. This mechanism of Na+ toxicity was first proposed by Oertli (1968), and direct supporting evidence has been provided by X-ray microanalysis measurements of Na+ concentrations in the apoplast of rice leaves (Flowers et al. 1991). These authors calculated that there was about 600 mM Na+ in the apoplast of leaves of rice plants that were moderately salt-stressed.
The cellular toxicity of Na+ causes another type of osmotic problem. Plants need to maintain internal water potential below that of the soil to maintain turgor and water uptake for growth. This requires an increase in osmotica, either by uptake of soil solutes or by synthesis of metabolically compatible solutes. This drought component of salinity poses a dilemma for plants: the major, cheap solutes in saline soils are Na+ and Cl–, but these are toxic in the cytosol. Compatible solutes are non-toxic, but are energetically much more expensive. With high concentrations of Na+ in the leaf apoplast and/or vacuole, plant cells have difficulty maintaining low cytosolic Na+ and, perhaps as importantly, low Na+/K+ ratios (Gorham et al. 1990; Dubcovsky et al. 1996).
29
Sodium chloride salinity also affects the nature of root development, by altering the Na+/Ca2+ ratio of the growth medium. Kurth et al.(1986) and Huang and Redman (1995) found that roots grown in high Na+ and low Ca2+ mediums are shorter and thicker than those grown in standard media. The Na+/Ca2+ ratio of the growth medium also has an effect on the development of root hairs, reducing their length and density (Shabala et al. 2003). A consistent feature of this body of experimentation is the ameliorative effect of increasing the concentration of Ca2+ in the growth medium. The addition of Ca2+ to a solution containing NaCl improves root elongation (Kent and Lauchli 1985; Kurth et al. 1986) and shoot growth (Cramer et al. 1989; Yeo et al. 1991; Cramer 1992), prevents symptoms of Ca2+ deficiency (Maas and Grieve 1987), reduces root thickening (Kurth et al. 1986) and restores the growth and development of root hairs (Shabala et al. 2003). At high salinities however, much of the growth inhibition by NaCl can be attributed to osmotic effects and these occur independently of the Ca2+ concentration of the growth medium (Cramer et al. 1989). Kinraide (1999) found that the negative effect of NaCl salinity on wheat roots could be restored by the addition of Ca2+ if the concentration of Na+ was 0.05). In Cairo, at ECFC 4.2 dS m1
, NaCl and CaCl2 and CNS treatments reduced the dry weight up to 23%, 26% and
15% respectively, whereas there was no significant difference between these treatments in Fiesta.
15
0.40
Gravimetric water content (g/g)
0.40
(b) Fiesta
(a) Cairo 0.35
0.35
0.30
0.30
0.25
0.25
0.20
0.20
0.15
0
2
4
6
8
10
12
14
Soil solution ECFC (dS m-1)
16
0.15
0
2
4
6
8
10
12
14
Soil solution ECFC (dS m-1)
Figure 4. The relationship between gravimetric water content of soil at harvest and ECFC of soil solution generated from NaCl in Cairo (a) and Fiesta (b). The upper dotted line indicates the soil water content at FC, the lower, short-dash line indicates PWP from matric potential alone (-900 kPa) and the solid line indicates the water content from a combined matric and osmotic potential of -1500 kPa where the mass of salt in solution remains constant (soil solution concentration increases as soil water content decreases). Values are means (n=4).
With increasing salinity, the difference between NaCl, CaCl2 and CNS treatments became less in Cairo but relative shoot dry matter of Fiesta in CNS3 treatments was 30% higher than NaCl. In Fiesta, the reduction in growth due to increased CaCl 2 was less compared to NaCl but in Cairo this was only seen at low and moderate salinity (Fig. 5 a, b). The whole shoot moisture content significantly decreased as the ECFC increased, with Cairo being about 20% lower than Fiesta (Fig. 5 c, d). In both varieties, the moisture content when grown with CNS was greater than when pant were grown with NaCl or CaCl2.
Shoot ionic concentration and leaf osmotic potential in relation to salinity For plants in non-saline soil, tissue Na+ concentration concentrations were approx 90 mmol kg-1 DW and no difference was apparent between the two genotypes. Tissue Na+ 16
16
concentration increased with salinity in both genotypes (P < 0.001), but the concentrations in the shoots of Fiesta were significantly less than those of Cairo. The Na+ concentration of plant tissue increased to 400 mmol kg-1 or by 4.5-fold for Fiesta and to 700 mmol kg-1 or 7.5-fold for Cairo over the range of ECFC levels in the NaCl treatment (Fig. 6a). Relative shoot dry matter declined with increasing Na+ concentration in both cultivars, but the decline was significantly less in Fiesta compared to Cairo (Fig. 6a). The concentration of Cl- also increased with the concentration of NaCl and the Cl- concentrations were consistently greater than the tissue Na+ concentrations in each NaCl treatment by up to 2.5-fold. The responses of the two genotypes differed significantly (Genotype × Treatment interaction, P < 0.01) (Fig. 6b). Chloride concentrations in Fiesta were up to 45% lower than those in Cairo (P < 0.01) consistently over all salinity levels. The reduction in relative shoot dry matter was highly correlated with increasing Cl- concentration in the whole shoot both in NaCl (r = -0.99, p 0.05). 13
These relationships changed at 150 mM NaCl. Shoot Na+ and Cl- concentration increased significantly under 150 mM NaCl but the range in concentrations among the genotypes was smaller and there was no significant relationship with salt tolerance (r = 0.09 for Na+ and r = - 0.17 for Cl- respectively). The concentration of K+ was also lower at the higher NaCl concentration but there was no relationship between the whole shoot K+ concentration salt tolerance under 150 mM NaCl (r = -0.01). However, in contrast to 75 mM, significant genotypic variation was observed in leaf osmotic potential among the 11 genotypes (P < 0.05) and it was highly correlated with salt tolerance (Fig 3f, r = 0.92, P < 0.001).
3.2
Experiment 2
3.2.1. Weather In 2008, rainfall during the May to October growing season was 71 mm less than the long-term average, whereas in 2009, it was just 0.3 mm less than average rainfall. The average rainfall in 2009 was about 14% higher than 2008, but the May-October rainfall in 2009 was 35% higher than 2008. In general, air temperatures were less in 2008 than in 2009 (Fig 2).
3.2.2 Genotypic variation in ion concentration and leaf osmotic potential in relation to grain yield In 2008, grain yield of the 6 genotypes ranged from 843 kg ha-1 in Manafest to 1370 kg ha-1 in Fiord. Significant genotypic variation occurred in Na+ and Cl- and K+ concentrations (Fig 4). Sodium concentrations varied widely, ranging from 105 to 322 mmol kg-1 DW. Chloride concentration varied about 2.5-fold ranging from 188 to 530 mmol kg-1 DW. As well, K+ concentration varied about 1.9-fold ranging from 490 to
14
900 mmol kg-1 DW. Grain yield was negatively and significantly correlated with leaf Na+ and Cl- concentrations (Fig 4a and b, P < 0.001) and the observed variations among the genotypes in K+ concentrations were negatively related to the Na+ concentration and positively related to the grain yield (Fig 4c and d, P < 0.001).
Large genotypic variation was also found among the genotypes in 2009 in grain yield, their tissue ionic concentrations and leaf osmotic potentials. Grain yield of the 11 genotypes ranged from 2923 kg ha-1 in accession 1477/4 to 3650 kg ha-1 in Nura. Significant genotypic variation occurred in Na+, Cl- and K+ concentrations (Fig 5). Sodium concentrations ranged from 195 in accessions 1487/7 and 1512/2 to 445 mmol kg-1 DW in Cairo and Manafest. Chloride concentration varied about 2.2-fold ranging from 265 in line 1512/2 to 588 mmol kg-1 DW in Manafest. As well, K+ concentration varied about 2-fold ranging from 375 to 790 mmol kg-1 DW. Leaf Na+ and Clconcentrations were negatively related to the grain yield (Fig. 5a and b, P < 0.001) and the observed variations among the genotypes in K+ concentrations were negatively related to the Na+ concentration and positively related to the grain yield. Significant genotypic variation (2-fold) was observed in leaf osmotic potential among the 11 genotypes (P < 0.01) and it was negatively correlated with grain yield (Fig 5d, r = -0.85, P < 0.001).
15
(a)
(b)
80 60 40
0
Fiord Cairo 1608/2 Farah 1477/4 Nura Manafest 1512/2 Icarus 1487/7 Fiesta
20
Cairo 1608/2 Manafest Farah 1477/4 Nura Icarus Fiord Fiesta 1512/2 1487/7
Salt tolerance (%)
100
100
(d)
Salt tolerance (%)
(c) 80 60 40 20 0
75 mM NaCl r = -0.96 150 mM NaCl r = n.s
300
600
75 mM NaCl r = -0.94 150 mM NaCl r = n.s
900 1200 1500 1800 2100 2400
300
Na+ concentration (mmol kg-1 DW)
Salt tolerance (%)
100
600
900 1200 1500 1800 2100 2400
Cl- concentration (mmol kg-1 DW)
(f)
(e)
80 60 40 20 0
75 mM NaCl r = 0.97 150 mM NaCl r = n.s 200
300
400
500
600
75 mM NaCl r = n.s 150 mM NaCl r = 0.92 700
K+ concentration (mmol kg-1 DW)
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
Leaf osmotic potential (-MPa)
Figure 3. The range in salt tolerance among 11 genotypes of faba bean grown at (a) 75 mM NaCl and (b) 150 mM NaCl, and the relationships between salt tolerance and shoot concentration of (c) Na+ concentration (mmol kg-1 DW), (d) Cl- concentration (mmol kg1
DW), (e) K+ (mmol kg-1 DW) and (f) leaf osmotic potential (-MPa) grown at 75 mM
(●) or 150 mM (○) NaCl for 42 days in an hydroponic system. Fitted curves are derived from linear regression. The vertical bars are LSD at 95%. Values are means (n=4). 16
1400
(a)
1300
Grain yield (kg ha-1)
Grain yield (kg ha-1)
1400
1200 1100 1000 900 800
r = -0.92
0
100 200 300 400 500 600
1200 1100 1000 900 800
1000 K+ concentration (mmol kg-1 DW)
Grain yield (kg ha-1)
1400
(c)
1200 1100 1000 900 800
r = 0.90
400
600
800
1000
K+ concentration (mmol kg-1 DW)
r = -0.94
0
100 200 300 400 500 600 Cl- concentration (mmol kg-1 DW)
Na+ concentration (mmol kg-1 DW)
1300
(b)
1300
(d)
800
600
400
r = -0.98
100
200
300
400
Na+ concentration (mmol kg-1 DW)
Figure 4. The relationship between grain yield and leaf concentration of (a) Na+ (mmol kg-1 DW), (b) Cl- (mmol kg-1 DW), (c) K+ (mmol kg-1 DW) and (d) the relationship between leaf Na+ and K+ concentration of 6 faba bean genotypes grown at Pinery site in 2008. The results are from youngest emerged leaves at full flowering. Fitted curves are derived from linear regression. The vertical bars are LSD at 95%. Values are means (n=4).
17
Grain yield (kg ha-1)
4000
(a)
(b)
3500 3000 2500 2000
r = -0.91
r = -0.84
1500 100
200
300
400
500
600
100
300
400
500
600
Cl- concentration (mmol kg-1 DW)
Na+ concentration (mmol kg-1 DW)
4000
(d)
(c) Grain yield (kg ha-1)
200
3500 3000 2500 2000 1500 200
r = -0.85
r = 0.87
400
600
800
1000
K+ concentration (mmol kg-1 DW)
1.0
1.2
1.4
1.6
1.8
2.0
2.2
Leaf osmotic potential (-MPa)
Figure 5. The relationship between grain yield and leaf concentration of (a) Na+ concentration (mmol kg-1 DW), (b) Cl- concentration (mmol kg-1 DW), (c) K+ (mmol kg-1 DW) and (d) leaf s (-MPa) of 11 faba bean genotypes grown at Pinery site in 2009. The results are from youngest emerged leaves at full flowering. Fitted curves are derived from linear regression. The vertical bars are LSD at 95%. Values are means (n=4).
The variation in the accumulation of compatible solutes was associated with variation in Na+ and Cl-. Among the 11 genotypes (Table 2), the five varieties that maintained lower leaf Na+ and Cl- concentrations also exhibited a stronger capacity for accumulation of organic solute that contributed to total leaf osmotic potential. For example in accessions 18
1487/7, 1512/2, Fiesta, Fiord and Icarus organic solutes contributed 29-37% to total leaf osmotic potential, while the contribution from inorganic solutes accounted for 55-64%. However, in Cairo, Manafest and Farah, the major contribution to total leaf osmotic potential was from the high concentrations of Na+ and Cl- which accounted for about 77-79% (Table 2).
4. Discussion This study was conducted to assess the variation among faba bean genotypes in response to salinity and also to examine which physiological traits can be used to assess salt tolerance among faba bean genotypes under both field and controlled conditions. Significant genotypic variation for salinity tolerance in faba bean was measured among the 11 varieties screened in hydroponics (Fig 1, Table 1S). The most tolerant entries were 1487/7, 1512/2 and Fiesta which are used regularly in the local faba bean breeding program for introgressing various other tolerance characteristics. A significant result from the screening is that the importance of Na+ and Cl- exclusion to salt tolerance varied with the severity of stress: it was important to salinity tolerance at low levels of salinity (75 mM NaCl) but its importance was diminished at 150 mM NaCl, when osmotic tolerance became more important (Fig 1). The two mechanisms – ion exclusion and tolerance to osmotic stress – were independent and altered the relative salt tolerance of genotypes depending on the severity of stress. For example, Fiesta and 1512/2 display similar levels of Na+ and Cl- exclusion at high and low levels of salinity (Fig 1, Table 1s). At 75 mM they also have similar levels of salt tolerance, but at 150 mM Fiesta had 11% greater salt tolerance than 1512/2 which was associated with a lower leaf osmotic potential suggesting it had also a greater level of osmotic tolerance.
19
Table 2. Estimated contribution of organic and inorganic ions to leaf osmotic potential (s). The contribution of individual solutes to measured s was determined using the van’t Hoff equation, where the calculated Ψs, was based on solute concentration on a fresh weight basis. The percentage value is based on the measured value of leaf s. s
Na+
Cl-
K+
Sucrose
Glucose
Fructose
Betaine
Proline
%Inorganic contribution
%Organic contribution
%Total
1477/4
-1.88
-0.48
-0.38
-0.42
-0.09
-0.08
-0.09
-0.09
-0.09
68
23
91
1487/7
-1.05
-0.21
-0.18
-0.31
-0.08
-0.05
-0.05
-0.02
-0.11
67
30
96
1512/2
-1.15
-0.15
-0.23
-0.25
-0.11
-0.07
-0.07
-0.05
-0.12
55
37
91
1608/2
-1.75
-0.45
-0.48
-0.39
-0.05
-0.08
-0.05
-0.07
-0.08
75
19
94
Cairo
-1.99
-0.55
-0.58
-0.41
-0.02
-0.05
-0.05
-0.09
-0.08
77
15
92
Farah
-1.55
-0.49
-0.51
-0.22
-0.05
-0.08
-0.05
-0.08
-0.05
79
20
99
Fiesta
-1.25
-0.21
-0.18
-0.31
-0.11
-0.12
-0.11
-0.08
-0.06
59
37
96
Fiord
-1.38
-0.21
-0.33
-0.35
-0.05
-0.09
-0.15
-0.05
-0.06
64
29
93
Icarus
-1.33
-0.22
-0.28
-0.35
-0.15
-0.12
-0.09
-0.08
-0.02
64
35
98
Manafest
-2.01
-0.51
-0.67
-0.35
-0.05
-0.05
-0.08
-0.05
-0.04
76
13
90
Nura
-1.33
-0.33
-0.32
-0.33
-0.05
-0.09
-0.07
-0.08
-0.05
74
26
99
Van’t Hoff equation Ψs (MPa) = –csRT, where cs = Osmolarlity (mol L–1), R = 0.0083143 L MPa mol–1 K–1, and T = 293 K were considered. Contribution = (Ψs calculated/Ψs measured) × 100.
20
Salt tolerance reflects the ability of the plant to exclude Na+ and Cl- as well as mechanisms associated with tolerance of the cells to high osmotic potential. As the concentration of salt increases, the ability to exclude salt may become less effective in protecting the plant from salt stress and other mechanisms, such as osmotic tolerance, become increasingly important. Salinity stress in the field is variable over time and space and so overall salt tolerance will depend on the relative contribution from the different mechanisms of salt tolerance, which in turn can vary with the severity of salinity stress (Tavakkoli et al. 2010b).
Improving the grain yield of crops is always a major target in plant breeding. Therefore, the evaluation of final grain yield and growth parameters determining grain yield is a critical aspect of breeding programs. Linear regressions adequately described the relationship between Na+ and Cl- concentrations in the plant and grain yields of genotypes, thus variation in yields was attributed to variation in ability of genotypes to exclude Na+ and Cl- ions (Fig 4 and 5).
A common mechanism in response to salt stress is the accumulation of compatible solutes which may be interpreted as a symptom of injury caused by stress or some type of adaptive response (Ashraf and Harris 2004). This poses the question of whether salttolerant genotypes also have a superior ability to accumulate higher concentrations of compatible solutes. In our study, salt stress caused an increase in ions and organic solutes in all genotypes, but the more salt tolerant varieties had significantly higher concentration of soluble sugars (glucose, fructose and sucrose), glycine betaine, proline and trigonelline. Cram (1976), showed that of the various organic osmotica, sugars contribute up to 50% of the total osmotic potential in glycophytes subject to saline 21
conditions. Ion accumulation in plants can also play a major role in osmotic adjustment to high salinities. It would seem, however, from the relationship between ion accumulation and water status observed here for genotypes that the simple accumulation of Na+ and Cl- alone can not account for the osmotic behaviour of these varieties (Table 2). While the ability to restrict Na+ and Cl- accumulation could prevent the development of an internal osmotic imbalance in some genotypes, the concentration of Na+ and Claccumulated when considered together with the reduction in shoot K+ would seem to necessitate the synthesis of additional osmotically active solute in order to prevent an osmotic imbalance with respect to the external salt in soil solution (Table 2). The increases observed in soluble sugars, glycine betaine and proline amount to a total of ~800µmol g-1 DW (data not shown) and could therefore represent an important component of the shoot osmotic potential (Table 2). Notwithstanding this caveat the present study suggests that salt exclusion coupled with a synthesis of organic solutes are important components of salt tolerance in the tolerate genotypes.
In order to assess the consistency in ranking of faba bean genotypes under both field and controlled condition, the salt tolerance ranking of 11 genotypes in Experiment 1 was compared with grain yield production under field condition in 2009. The rankings of genotypes based on their salt tolerance in controlled condition at 75 mM NaCl (but not 150 mM NaCl) and grain yield production in the field (P=0.006, n = 11) (Table 3), and the crucial parameters of leaf Na+ and Cl- concentration (rs = 0.88; P = 0.002; n = 11, data not shown) were significantly correlated. Moreover, in the hydroponic experiment the rankings of genotypes based on their salt tolerance in the two salt concentrations was only just significant (rs = 0.68, P = 0.04). This consistency of the
22
rankings between hydroponics (especially at the lower NaCl concentration) and field supports the robustness of the overall results. Table 3. The Spearmans’s rank correlations between grain yield of field-grown plants and salt tolerance of plants grown in the hydroponic (n=11). Field Grain yield Field
Hydro 75 mM Salt tolerance
Grain yield
Hydro 75 mM
Salt tolerance
0.85 (P=0.006)
Hydro 150 mM
Salt tolerance
0.47 (P=0.128)
0.68 (P=0.04)
To answer the question of which physiological screening criteria can enable an accurate ranking of faba bean genotypes under both field and control conditions, the correlation coefficient between the grain yield and the scores of different physiological parameters were analysed by simple linear correlation (Table 4). The grain yield production in the field was significantly associated with the salt tolerance index of plants grown in 75 mM NaCl and the exclusion of Na+ and Cl− in leaves. Interestingly, the results in this study indicate that K+ content in plants demonstrated a great genotypic variation and was well correlated with the salt tolerance ranked by using grain yield (Table 4). Under saline conditions, low osmotic potentials of the soil solution induce water deficit in plant tissue. As a consequence, the turgor in plants may decrease. Leaf osmotic potential was significantly correlated with grain yield in the field (Table 4).
23
Table 4. Correlation coefficients (r) between pairs of physiological attributes of salt stressed faba bean plants grown in the field in 2009 and at 75 mM of 150 mM NaCl in hydroponics . *, **, *** = significant at 0.05, 0.01, and 0.001 levels, respectively. n = 11.
Field Grain Yield
Field Na
+
Field K
+
Field Cl
-
Field
s
Hydro 75mM ST
Hydro 75mM Na
+
Hydro 75mM K
+
Hydro 75mM Cl
Hydro 75mM
s
-
Field (2009)
yield
Field (2009)
Na+
-0.84**
Field (2009)
K+
0.87**
-0.89**
Field (2009)
Cl
-
-0.91***
0.91***
-0.95
Field (2009)
s
-0.86**
0.89**
-0.93***
0.94***
Hydro 75mM
ST
0.85**
-0.95***
0.86**
-0.85**
-0.89**
Hydro 75mM
Na
+
-0.84**
0.92***
-0.81**
0.81**
0.84**
-0.97***
Hydro 75mM
K
+
0.76**
-0.91***
0.84**
-0.79**
-0.87**
0.97***
-0.92***
Hydro 75mM
Cl-
-0.79**
0.92***
-0.73**
0.75**
0.78**
-0.94***
0.94***
-0.92***
Hydro 75mM
s
-0.70*
0.92***
-0.88**
0.84**
0.87**
-0.88**
0.88**
-0.90**
0.89**
Hydro 150mM
ST
0.53
-0.43
0.51*
-0.44
-0.55*
0.66*
-0.63*
0.64*
-0.56*
-0.51*
Hydro 150mM
Na+
0.03
0.21
-0.10
0.17
0.25
-0.15
0.16
-0.09
0.28
0.18
Hydro 150mM
K
+
0.20
-0.03
-0.14
0.08
0.09
0.04
-0.22
-0.11
-0.03
0.09
Hydro 150mM
Cl-
-0.67
0.91
-0.71**
0.80**
0.77**
-0.88**
0.85**
-0.82**
0.95***
0.83**
Hydro 150mM
s
-0.40
0.39
-0.43
0.33
0.53*
-0.63*
0.62*
-0.68*
0.57*
0.52*
24
Because of the spatial and temporal variation in salt stress at field level, plants are exposed to varying levels of salt stress, whereas much of the past work has focussed on using one mechanism to improve salt tolerance. The present results suggest mechanisms of exclusion are important under low-moderate stress and osmotic effects more important at higher levels of stress. While Na+ and Cl- exclusion is an important primary mechanism of salt tolerance (Tavakkoli et al. 2010a), the development of more salttolerant germplasm is likely to be accelerated if screening based on ion exclusion also takes into account genotypic differences in osmotic tolerance. It is also important to emphasise that while use of hydroponic-based screening in this study was shown to be well-correlated with results from the field experiment for faba bean, previous studies in cereals such as barley demonstrated that solution culture may not allow differences in salt tolerance between genotypes to be discerned and the physiological responses are not the same when the same materials were grown in soil (Tavakkoli et al. 2010b).
In conclusion, the tested physiological traits showed significant genotypic variation, indicating that the traits that have a significant genotypic variation may possibly be used as screening criteria. The increased production of faba bean under rainfed conditions on saline-sodic soils highlights the importance of improving salinity tolerance through breeding. The availability of large and useful genotypic variation as shown in this study, and the high association of Na+ and Cl- exclusion and K+/Na+ ratio with biomass indicates that the introduction of low Na+ and Cl- accumulation into modern cultivars should be possible as part of a faba bean breeding program. This study also clearly shows that several processes are involved in salt tolerance and that the relative importance of these traits may differ with the severity of the salt stress. The osmotic stress was the predominant cause of reduced growth at high levels of salinity, while 25
specific-ion toxicity was more important under mild salinity stress. It is likely that stress-tolerant faba bean plants accumulate compatible solutes and further field tests of these plants under stress conditions will help to verify their potential utility in cropimprovement programs.
Acknowledgements This work was supported by a grant from the Grains Research and Development Corporation to ET and by the University of Adelaide. We thank Mr. S. Coventry (The University of Adelaide) for excellent HPLC guidance and Mr K. James for his support with the field studies.
26
References Ashraf M, Harris PJC (2004) Potential biochemical indicators of salinity tolerance in plants. Plant Science 166, 3-16. Bernstein L (1961) Osmotic adjusment of plants to saline media. I. steady state. American journal of botany 48, 909-918. Boyer JS, James RA, Munns R, Condon TAG, Passioura JB (2008) Osmotic adjustment leads to anomalously low estimates of relative water content in wheat and barley. Functional Plant Biology 35, 1172-1182. Cram WJ (1976) Negative feedback regulation of transport in cells. The maintenance of turgor, volume and nutrient supply. In 'Encyclopaedia of Plant Physiology'. (Eds U Luttge and MG Pitman) pp. 284-316. (Springer-Verlag: Berlin) Del Pilar Cordovilla M, Ligero F, Lluch C (1999) Effect of salinity on growth, nodulation and nitrogen assimilation in nodules of faba bean (Vicia faba L.). Applied Soil Ecology 11, 1-7. Duc G, Bao S, Baum M, Redden B, Sadiki M, Suso MJ, Vishniakova M, Zong X (2010) Diversity maintenance and use of Vicia faba L. genetic resources. Field Crops Research 115, 270-278. Epstein E, Norlyn JD, Rush DW, Kingsbury RW, Kelley DB, Cunningham GA (1980) Saline culture of crops: a genetic approach. Science 210, 399-404. Flowers TJ (2004) Improving crop salt tolerance. Journal of Experimental Botany 55, 307-319. Genc Y, McDonald GK, Tester M (2007) Reassessment of tissue Na+ concentration as a criterion for salinity tolerance in bread wheat. Plant, Cell and Environment 30, 14861498. Hall J, Maschmedt D, Billing B (2009) 'The soils of Southern South Australia ' (Department of water, land and biodiversity conservation, Government of South Australia) Isbell RF (1996) 'The Australian Soil Classification.' (CSIRO: Melbourne) Jensen ES, Peoples MB, Hauggaard-Nielsen H (2010) Faba bean in cropping systems. Field Crops Research 115, 203-216. Kebebew F, McNeilly T (1996) The genetic basis of variation in salt tolerance in Pearl Millet, Pennisetum americanum (L.) Leeke. J Genet and Breed 50, 129-136. Krishnamurthy L, Serraj R, Hash C, Dakheel A, Reddy B (2007) Screening sorghum genotypes for salinity tolerant biomass production. Euphytica 156, 15-24.
27
Li-juan L, Zhao-hai Y, Zhao-jie Z, Ming-shi X, Han-qing Y (1993) Study and utilization of faba bean germplasm resources. In 'Faba bean in China: State-of-the art review.'. (Eds MC Saxena, S Weigand and L Li-Juan) pp. 51-63. (ICARDA Press) Malhotra RS (1997) Evaluation techniques for abiotic stresses in cool season food legumes. In 'Recent Advances in Pulses Research'. (Eds AN Asttranu and A Masood) pp. 459-473. ( Indian Society of Pulses Research and Development: Kanpur, India) Marigo G, Peltier JP (1996) Analysis of the diurnal change in osmotic potential in leaves of Fraxinus excelsior L. Journal of Experimental Botany 47, 763-769. Meier U (Ed.) (2001) 'Growth stages of mono-and dicotyledonous plants: BBCH Monograph.' (Federal Biological Research Centre for Agriculture and Forestry: Germany) Morgan JM, Rodriguez-Maribona B, Knights EJ (1991) Adaptation to water-deficit in chickpea breeding lines by osmoregulation: relationship to grain yields in the field. 27, 61-70. Munns R, Hare RA, James RA, Rebetzke GJ (2000) Genetic variation for improving the salt tolerance of durum wheat. Aust. J. Agric. Res. 51, 69-74. Munns R, James RA (2003) Screening methods for salinity tolerance: a case study with tetraploid wheat. Plant and soil 253, 201-218. Munns R, Tester M (2008) Mechanisms of salinity tolerance. Annual Review of Plant Biology 59, 651-681. Noble CL, Halloran GM, West DW (1984) Identification and selection for salt tolerance in Lucerne (Medicago sativa L.). Australian Journal of Agricultural Research 35, 239252. Noble CL, Rogers ME (1992) Arguments for the use of physiological criteria for improving the salt tolerance in crops. plant Physiol 146, 99-107. Norlyn JD, Epstein E (1984) Variability in salt tolerance of four tritcale lines at germination and emergence. Crop Sci 24, 1090-1092. Poustini K, Siosemardeh A (2004) Ion distribution in wheat cultivars in response to salinity stress. Field crops research 85, 125-133. R Development Core Team (2006) A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna. Richards RA (1983) Should selection for yield in saline regions be made on saline or non-saline soils? Euphytica 32, 431-438. Rispail N, Kaló P, et al. (2010) Model legumes contribute to faba bean breeding. Field Crops Research 15, 253-269. 28
Robson MC, Fowler SM, Lampkin NH, Leifert C, Leitch M, Robinson D, Watson CA, Litterick AM (2002) The agronomic and economic potential of break crops for ley/arable rotations in temperate organic agriculture. Adv Agron 77, 369-472. Rowell D (1994) 'Soil science : methods and applications.' (Wiley: New York) Tavakkoli E, Rengasamy P, Mcdonald GK (2010a) High concentrations of Na+ and Clions in soil solution have simultaneous detrimental effects on growth of faba bean under salinity stress. Journal of Experimental Botany 61, 4449–4459. Tavakkoli E, Rengasamy P, Mcdonald GK (2010b) The response of barley to salinity stress differs between hydroponics and soil systems. Functional Plant Biology 37, 621633. Turner NC, Wright GC, Siddique KHM (2001) Adaptation of grain legumes (pulses) to water-limited environments. In 'Advances in Agronomy' pp. 193-231. (Academic Press) Verma OPS, Yadava RBR (1986) Salt tolerance of some oats (Avena sativa L.) varieties at germination and seedling stage. J. Agronomy & Crop Science 156, 123-127. Yeo AR, Flowers TJ (1983) Varietal differences in the toxicity of sodium ions in rice leaves. Physiologia Plantarum 59, 189-195.
29
Table1S. Salt tolerance (ST), shoot dry matter, Na+, K+, Cl- concentration and s of genotypes grown at 75 and 150 mM NaCl for 49 days. Genotypes are arranged in ascending order of salt tolerance. Genotype
Shoot dry matter (g plant-1)
ST (%)
Na+
K+
Cl-
s
1888
-1.51
Cairo
Control 4.99
Salt 2.02
41
75 mM NaCl 1754 401
1608/2
3.47
1.50
45
1658
407
1655
-1.51
Manafest
4.51
2.30
52
1813
401
1911
-1.53
Farah
4.31
2.35
56
1584
453
1233
-1.49
1477/4
3.62
2.15
59
1411
453
1215
-1.51
Nura
3.75
2.22
60
1653
453
1656
-1.50
Icarus
4.12
3.01
73
1565
455
1255
-1.51
Fiord
3.82
2.73
75
1501
553
988
-1.47
Fiesta
4.29
3.48
81
1126
628
858
-1.46
1512/2
3.93
3.28
85
1153
657
834
-1.40
1487/7
3.66
3.34
93
1253
720
730
-1.43
1188
-2.65
Fiord
3.82
1.28
35
150 mM NaCl 1956 446
Cairo
4.99
1.94
39
2356
225
1724
-2.55
1608/2
3.47
1.44
43
1741
323
1353
-2.45
Farah
4.31
2.00
48
2019
516
1858
-2.35
1477/4
3.62
1.93
53
2192
686
1939
-2.40
Nura
3.75
2.06
55
2221
432
1699
-2.44
Manafest
4.51
2.58
58
2124
422
1956
-2.35
1512/2
3.93
2.20
61
2105
453
1299
-2.22
Icarus
4.12
2.61
63
2115
455
1205
-2.14
1487/7
3.66
2.76
72
2109
308
1264
-2.05
Fiesta
4.29
3.11
72
1988
358
1258
-1.95
0.136
2.1
7.2
7.19
11.2
0.047
LSD0.05
(genotype×salt)
30
Table 2S. The grain yield (kg ha-1) and leaf Na, Cl and K concentration (mmol kg-1 DW) of 6 faba bean genotypes grown at Pinery site in 2008. Values are averages (n=4).
Cairo Farah Fiesta Fiord Manafest Nura
Yield (kg/ha)
Na
K
Cl
1050.0 1170.4 1247.2 1371.0 843.8 1139.5
309.7 195.0 113.0 105.3 322.0 219.7
503.7 718.7 887.3 902.0 491.0 655.0
465.0 278.7 227.0 188.3 530.0 394.7
31
Table 3S. The grain yield (kg ha-1) and leaf Na, Cl and K concentration (mmol kg-1 DW) of 11 faba bean genotypes grown at Pinery site in 2009. Values are averages (n=4).
1477/4 1487/7 1512/2 1608/2 Cairo Farah Fiesta Fiord Ic*As/15/1-20 Manafest Nura
yield
Na
K
Cl
2923 3575 3512 2115 2055 2488 3314 3350 3150 2350 3650
378 198 195 425 445 385 295 225 261 445 355
405 745 788 450 385 411 588 601 605 374 656
523 277 265 530 544 502 355 311 291 585 305
32
Chapter 9
Chapter 9 General discussion and conclusions 9.1
Introduction
Broadacre cropping in Australia is based on rainfed systems in a semiarid environment, where the efficient uptake and use of water is the main driver of productivity, but the presence of subsoil constraints such as salinity and sodicity in many soils reduces the amount of water and nutrients plants can obtain from the soil (Hochman et al. 2007; Rengasamy 2010a). More than 60% of the 20 million ha of cropping soils in Australia are sodic, which together with low rainfall and high rates of evapotranspiration have contributed to the development of transient salinity (Rengasamy 2002).
Salinity stress inhibits plant growth for two reasons (Munns et al. 2006). First, the presence of salt in the soil solution reduces the ability of the plant to take up water and this leads to slower growth (osmotic stress). In soil, the effect may be exacerbated by the influence of the soil matrix on water retention (the soil matric potential). Second, excessive amounts of salt can accumulate in leaves and this may further reduce growth (ion specific toxicity) (Munns and Termaat 1986; Munns et al. 2002; Munns et al. 2006). The physiology of plant responses to salinity and their relation to salinity resistance have been thoroughly researched and frequently reviewed in recent years (Munns 1993; Flowers and Yeo 1995; Munns 2002; Tester and Davenport 2003; Flowers 2004; Colmer et al. 2005; Munns et al. 2006; Apse and Blumwald 2007; Munns and Tester 2008; Dang et al. 2010; Nuttall et al. 2010; Rengasamy 2010b). A common theme is that plants with increased salinity tolerance are expected to maintain higher rates of growth than less tolerant plants under equivalent levels of salinity, 70
but there is some controversy regarding the importance of the different mechanisms of salinity tolerance to effect improvements in growth. This controversy reflects the difficulty in separating osmotic effects from specific ion effects. It was often impossible to assess with any confidence the relative importance of ion excess and osmotic stress partly because after the initial osmotic stress of low levels of salinity, osmotic stress and ion specific stress develop concurrently as salt stress increases (Munns et al. 2006).
Soil is a heterogeneous medium and this presents a number of challenges in developing strategies to generate salt-tolerant varieties. The severity of salt stress in the field is inherently variable, both over time and space, and there are also interactions with other soil properties such as soil texture that influence how plants respond to saline-sodic conditions. Given that the cause of reductions in growth under salt stress varies with the severity of the stress, it is difficult to say which is the major cause of yield reductions under salt stress and perhaps unrealistic to specify a single trait which should be targeted for plant breeding. Additionally, the development of appropriate strategies for the management of broad acre crops produced on saline-sodic soils has been hampered by uncertainty regarding the principal mechanisms by which saline-sodic soils limit crop performance. The physical characteristics of saline soils affects water holding capacity and soil water availability and the presence of high concentrations of salt may exacerbate the poor soil-plant water relations of these soils. Despite the fact that the osmotic effect on growth of the more-tolerant species such as wheat and barley is much greater than the salt-specific effect, the relative importance of the mechanisms that regulate the growth are not yet well understood (Munns et al. 2006).
There has also been some recent debate about the importance of soil Cl, and by implication plant Cl- uptake, as predictors of damage and yield loss, rather than electrical conductivity. 71
Where NaCl is high, increased uptake of Na+ ions will be associated with high uptake of Clions (Dang et al. 2006a; Dang et al. 2006b; Teakle and Tyerman 2010). Soil Cl- has been suggested to be a better predictor than Na+ of growth and grain yield of dryland cereal crops under salt stress in southwest Queensland where there are high levels of gypsum (CaSO4) in the soil (Dang et al. 2006a). Under such conditions, electrical conductivity is high because of the high concentrations of CaSO4 in the soil but, Na+ uptake may be relatively low and thus the uptake of Cl- relative to Na+ may be high. However the soils of southern Australia have lower concentrations of gypsum and the importance of Cl- to salt stress in annual grain crops has generally been overlooked. Chloride toxicity is known to be important in some species, especially perennial plants (Wieneke and Läuchli 1979; Hajrasuliha 1980; White and Broadley 2001), but there is little information on its impact on most broadacre crops. Little is known concerning the primary acquisition mechanisms of Cl- by plants, and knowledge about its subcellular distribution and flux dynamics is scarce (Britto et al. 2004).
Reliable and effective screening techniques for salt tolerance to predict field performances are important for breeding programmes. While much of the work on salt tolerance has been conducted under controlled conditions, it is important to verify whether or not the laboratory conditions can predict responses to field stresses. Studies using solution culture methods have failed to address the actual plant response under field conditions, where several environmental factors are also involved. Thus, there is a clear need to characterise the relative contributions of the various soil and plant factors that reduce growth and yield over the length of a growing season.
72
The studies conducted in this thesis aimed to quantify the effects of salinity on the growth and yield of barley and faba bean in relation to the mechanisms responsible for these effects. The main objectives of this research were to:
quantify the relative importance of ion (Na+ and/or Cl-) toxicity and osmotic stress on growth and yield reduction under different levels of salinity
determine which of the two ions most frequently implicated in salinity, Na+ and Cl-, is more toxic to barley and faba bean
investigate whether hydroponic and pot experiments under controlled environmental conditions are useful plausible surrogates for evaluating whole-plant response to salinity under field conditions.
The value of the research outcomes of this thesis lie in their potential to improve the understanding of the soil processes that limit crop productivity in saline-sodic soils. The experiments described in Chapters 3 and 4 were designed to determine the relative importance of osmotic stress and specific ion toxicity in barley and faba bean. The results of these experiments were also used to inform the design of subsequent glasshouse experiments in Chapters 5 and 6, which determined the extent to which high concentrations of Na+ and Cllimit the growth of crop plants. In Chapters 7 and 8 the physiological responses of a relatively large number of barley and faba bean genotypes to salinity were examined under controlled conditions and the results compared to ion uptake and yield for these genotypes under field conditions. These results have been used in the interpretations of the results of glasshouse studies in Chapters 3-6.
In terms of growth conditions of plants in Chapters 5-8 some precautions were taken to minimise probable, often unrecognised, artefacts that exist in pot experiments, especially 73
those related to the effects of the water relations and oxygen status of the soil on the functioning of plants. This has been done by using larger pots (10 cm in diameter and 32cm in height) in Chapters 5-8 compared to the pots (15 cm in diameter and height) used in Chapters 3 and 4. As well, using a two-layer soil design (saline-subsoil and non-saline topsoil) and limiting watering to the saline sub-soil allowed the experiments to simulate the effects of saline subsoil that occur in the field. In this way it was possible to draw conclusions regarding soil processes responsible for limitations to crop growth and nutrition in salinesodic soils and the results of field trials. The implications of the results of the experiments in Chapters 3-8 are outlined below.
9.2
Relative importance of ion (Na+ and/or Cl-) toxicity and osmotic
effect to growth and yield reduction under different levels of salinity Salinity stress is dynamic as osmotic and ionic stresses vary over time (Munns 2002). Consequently the response of plants to salt stress changes as the importance of the different components of salt stress – osmotic stress and ion toxicity - and the corresponding tolerance mechanisms varies. This dynamic nature of salt stress is often overlooked in many studies which focus on one specific mechanism to improve salt tolerance. Moreover, as different genotypes have different abilities to exclude Na+ and Cl- the relative effects of ionic and osmotic effects will also differ.
The experiments in Chapters 3 and 4 were designed to investigate the relative importance of osmotic stress and ion toxicity in genotypes with different abilities to exclude Na+ and Cl-. The osmotic effect was examined by using concentrated nutrient solution at similar EC as the salt treatments. In Chapter 4 a non-destructive, real-time method using image capture and analysis equipment (LemnaTec ‘Scanalyser 3D’) was used to assess the growth of plants 74
during a greenhouse experiment. These measurements were done in parallel with measurements of plant water use. The results clearly showed that the relative importance of ion toxicity and osmotic stress varied with: (1) the degree of stress encountered (mild, moderate, or severe); (2) the variety which is investigated; (3) the duration of the stress and (4) the time of sampling. These results were consistently observed in both barley and faba bean and also in Chapters 7 and 8 in which 15 genotypes of barley and 10 genotypes of faba bean were screened for salt tolerance at different levels of salinity stress. Plant growth was strongly reduced by salinity, but non-excluding genotypes showed a greater reduction in growth than excluding genotypes. Under mild salinity stress (ECFC =4.2 dS m-1), growth of excluding varieties of barley and faba bean (Clipper for barley and Fiesta for faba bean) was reduced mainly by osmotic stress due to efficient exclusion of Na+ and Cl-. However, the efficiency of ion exclusion declined at higher levels of salinity (ECFC=8 and 15 dS m-1) and specific ion toxicity appeared to be the main cause of reduced growth. At high levels of salinity stress (15 dS m-1), exclusion of Na+ and Cl- in the excluding genotypes, Clipper and Fiesta, appeared to contribute less to salt tolerance, however, Fiesta showed a high level of tolerance to osmotic stress which was reflected its ability to maintain a relatively high plant moisture content uner increasing salt stress. On the other hand, the reduction in growth of Cairo (faba bean) compared to Fiesta and Sahara (barley), compared to Clipper, was a result of NaCl treatments rather than pure osmotic treatments at low salinity stress. However, the osmotic stress became the major cause of growth reduction with increasing levels of soil salinity.
The studies in Chapter 8 were conducted to assess the variation among faba bean genotypes in response to salinity and also to examine which physiological traits can be used to assess salt tolerance among faba bean genotypes under both field and controlled conditions. The 75
results of screening faba bean genotypes in two levels of salinity stress showed that the importance of Na+ and Cl- exclusion to salt tolerance varied with the severity of stress: it was important to salinity tolerance at low levels of salinity (75 mM NaCl) but its importance was diminished at 150 mM NaCl, when osmotic tolerance became more important (Figure 1 of Chapter 8). The two mechanisms – ion exclusion and tolerance to osmotic stress – were independent and altered the relative salt tolerance of genotypes depending on the severity of stress.
In conclusion, an interpretation of the results in Chapters 3, 4, 7 and 8 is that the difference between the genotypes at low ECFC (~ 4-8 dS m-1) values is an ion-specific effect, while the rapid decline in growth above EC 8.5 dS m-1 is a result of both osmotic and ion-toxicity effects. Osmotic stress was the predominant cause of differences in growth between genotypes at high levels of salinity, while specific-ion toxicity was more important under mild salinity stress. It was also shown that genotypes which maintained greater whole-plant tolerance to salinity had two mechanisms such as tissue tolerance and osmotic adjustment (Chapter 3) or ion exclusion and osmotic adjustment (Chapter 4), compared to sensitive genotypes. As the importance of different mechanisms to salinity tolerance differs by the severity of stress, robust levels of salt tolerance may depend on more than one mechanism and selection for improved salt tolerance therefore needs to be able to identify these.
9.3
Relative importance of Na+ and Cl- toxicity in growth reduction of
barley and faba bean Despite the fact that most plants accumulate both sodium (Na+) and chloride(Cl-) ions in high concentration in their shoot tissues when grown in saline soils, most research on salt tolerance in annual plants has focused on the toxic effects of Na+ accumulation. Chapters 5 76
and 6 were designed to determine the extent to which specific ion toxicities of Na+ and Clreduce the growth of barley and faba bean plants and to clarify the controversy which exists in the current literature of salinity research concerning the toxic effects of these two ions on plant growth (Kingsbury and Epstein 1986; Munns et al. 1988; Kinraide 1999; Dang et al. 2006a; Dang et al. 2008; Slabu et al. 2009). A soil-based design was employed using a combination of different salts to produce soils enhanced with Na+, Cl- and NaCl with similar soil solution EC and ΨOThis differs from many earlier studies comparing the effects of Na+ and Cl− in grain crops in which plants were grown under salinity stress and have mainly used short-term hydroponic experiments (ranging from 2-14 days) with a single salt of Na+ or Cland one or two genotypes (Kingsbury and Epstein 1986; Lin and Kao 2001; Tsai et al. 2004; Luo et al. 2005; Slabu et al. 2009). Sodium gluconate and the Cl- inhibitor DIDS were also used to separate the effects of Na+ and Cl- on growth.
9.3.1
Barley
In order to determine which of the two ions is more toxic to growth of barley three experiments were conducted (Chapter 5). A solution culture experiment was conducted to assess the effect of different concentrations of Na+ and Cl- ions (0-150mM) on the growth of Barque73 and Tadmor, which differed in Cl- uptake. In a following solution culture experiment the relative importance of Na+ and Cl- ions to salt toxicity was assessed in 4 genotypes of barley. The effect of Na+ independently of Cl- was examined in three ways: by using DIDS, which is a non-permeating amino acid that inhibits Cl- transport (Lin 1981; Lin and Kao 2001); by using Na+-gluconate because gluconate is an anion that is unable to permeate the cell membrane; and by using the Na+-Hoagland and Cl--Hoagland solutions. The Na+-dominant and Cl--dominant Hoagland’s solutions were designed to provide equimolar concentrations of the Na+ and Cl- ions generated from various salts of Na+ and Cl77
to avoid increasing particular counteranions/countercations. Using barley varieties with known genetic variation in salinity tolerance and in Na+ and Cl- uptake also assisted in distinguishing the toxic effects of Na+ from Cl-. A third soil based experiment was designed to simulate the responses in field-grown plants. The method employed here maintained a constant EC and ΨObut used a combination of different salts to produce soils enhanced with Na+, Cl- and NaCl.
The results of these studies and also the results from Chapters 3 and 7 indicated that growth of barley under NaCl stress was caused by the additive effect of the reductions due to Na+ and Cl-. Moreover, the responses to Na+ and Cl- stress among the genotypes were independent and so similar levels of tolerance to NaCl could be achieved by different combinations of responses to Na+ and Cl- (Tables 4, 5 and 6 of Chapter 5). This result is clearly at odds with previous studies that have dismissed Cl- toxicity as a contributing factor to salt damage (Kingsbury and Epstein 1986; Kinraide 1999; Lin and Kao 2001; Tsai et al. 2004). These previous experiments either used sole counter-anions such as 120 mM nitrate (Kingsbury and Epstein 1986), which at high concentrations can be phytotoxic (Chen et al. 2004), or were short-term studies lasting less than a week (Kinraide 1999). On the basis of two-phase model of plant response to salinity stress (Munns et al. 1995) the specific ion toxicity develops over several weeks when the accumulated Na+ and Cl- reach the toxic concentration and therefore any interpretation on the basis of short-term experiments may not be valid.
9.3.2
Faba bean
To assess the relative importance of toxicity of Na+ versus Cl− in faba bean an experiment was conducted in a field soil using two varieties of faba bean, Nura and line 1487/7 differing in their ion exclusion mechanism and salt tolerance (Chapter 6). Salinity reduced biomass 78
production and water uptake of faba bean plants (Figure 1, Table 2 of Chapter 5) and from the results it was clear that plants were more sensitive to Cl- than to Na+. However, the data also show that when leaf Cl- concentration is high, the presence of Na+ as the dominant cation exacerbates the severity of the effects (Figure 1 of Chapter 6 and Figures 1 and 2 of Chapter 4).
According to the results of the experiments, it is concluded that concentrations of Na+ and Clhave an additive (barley) and/or an interactive (faba bean) effect and high concentrations of both ions can be harmful for plant growth in saline soils. The results also demonstrated that Na+ and Cl- exclusion are independent mechanisms, and different genotypes expressed different combinations of the two mechanisms. Faba bean was more sensitive to Cl- toxicity than Na+ but the presence of Na+ as dominant cation exacerbates the severity of the alterations.
The work also showed that the toxic effects of Na+ and Cl- operated through different mechanisms. A significant correlation between reduced leaf chlorophyll content and the parameters of chlorophyll fluorescence occurred with increasing Cl- concentration but not with Na+ concentrations. Using simultaneous measurements of leaf gas exchange and chlorophyll fluorescence it was shown that there was a significant reduction in both the efficiency of light harvesting of PSII (F′v/F′m) and actual quantum efficiency of PSII (PSII) due to high concentrations of Cl and NaCl in soil solution. Moreover, while a significant reduction in F′v/F′m, PSII and qP of sensitive genotypes under Cl- and NaCl stress was indicated, the Cl- excluding varieties maintained a higher capacity of PSII system. These results together with findings in Chapter 3 and 4 on the effects of CaCl2 treatment in growth dynamics and gas exchange of barley and faba bean, suggest that a high concentration of Clis damaging the photosynthetic apparatus, and that Cl- exclusion is an important mechanism 79
under saline conditions to maintain the function of the chloroplasts. In contrast, Na+ toxicity operated mainly through stomatal effects and leaf chlorophyll content was not affected or increased.
The interpretation of these results in relation to research on salt tolerance of crops is that the toxicity of NaCl is not merely the result of uptake of excess Na+, a belief that lies behind many attempts to select for salt tolerance on the basis of tissue Na+ levels in grain crops. The high concentrations of Cl- also can be damaging to crop growth and needs to be taken into account for the understanding of salt damage and manipulation of salt tolerance.
9.4
Evaluation of crop salt tolerance in solution and soil cultures under controlled
environmental conditions: Are they good surrogates for evaluating whole-plant response to salinity under field conditions?
Many studies on the mechanisms of salt tolerance have been conducted in nutrient solution or supported hydroponics (sand or inert growth media flushed with nutrient solution several times a day) (Munns et al. 2002; Genc et al. 2007). While such systems facilitate the selection and maintenance of plants, the results do not cover all processes relevant under field conditions (Vetterlein et al. 2004). In fact there has been little or no work that has directly compared the responses of different varieties to salt in different growth media or under controlled conditions and the field. The experiments described in Chapters 3, 7 and 8 compared the responses to salinity in hydroponics and in soil in several varieties of barley and faba bean known to differ in their salt tolerance and ability to exclude Na+ and to assess the importance of different mechanisms of salinity tolerance in the two systems under both controlled and field conditions.
80
9.4.1
Barley
Genetic differences in Na+ exclusion, Na+: K+ discrimination and tissue tolerance between two barley genotypes, Clipper and Sahara were not expressed at any EC level in hydroponics (Chapter 3). Similar results were obtained in another screening experiment using supported hydroponics where no significant differences in Na+ exclusion were found between Clipper (shoot Na+ concentration = 2258 mmol kg-1) and Sahara (shoot Na+ concentration = 2364 mmol kg-1) (Chapter 7). In soil, however, genetic differences in Na+ exclusion between these two varieties were expressed. Genetic differences in Na+ exclusion have been previously demonstrated in hydroponics studies (Schachtman et al. 1991; Munns et al. 2006; Genc et al. 2007), but the different results for selected genotypes in this study (Chapters 3 and 7) suggest that the range in Na+ and Cl- concentration was less in hydroponics than in soil.
The results in Chapters 3 and 7 confirmed the hypothesis that there are differences in the responses to salinity between plants grown in hydroponic and soil systems, a result that has important implications for the development of salt tolerant germplasm and for elucidating the relative importance of the mechanisms of salt tolerance in the field (Chapters 7). These studies on salt tolerance of barley demonstrated that the genotypic variation observed in solution culture for differences in salt tolerance, Na+ and Cl- do not reflect those in soil and the physiological responses are not the same when the same materials were grown in soil in both pot experiments under controlled conditions and in the field.
While the majority of salt tolerance experiments were conducted in hydroponics and sand cultures in solution with pH ranges of of 6-7 (Munns 2002; Munns et al. 2006; Genc et al. 2007; Genc et al. 2010a; Genc et al. 2010b), the interactions between root-zone environments and plant responses to increased osmotic pressure or specific ion concentrations in the field are complicated by many soil processes such as soil water dynamics, soil structural stability, 81
and solubility of compounds in relation to pH and pE (electron concentration related to redox potential) (Rengasamy 2010b). However, the effect of pH on salt tolerance and growth of crops in salt-affected soils has not well understood. Under field conditions, the topsoil can be sodic while the subsoil is alkaline saline–sodic (Chapter 7). There were differences in pH among the three methods of assessment but specially between hydroponics and soil and the effect of this on the consistency between hydroponics and soil is not known. When a salt tolerant wheat variety was grown in this type of saline-sodic soil, the yield was similar to that of a less salt-tolerant variety. On further investigation it was found that topsoil sodicity and subsoil alkaline pH (9.6) prevented the roots from reaching the saline subsoil layer (Cooper 2004; Rengasamy 2010b), salt tolerance character of the wheat variety being not utilised. As the pH of the soil increases above 8, soil becomes alkaline and carbonates dominate the anions. Thus, salinity affect plants through adverse soil properties of alkalinity and sodicity, properties imposed on the soil by mobile salts. Further, alkaline pH induces severe soil structural problems than neutral sodic soils at comparable sodicity (SAR) levels. A preliminary investigation showed also that chemistry of aluminium and carbonates in soils is completely different when the soil pH is above 9.5 compared with pH between 8.2 and 9.5 (Rengasamy 2010b). Further research is needed to assess how pH affects the productivity of crop growth in saline soils, an area which has been underestimated in current literature.
As well, measuring the biomass production at 70 days after germination has provided an accurate screen for tolerance of the relative biomass production under saline conditions, and has revealed substantial variation among genotypes at both levels of salinity stress which was also predictive of grain yield production in the field (Table 3 of Chapter 7). However, there was no significant relationship between relative shoot growth of different genotypes and their salt tolerance at earlier harvests. This finding indicated the unsuitability of using an early assessment of salinity tolerance at the seedling stage. 82
9.4.2
Faba bean
The studies in Chapter 8 were conducted to assess the variation among faba bean genotypes in response to salinity and also to examine which physiological traits can be used to assess salt tolerance among faba bean genotypes under both field and controlled conditions using supported hydroponics system. The ranking of 11 genotypes of faba bean under both hydroponics and field conditions was compared with grain yield production under field condition in 2008 and 2009. The rankings of genotypes based on their salt tolerance in controlled condition at 75 mM NaCl and grain yield production in the field (rs = 0.85, P=0.006, n = 11) (Table 3 of Chapter 8), and the crucial parameters of leaf Na+ and Clconcentration (rs = 0.88; P = 0.002; n = 11) were significantly correlated. The consistency of results obtained for physiological responses to salinity stress in hydroponics and soil in faba bean (Chapters 8) is in contrast to the poor relationship in barley (Chapter 7) and highlights that the most appropriate screening methods may need to vary with different crop species.
There has been the hope that a better understanding of the physiological basis of salinity tolerance will also result in the identification of critical genes for which breeders might select, or new genetic resources that could be manipulated by the tools of molecular biology, but intensive research have not yet identified a single gene or genetic resource that has been used by breeders to improve salinity tolerance. Research has been fruitful in identifying characteristics that are important in accounting for differences in salt tolerance between and within species (Rawson et al. 1988; Munns and Tester 2008) but to date these have not been routinely used in breeding to improve salt tolerance. There are fundamental and major differences between experiments in the glasshouse and/or growth chambers and those in the 83
field as shown for barley in this study. For example plants are or can be subjected to various stresses, but rarely to several of these at the same time when grown under controlled conditions. This will of course be the case in nature, where a combination of abiotic stresses affects overall plant growth. In this respect, although uniform growth conditions are important to compare results, it is equally or even more important to determine if growth differences are also apparent under various conditions.
9.5
Conclusions
The research reported in this thesis investigated how the osmotic and specific toxic effects of salinity interact to reduce plant growth and soil water extraction. By better understanding the mechanisms by which salinity affects plants and the interactions between soil, plant and water under saline conditions, improved management of saline soils will be possible. The results presented here allow greater understanding of the osmotic and toxic components of salinity, and the influences of environmental conditions which allow further development of the two phase salinity model of growth crops under salt stress (Munns et al. 1995, Munns and Tester 2008). In conclusion, the results of this study indicated that:
1. The relative effects of osmotic stress and ion toxicity and the genetic tolerance to these stresses change as the severity of salinity stress varies. Osmotic stress was the predominant cause of reduced growth at high levels of salinity, while specific-ion toxicity was more important under mild salinity stress. This has important implications for interpreting responses in the field and for the development of screening techniques because robust levels of salt tolerance may depend on more than one mechanism and so selection for improved salt tolerance therefore needs to be able to identify these. 84
2. It was shown that the contribution of Cl- toxicity to salinity stress may have been underestimated in barley and faba bean and tolerance to high concentrations of Na+ and Cl- are independently controlled. High Na+ interferes with K+ and Ca2+ nutrition and stomatal regulation, while high Cl- concentration reduces the photosynthetic capacity due to chlorophyll degradation.
3. In barley, it was indicated that responses to salinity in hydroponic screening are fundamentally different to those observed in soil. The diverse genotypic variation in ion exclusion and salinity tolerance found in hydroponics did not correlate with the result so of pot and field experiments. Also the unsuitability of using early assessment of salinity tolerance at seedling stage was demonstrated.
4. In contrast to barley, the study of screening faba bean genotypes in hydroponics (at 75 mM NaCl) was shown to be predictive of responses in the field. These results clearly show that the suitability of a screening method for salinity tolerance differs with the crop species.
5. In both barley and faba bean, the exclusion of Na+ and Cl- significantly contributed to salt tolerance and grain yield production in pot and field studies but at high (ECFC>10dS m-1) levels of salinity the capacity of exclusion alone to maintain salt tolerance was reduced.
6. It was indicated that salt exclusion coupled with a synthesis of organic solutes are important components of salt tolerance in the tolerant genotypes and further field tests 85
of these plants under stress conditions will help to verify their potential utility in cropimprovement programs.
9.6
Recommended future research
The experiments described in this thesis have determined soil processes that limit the growth of crops in saline-sodic soils. The difference in Na+ and Cl- accumulation between the high and low Na+ and/or Cl- genotypes was obvious; however, there was no beneficial effect of the low Na+ and Cl- trait at the highest salinity level (150 mM NaCl or ECFC > 10 dS m-1). To increase yield at such high salinity, it may be necessary to have additional traits for adaptation to the osmotic stress such as those resulting in increased water use efficiency to conserve soil moisture and minimise transient increases in salinity in saline subsoils. Pyramiding of different traits should result in further increments of salt tolerance.
The thesis has highlighted the lack of information on the contribution of Cl- to salt tolerance mainly because more is known about Na+ transport mechanisms compared with Cl-. It was demonstrated that tolerance to soil salinity is complex and strongly linked to Na+ and Cltransport, therefore reaffirming the importance of studying anion and cation transport in parallel. Plant responses to salinity and Cl- transport processes associated with salt tolerance will vary depending on the species, and even the genotype within a species, as we have shown for barley and faba bean. This highlights the need for genotypic comparisons of ion transport processes at the whole plant, organ and cellular level. More accurate measurements of Cl- concentrations in the vacuole versus cytoplasm in genotypes that vary in salt tolerance may help identify Cl- transport processes important for salt tolerance. The results also suggest that salt exclusion coupled with a synthesis of organic solutes are important components of 86
salt tolerance in the tolerant genotypes and further field tests of these plants under stress conditions will help to verify their potential utility in crop-improvement programs.
Current knowledge of salt tolerance of plants is based on saturation water contents of soils. However, in dryland conditions, soils are never saturated with water and the field soil water content changes with seasonal weather conditions. Thus, it is essential to develop new guidelines on salt tolerance of plant species taking into account soil water dynamics. There is also a gap in our knowledge in identifying the predominant, or a common, factor when different issues cause constraints to plant growth in different soil layers. The uncertainty in our ability to separate effects of these factors will need to be overcome for developing varieties adapted to various physicochemical constraints, in addition to salinity of soil layers.
Most (>85%) saline-sodic soils in Australia have dense subsoils with an alkaline pH which alter the electron and proton activities (pE and pH) leading to nutrient ion transformations which render them unavailable for plant uptake. Rengasamy (2010) proposed a scheme for classifying salt affected soils based upon key soil properties, namely sodium adsorption ratio (SAR), electrical conductivity (EC) and pH. Although the impacts of SAR and EC on the structural behaviour and surface properties of saline-sodic soils have long been recognised, research on pH-plant salt tolerance related constraints has received little attention. Future research is needed to identify and quantify the influence of pH on crop salt tolerance.
The discussion on salt tolerance in this thesis clearly shows that a multitude of processes is involved and this would make it a polygenic trait. Despite the many studies investigating the mechanisms of salt tolerance, this issue is far from resolved. If multiple salt tolerance mechanisms exist in plants, they would presumably be encoded by different genes, and not all 87
sources of salt tolerance might be identified with one screening method. Thus, using a single method to identify salt tolerant accessions could be misleading. Therefore, future work on salt tolerance needs to focus on using a combination of soil-based screening and field evaluation to identify salt tolerant genotypes possessing multiple salt tolerance mechanisms.
88
References for literature review and general discussion Almansouri M, Kinet JM,Lutts S (2001). Effect of salt and osmotic stresses on germination in durum wheat (Triticum durum Desf.). Plant and Soil 231(243-254), 547-550. Apse MP,Blumwald E (2007). Na+ transport in plants. FEBS Letters 581, 2247-2254. Asch F, Dingkuhn M, Dörffling K,Miezan K (2000). Leaf K/Na ratio predicts salinity induced yield loss in irrigated rice. Euphytica 113(2), 109-118. Ashraf M (1994). Breeding for salinity tolerance in plant. Crit. Rev. Plant Sci. 13, 17-42. Ashraf M (2001). Relationships between growth and gas exchange characteristics in some salttolerant amphidiploid Brassica species in relation to their diploid parents. Environmental and Experimental Botany 45(2), 155-163. Ashraf M (2004). Some important physiological selection criteria for salt tolerance in plants. Flora 199, 361-376. Ashraf M,Harris PJC (2004). Potential biochemical indicators of salinity tolerance in plants. Plant Science 166(1), 3-16. Ashraf M,McNeilly T (1988). Variability in salt tolerance of nine spring wheat cultivars. J Agron Crop Sci 160, 14-21. Aslam M, Muhammad N, Qureshi RH, Ahmad Z, Nawaz S,Akhtar J (2003). Calcium and salttolerance of rice. Communications in Soil Science and Plant Analysis 34(19/20), 3013-3031. Aslam M, Qureshi RH,Ahmed N (1993). A rapid screening technique for salt tolerance in rice (Oryza sativa L.). Plant and Soil 150, 99-107. Aslam Z, Jeschke WD, Barrett-Lennard EG, Greenway H, Setter TL,Watkin E (1986). Effects of external NaCl on the growth of Atriplex amnicola and the ion relations and carbohydrate status of the leaves. Plant Cell and Environment 9, 571-580. Babourina O, Leonova T, Shabala S,Newman I (2000). Effect of sudden salt stress on ion fluxes in intact wheat suspension cells. Annals of Botany 85, 759-767. Bains SS,Fireman M (1964). Effect of exchangeable sodium percentage on the growth and absorption of essential nutrients and sodium by five crop plants. Agronomy Journal 56, 432-435. Barrett-Lennard EG (1986). Wheat growth on saline waterlogged soils. Department of Agriculture, Western Australia Journal of Agriculture 27, 118-119. Barrett-Lennard EG (2003). The interaction between waterlogging and salinity in higher plants: causes, consequences and implications. Plant and Soil 253, 35-54. Bartels D,Sunkar R (2005). Drought and salt tolerance in plants. Critical reviews in plant sciences 24, 23-58.
89
Belkhodja R, Morales F, Abadia A, Gomez-Aparisi J,Abadia J (1994). Chlorophyll fluorescence as a possible tool for salinity tolerance screening in barley (Hordeum vulgare L.). Plant Physiol. 104(2), 667-673. Ben-Hayyim G, Kafkafi U,Ganmore-Neumann R (1987). Role of Internal Potassium in Maintaining Growth of Cultured Citrus Cells on Increasing NaCl and CaCl2 Concentrations. Plant Physiol. 85(2), 434-439. Bernstein L (1975). Effects of salinity and sodicity on plant growth. Annu. Rev. Phytopathol. 13, 295312. Bernstein L,Pearson GA (1956). Influence of exchangeable sodium on the yield and chemical composition of plants 1. Soil Science 82, 247-258. Berry JA,Downton WJS (1982). Environmental regulation of photosynthesis. Photosynthesis: development, carbon metabolism, and plant productivity. Govindjee. New York Londo, Academic Press. 2: 263-343. Bhandal IS,Malik CP (1988). Potassium estimation, uptake, and its role in the physiology and metabolism of flowering plants. International Review of Cytology 110, 205-254. Blaha G, Stelzl U, Spahn CMT, Agrawal RK, Frank J,Nierhaus KH (2000). Preparation of functional ribosomal complexes and effect of buffer conditions on tRNA positions observed by cryoelectron microscopy. Rna-Ligand Interactions Pt A. 317: 292-309. Bond WJ (1998). Effluent irrigation – an environmental challenge for soil science. Aust. J. Soil Res 36, 643-555. Bongi G,Loreto F (1989). Gas exchange properties of salt-Stressed olive (Olea europea L.) leaves. Plant Physiol. 90(4), 1408-1416. Boyer JS (1976). Photosynthesis at low water potentials. Philosophical transactions of the royal society 273, 501-512. Bradford MJ, Ferris JE,Remley PA (1987). Interrill soil erosion processes: I. Effect of surface sealing on infiltration, runoff and soil splash detachment. Soil Sci Soc Am J 51, 1566-1571. Bresler E, McNeal BL,Carter DL (1982). Saline and sodic soils. Principles - dynamics - modeling. New York, Springer-Verlag. Britto DT, Ruth TJ, Lapi S,Kronzucker HJ (2004). Cellular and whole-plant chloride dynamics in barley: insights into chloride-nitrogen interactions and salinity responses. Planta 218, 615-622. Campbell GS (1988). Soil water potential measurement: An overview. Irrigation Sciemce 9, 265-273. Castillo EG, Tuong TP, Ismail AM,Inubushi K (2007). Response to salinity in rice: comparative effects of osmotic and ionic stresses. Plant Production Science 10, 159-170. Chauhan R,Chauhan C (1985). Effect of P fertiliser on alleviating chloride toxicity in wheat. Fertiliser Research 6, 171-176. Chavan PD,Karadge BA (1980). Influence of sodium-chloride and sodium-sulfate salinities on photosynthetic carbon assimilation in peanut. Plant and Soil 56, 201-207.
90
Chaves MM, Flexas J,Pinheiro C (2009). Photosynthesis under drought and salt stress: regulation mechanisms from whole plant to cell. Ann Bot 103(4), 551-560. Cheeseman JM (1988). Mechanisms of Salinity Tolerance in Plants. Plant Physiol. 87(3), 547-550. Chen BM, Wang ZH, Li SX, Wang GX, Song HX,Wang XN (2004). Effects of nitrate supply on plant growth, nitrate accumulation, metabolic nitrate concentration and nitrate reductase activity in three leafy vegetables. Plant Science 167(3), 635-643. Chen S, Li J, Wang T, Wang S, Polle A,Hüttermann A (2003). Osmotic stress and ion-specific effects on xylem abscisic acid and the relevance to salinity tolerance in Poplar. Journal of Plant Growth Regulation 21(3), 224-233. Chen Z, Zhou M, Newman IA, Mendham NJ, Zhang G,Shabala S (2007). Potassium and sodium relations in salinised barley tissues as a basis of differential salt tolerance. Functional Plant Biology 34, 150-162. Chinnusamy V, Jagendorf A,Zhu J-K (2005). Understanding and Improving Salt Tolerance in Plants. Crop Sci 45(2), 437-448. Colmer TD, Munns R,Flowers TJ (2005). Improving salt tolerance of wheat and barley: future prospects. Australian Journal of Experimental Agriculture 45, 1425-1443. Cooper DS (2004). Genetics and agronomy of transient salinity in Triticum durum and T.aestivum. PhD Thesis, The University of Adelaide, Australia. Cram WJ (1976). Negative feedback regulation of transport in cells. The maintenance of turgor, volume and nutrient supply. Encyclopaedia of Plant Physiology. U Luttge,MG Pitman. Berlin, Springer-Verlag. 2: 284-316. Cramer GR (1992). Kinetics of maize leaf elongation. 2. Responses of a Na+-excluding cultivar and a Na+-including cultivar to varying Na+ / Ca2+ salinities. Journal of Experimental Botany 43, 857-864. Cramer GR (2002). Response of abscisic acid mutants of Arabidopsis to salinity. Functional Plant Biology 29, 561-567. Cramer GR, Alberico GJ,Schmidt C (1994). Salt tolerance is not associated with the sodium accumulation of two maize hybrids. Functional Plant Biology 21(5), 675-692. Cramer GR, Epstein E,Lauchli A (1989). Na-Ca interactions in barley seedlings: relationship to ion transport and growth. Plant, Cell and Environment 12(5), 551-558. Cramer GR,Lauchli A (1986). Ion Activities in Solution in Relation to Na+ -Ca2+Interactions at the Plasmalemma. J. Exp. Bot. 37(3), 321-330. Cramer GR, Lauchli A,Epstein E (1986). Effects of NaCl and CaCl2 on Ion Activities in Complex Nutrient Solutions and Root Growth of Cotton. Plant Physiol. 81(3), 792-797. Cramer GR, Lauchli A,Polito VS (1985). Displacement of Ca2+ by Na+ from the Plasmalemma of Root Cells : A Primary Response to Salt Stress? Plant Physiology. 79(1), 207-211. Cramer GR, Lynch J, Lauchli A,Epstein E (1987). Influx of Na +, K+, and Ca2+ into Roots of SaltStressed Cotton Seedlings : Effects of Supplemental Ca2+. Plant Physiol. 83(3), 510-516.
91
Cresswell HP, Green TW,McKenzie NJ (2008). The adequacy of pressure plate apparatus for determining soil water retention. Soil Sci Soc Am J 72(1), 41-49. Cruz-Romero G,Coleman NT (1975). Reactions responsible for high pH of Na-saturated soils and clays. Soil Science 25, 196-175. Dang Y, McDonald M, Routley R, Dalal RC, Singh D, Orange D,Mann M (2006a). High chloride in subsoil: a key indicator for potential grain yield losses in southwest Queensland. The Australian society of agriculture. Dang YP, Dalal RC, Buck SR, Harms B, Kelly R, Hochman Z, Schwenke GD, Biggs W, Ferguson NJ, Norrish S, Routley R, McDonald M, Hall C, Singh DK, Daniells IG, Farquharson R, Manning W, Speirs S, Grewal HS, Cornish P, N. B,Orange D (2010). Diagnosis, extent, impacts, and management of subsoil constraints in the northern grains cropping region of Australia. Australian Journal of Soil Research 48(2), 105-119. Dang YP, Dalal RC, Mayer DG, McDonald M, Routley R, Schwenke GD, Buck SR, Daniells IG, Singh DK, Manning W,Ferguson N (2008). High subsoil chloride concentrations reduce soil water extraction and crop yield on Vertisols in north-eastern Australia. Australian Journal of Agricultural Research 59, 321-330. Dang YP, Dalal RC, Routley R, Schwenke GD,Daniells I (2006b). Subsoil constraints to grain production in the cropping soils of the north-eastern region of Australia: an overview. Australian Journal of Experimental Agriculture 46(1), 19-35. Del Pilar Cordovilla M, Ligero F,Lluch C (1999). Effect of salinity on growth, nodulation and nitrogen assimilation in nodules of faba bean (Vicia faba L.). Applied Soil Ecology 11(1), 1-7. Devitt D, Stolzy LH,Jarrell WM (1984). Response of sorghum and wheat to different K +/Na+ ratios at varying osmotic potentials. Agronomy Journal 76, 681-688. Diego HS, Felix L, Henning R, Matthew AH, Alexander E, Ute K, Joachim K,Michael KU (2008). Integrative functional genomics of salt acclimatization in the model legume Lotus japonicus. The Plant Journal 53(6), 973-987. Downton WJS (1977). Photosynthesis in salt-stressed grapevines. Aust. J. Plant Physiol. 4, 183-192. Drew MC,Lauchli A (1985). Oxygen-dependent exclusion of sodium ions from shoots by roots of Zea mays (cv Pioneer 3906) in relation to salinity damage. Plant Physiol. 79(1), 171-176. Drew MC,Saker LR (1984). Uptake and long-distance transport of phosphate, potassium and chloride in relation to internal ion concentrations in barley: evidence of non-allosteric regulation. 160(6), 500507. Dubcovsky J, Santa Maria G, Epstein E, Luo MC,Dvoøák J (1996). Mapping of the K+/Na+ discrimination locus Kna1 in wheat. Theoretical and Applied Genetics 92, 448-454. Duc G, Bao S, Baum M, Redden B, Sadiki M, Suso MJ, Vishniakova M,Zong X (2010). Diversity maintenance and use of Vicia faba L. genetic resources. Field Crops Research 115(3), 270-278. Dudley LM (1994). Salinity in the environment. Handbook of plant and crop stress. M Pessarakli. New York, USA, Marcel Dekker, Inc: 13-30.
92
Dvořak J, Noaman MM, Goyal S,Gorham J (1994). Enhancement of the salt tolerance of Triticum turgidum L. by the Kna1 locus transferred from the Triticum aestivum L. chromosome 4D by homoeologous recombination. TAG Theoretical and Applied Genetics 87(7), 872-877. Dyer CL, Kopittke PM, Sheldon AR,Menzies NW (2008). Influence of Soil Moisture Content on Soil Solution Composition. Soil Sci Soc Am J 72(2), 355-361. Eaton FM (1942). Toxicity and accumulation of chloride and sulfate salts in plants. Agricultural research 64(7), 357-399. Eaton FM (1966). Chlorine. Diagnostic criteria for plants and soils. HD Chapman. Riverside, University of California Division of agricultural sciences: 98-135. El-hendawy SE, Hu Y,Schmidhalter U (2005). Growth, ion content, gas exchange, and water relations of wheat genotypes differing in salt tolerances. Australian Journal of Agricultural Research 56, 123134. Epstein E (1961). The essential role of calcium in selective cation transport by plant cells. Plant Physiology 36, 434-444. Epstein E (1966). Dual pattern of ion absorption by plant cells and by plants. Nature 212, 1324-1327. Epstein E, Norlyn JD, Rush DW, Kingsbury RW, Kelley DB,Cunningham GA (1980). Saline culture of crops: a genetic approach. Science 210, 399-404. Ezlit YD, Smith RJ,Raine SR (2010). A Review of Salinity and Sodicity in Irrigation. CRC for Irrigation Futures Irrigation Matters Series No. 01/10, University of Southern Queensland. Farquhar GD,Sharkey TD (1982). Stomatal conductance and photosynthesis. Annu. Rev. Plant Physiol. 33, 317-345. Felle H (1994). The H+/Cl- symporter in root-hair cells of Sinapis alba. An electrophysiological study using ion-selective microelectrodes. Plant Physiol. 106, 1131-1136. Fitzpatrick RW, Boucher SC, Naidu R,Fritsch E (1994). Environmental consequences of soil sodicity. Aust. J. Soil Res 32, 1069-1093. Flexas J, Bota J, Escalona JM, Sampol B,Medrano H (2002). Effects of drought on photosynthesis in grapevines under field conditions: an evaluation of stomatal and mesophyll limitations. Functional Plant Biology 29, 461-471. Flexas J, Bota J, Loreto F, Cornic G,Sharkey TD (2004). Diffusive and metabolic limitations to photosynthesis under drought and salinity in C3 plants. Plant Biology 6(3), 269-279. Flexas J, Diaz-Espejo A, Galmes J, Kaldenhoff R, Medrano H,Ribas-Carbo M (2007). Rapid variations of mesophyll conductance in response to changes in CO2 concentration around leaves. Plant, Cell & Environment 30, 1284-1298. Flowers TJ (1988). Chloride as a nutrient and as an osmoticum. Adv Plant Nutr 3, 55-78. Flowers TJ (2004). Improving crop salt tolerance. Journal of Experimental Botany 55(396), 307-319. Flowers TJ, Garcia A, Koyama M,Yeo AR (1997). Breeding for salt tolerance in crop plant- the role of molecular biology. Acta physiologiae plantarum 19(4), 427-433.
93
Flowers TJ, Hajibagheri MA,Yeo AR (1991). Ion accumulation in the cell walls of rice plants growing under saline conditions – evidence for the Öertli hypothesis. Plant, Cell and Environment 14, 319-325. Flowers TJ, Troke PF,Yeo AR (1977). Mechanism of salt tolerance in halophytes. Annual Review of Plant Physiology 28, 89-121. Flowers TJ,Yeo AR (1995). Breeding for Salinity Resistance in Crop Plants: Where Next? Functional Plant Biology 22(6), 875-884. Forster BP, Ellis RP, Thomas WTB, Newton AC, Tuberosa R, This D, El-Enein RA, Bahri MH,Ben Salem M (2000). The development and application of molecular markers for abiotic stress tolerance in barley. J. Exp. Bot. 51(342), 19-27. Forster BP, Russell JR, Ellis RP, Handley LL, Robinson D, Hackett CA, Nevo E, Waugh R, Gordon DC, Keith RP,Powell W (1997). Locating genotypes and genes for abiotic stress tolerance in barley: a strategy using maps, markers and the wild species. New Phytologist 137, 141-147. Fortmeier R,Schubert S (1995). Salt tolerance of maize (Zea mays L.): the role of sodium exclusion. Plant, Cell and Environment 18(9), 1041-1047. Francois LE, Grieve CM, Maas EV, Donovan TJ,Lesch SM (1994). Time of salt stress affects growth and yield components of irrigated wheat. Agronomy Journal 86, 100-107. Francois LE,Maas EV (1999). Crop response and management of salt-affected soils. Handbook of Plant and Crop Stress. M Pessarakli. New York, Marcel-Dekker. 2nd edn: 169-201. Garcia A, Senadhira D, Flowers TJ,Yeo AR (1995). The effects of selection for sodium transport and of selection for agronomic characteristics upon salt resistance in rice (Oryza sativa L.). TAG Theoretical and Applied Genetics 90(7), 1106-1111. Genc Y, McDonald GK,Tester M (2007). Reassessment of tissue Na+ concentration as a criterion for salinity tolerance in bread wheat. Plant, Cell and Environment 30(11), 1486-1498. Genc Y, Oldach K, Verbyla AP, Lott G, Hassan M, Tester M, Wallwork H,McDonald GK (2010a). Sodium exclusion QTL associated with improved seedling growth in bread wheat under salinity stress. Theor Appl Genet 121, 877-894. Genc Y, Tester M,Mcdonald GK (2010b). Calcium requirement of wheat in saline and non-saline conditions. Plant & Soil 327, 331-345. Genty B, Briantais JM,Baker NR (1989). The relationship between the quantum yield of photosynthetic electron transport and quenching of chlorophyll fluorescence. Biochim. Biophys. Acta 990, 87-92. Ghassemi F, Jakeman AJ,Nix HA (1995). Salinization of land and water resources. Human causes, extent, management and case studies. Sydney: University of New South Wales Press Ltd. Ghoulam C, Foursy A,Fares K (2002). Effects of salt stress on growth, inorganic ions and proline accumulation in relation to osmotic adjustment in five sugar beet cultivars. Environ. Exp. Bot. 47, 3950.
94
Glen EP,Brown JJ (1998). Effects of soil salt levels on the growth and water use efficiency of Atriplex canescens (Chenopodiaceae) varieties in drying soil. Am. J. Bot 85, 10-16. Gorham J, Jones RGW,Bristol A (1990). Partial Characterization of the Trait for Enhanced K+-Na+ Discrimination in the D-Genome of Wheat. Planta 180(4), 590-597. Gorham J, Papa R,Aloy-Lleonart M (1994). Varietal differences in sodium uptake in barley cultivars exposed to soil salinity or salt spray. Journal of Experimental Botany 45(7), 895-901. Greenway H,Munns R (1980). Mechanisms of salt tolerance in nonhalophytes. Ann. Rev. Plant Physiol. 31, 149-190. Groenevelt PH, Grant CD,Murray RS (2004). On water availability in saline soils. Australian Journal of Soil Research 42, 833-840. Hajrasuliha S (1980). Accumulation and toxicity of chloride in bean plants. Plant and soil 55, 133138. Hasegawa PM, Bressan RA, Zhu J-K,Bohnert HJ (2000). Plant cellular and molecular responses to high salinity. Annual Review of Plant Physiology and Plant Molecular Biology 51(1), 463-499. Hochman Z, Dang YP, Schwenke GD, Dalgliesh NP, Routley R, McDonald M, Daniells IG, Manning W,Poulton PL (2007). Simulating the effects of saline and sodic subsoils on wheat crops growing on Vertosols. Australian Journal of Agricultural Research 58(8), 802-810. Huang J,Redmann RE (1995). Responses of growth, morphology and anatomy to salinity and calcium supply in cultivated and wild barley. Canadian Journal of Botany 73, 1859-1866. Husain S, Munns R,Condon AG (2003). Effect of sodium exclusion trait on chlorophyll retention and growth of durum wheat in saline soil. Australian Journal of Agriculture Research 54, 589-597. Hutson JL (1990). Modelling transient root zone salinity. Agricultural salinity assessment and management. KK Tanji. New York, American Society of Civil Engineers: 482–503. Isbell RF (1996). The Australian Soil Classification. Melbourne, CSIRO. Isla R, Aragüés R,Royo A (1998). Validity of various physiological traits as screening criteria for salt tolerance in barley. Field Crops Research 58(2), 97-107. Iwata S, Tabuchi T,Warkentin BP (1994). Soil-water interactions: mechanism and applications. New York, Marcel Dekker. James R, Rivelli AR, Munns R,Caemmere SV (2002). Factors affecting CO2 assimilation, leaf injury and growth in salt-stressed durum wheat. Functional Plant Biology 29, 1393-1403. James RA, Caemmerer SV, Condon AGT, Zwart AB,Munns R (2008). Genetic variation in tolerance to the osmotic stress component of salinity stress in durum wheat. Functional Plant Biology 35(2), 111-123. James RA, Davenport RJ,Munns R (2006). Physiological charaterisation of two genes for Na + exclusion in durum wheat, NAx1 and Nax2. Plant Physiol. 142, 1537-1547. Jayawardane NS,Chan KY (1994). The management of soil physical properties limiting crop production in Australian sodic soils: a review. Australian Journal of Soil Research 32, 13-44.
95
Jensen ES, Peoples MB,Hauggaard-Nielsen H (2010). Faba bean in cropping systems. Field Crops Research 115(3), 203-216. Jeschke WD,Pate JS (1991). Cation and chloride partitioning through xylem and phloem within the whole plant of Ricinus communis L. under conditions of salt stress. J. Exp. Bot. 42(9), 1105-1116. Jones HG (1976). Moderate-term water stresses and associated changes in some photosynthetic parameters in cotton. New Phytologist 72, 1095-1105. Kafkafi U, Siddiqi MY, Ritchie RJ, Glass ADM,Ruth TL (1992). Reduction of nitrate (13NO3) influx and nitrogen (13N) translocation by tomato and melon varieties after short exposure to calcium and potassium chloride salts. Plant Nutr 15, 959-975. Kapulnik Y, Tueber LR,Phillips DA (1989). Lucerne (Medicago sativa L.) selected for vigor in a nonsaline environment maintained growth under salt stress. Australian Journal of Agricultural Research 40, 1253-1259. Kazman Z, Shainberg I,Gal M (1983). Effects of low levels of exchangable Na and applied phosphogypsum on the infiltration rate of various soils. Soil Science 35, 184-192. Kelly J,Rengasamy P (2006). Diagnosis and management of soil constraints: transient salinity, sodicity and alkalinity. The University of Adelaide and Grain Research and Development Corporation, Australia. Available at http://www.arris.com.au/index.php?id=39 [Verified 24 May 2010]. Kendall HW,Pimentel D (1994). Constraints on the expansion of the global food supply. Ambio 23, 198-205. Kent IM,Lauchli A (1985). Germination and seedling growth of cotton: salinity-calcium interactions. Plant, Cell and Environment 8, 155-159. Kingsbury R,Epstein E (1986). Salt Sensitivity in Wheat. A case for specific ion toxicity. Plant Physiology 80, 651-654. Kingsbury R, Epstein E,Pearcy RW (1984). Physiological responses to salinity in selected lines of wheat. Plant Physiology 74, 417-423. Kinraide TB (1998). Three Mechanisms for the Calcium Alleviation of Mineral Toxicities. Plant Physiol. 118(2), 513-520. Kinraide TB (1999). Interactions among Ca2+, Na+ and K+ in salinity toxicity: quantitative resolution of multiple toxic and ameliorative effects. Journal of Experimental Botany 50(338), 1495-1505. Krishnamurthy L, Serraj R, Hash C, Dakheel A,Reddy B (2007). Screening sorghum genotypes for salinity tolerant biomass production. Euphytica 156(1), 15-24. Kurth E, Cramer GR, Lauchli A,Epstein E (1986). Effects of NaCl and CaCl2 on Cell Enlargement and Cell Production in Cotton Roots. Plant Physiol. 82(4), 1102-1106. Lauchli A,Wieneke JZ (1979 ). Studies on growth and distribution of Na, K and CI in soybean varieties differing in salt tolerance. Pflanzenernaehr. Bodenkd 142, 3-13.
96
Lawlor DW (1970). Absorption of polyethylene glycols by plants and their effects on plant growth. New Phytologist 69, 501-513. Lawlor DW,Cornic G (2002). Photosynthetic carbon assimilation and associated metabolism in relation to water deficits in higher plants. Plant, Cell & Environment 25, 275-294. Lea-Cox JD,Syvertsen JP (1993). Salinity Reduces Water Use and Nitrate-N-use Efficiency of Citrus. Ann Bot 72(1), 47-54. Li-juan L, Zhao-hai Y, Zhao-jie Z, Ming-shi X,Han-qing Y (1993). Study and utilization of faba bean germplasm resources. Faba bean in China: State-of-the art review. MC Saxena, S Weigand,L Li-Juan, ICARDA Press: 51-63. Lin CC,Kao CH (2001). Relative importance of Na+, Cl−, and abscisic acid in NaCl induced inhibition of root growth of rice seedlings. Plant and soil 237(1), 165-171. Lin W (1981). Inhibition of anion transport in corn root protoplast. Plant Physiol 68, 435-438. Longstreth DJ,Nobel PS (1979). Salinity effects on leaf anatomy. Consequences for photosynthesis. Plant Physiol 63, 700-703. Luo Q, Yu B,Liu Y (2005). Differential sensitivity to chloride and sodium ions in seedlings of Glycine max and G. soja under NaCl stress. Journal of Plant Physiology 162(9), 1003-1012. Maas EV,Grieve CM (1987). Sodium-induced calcium deficiency in salt-stressed corn. Plant, Cell & Environment 10(7), 559-564. Maas EV,Hoffman GJ (1977). Crop salt tolerance-current assessment. J. Irrig. Drain. Div. ASCE 103, 115-134. Malhotra RS (1997). Evaluation techniques for abiotic stresses in cool season food legumes. Recent Advances in Pulses Research. AN Asttranu,A Masood. Kanpur, India, Indian Society of Pulses Research and Development: 459-473. Manchanda HR,Sharma SK (1989). Tolerance of chloride and sulfate salinity in chickpea (Cicer arietinum). Journal of Agricultural Science 113, 407-410. Marschner H (1995). Mineral nutrition of Higher Plants, Academic Press. McCown RL, Murtha GG,Smith GD (1976). Assessment of available water storage capacof soils with restricted permeability. Water Resource Research 12, 1255-1259. McCree K,Richardson S (1987). Salt increases the water use efficiency in water stressed plants. Crop Sci 27, 543-547. McIntyre DS (1979). Exchangeable sodium, subplasticity and hydraulic conductivity of someAustralian soils. Australian Journal of Soil Research 17, 115-120. Melesse T,Caesar K (1992). Stomatal and non-stomatal effects of salinity on photosynthesis in Faba Beans (Vicia faba L.). Journal of Agronomy and Crop Science 168(5), 345-353. Mittler R,Blumwald E (2010). Genetic engineering for modern agriculture: challenges and perspectives. Annu. Rev. Plant Biol 61, 443-462.
97
Moller IS, Gilliham M, Jha D, Mayo GM, Roy SJ, Coates JC, Haseloff J,Tester M (2009). Shoot Na+ exclusion and increased salinity tolerance engineered by cell Type-Specific alteration of Na+ transport in Arabidopsis. Plant Cell, tpc.108.064568. Morgan JM, Rodriguez-Maribona B,Knights EJ (1991). Adaptation to water-deficit in chickpea breeding lines by osmoregulation: relationship to grain yields in the field. 27, 61-70. Munns R (1985). Na+, K+ and Cl- in xylem sap flowing to shoots of NaCl-treated barley. Journal of Experimental Botany 36(168), 1032-1042. Munns R (1993). Physiological processes limiting plant growth in saline soils: some dogmas and hypotheses. Plant cell environment 16, 15-24. Munns R (2002). Comparative physiology of salt and water stress. Plant, Cell and Environment 25, 239-250. Munns R (2007). Utilizing genetic resources to enhance productivity of saltprone land: published as part of a theme on salt-prone land resources. CAB Reviews: Perspectives in Agriculture, Veterinary Science, Nutrition and Natural Resources 2, 1-11. Munns R, Gardner PA, Tonnet ML,Rawson HM (1988). Growth and development in NaCl-Treated plants. II. Do Na+ or Cl- concentrations in dividing or expanding tissues determine growth in barley? Functional Plant Biology 15(4), 529-540. Munns R, Hare RA, James RA,Rebetzke GJ (2000a). Genetic variation for improving the salt tolerance of durum wheat. Aust. J. Agric. Res. 51, 69-74. Munns R, Husain S, Rivelli AR, James R, Condon AG, Lindsay M, Lagudah ES, Schachtman DP,Hare RA (2002). Avenues for increasing salt tolerance of crops, and the role of physiologically based selection traits. Plant and Soil 247, 93-105. Munns R,James RA (2003). Screening methods for salinity tolerance: a case study with tetraploid wheat. Plant and soil 253, 201-218. Munns R, James RA,Lauchli A (2006). Approaches to increasing the salt tolerance of wheat and other cereals. Journal of Experimental Botany 57(5), 1025-1043. Munns R, Passioura JB, Guo J, Chazen O,Cramer GR (2000b). Water relations and leaf expansion: importance of time scale. J. Exp. Bot. 51(350), 1495-1504. Munns R, Rebetzke GJ, Husain S, James R,Hare R (2003). Genetic control of sodium exclusion in durum wheat. Australian Journal of Agriculture Research 54, 627-635. Munns R, Schachtman DP,Condon AG (1995). The significance of a two-phase growth response to salinity in wheat and barley. Australian Journal of Agricultural Research 22, 561-569. Munns R,Termaat A (1986). Whole-plant responses to salinity. Australian Journal of Agricultural Research 13, 143-160. Munns R,Tester M (2008). Mechanisms of salinity tolerance. Annual Review of Plant Biology 59, 651-681.
98
Murillo-Amador B, Lopez-Aguilar R, Kaya C, Larrinaga-Mayoral J,Flores-Hernandez A (2002). Comparative effects of NaCl and polyethylene glycol on germination, emergence and seedling growth of cowpea. Journal of Agronomy and Crop Science 188, 235-247. Nature E (2010). How to feed a hungry world. Nature 466(7306), 531-532. Netondo GW, Onyango JC,Beck E (2004). Sorghum and Salinity: II. Gas Exchange and Chlorophyll Fluorescence of Sorghum under Salt Stress. Crop Sci 44(3), 806-811. Nonami H,Boyer JS (1990). Primary events regulating stem growth at low water potentials. Plant Physiol. 93(4), 1601-1609. Norlyn JD,Epstein E (1982). Barley production: irrigation with seawater on coastal soil. Biosaline research: a look to the future. A San Pietro. New York, Plenum: 525-529. Nuttall JG, Armstrong RD,Connor DJ (2003). Evaluating physicochemical constraints of Calcarosols on wheat yield in the Victorian southern Mallee. Australian Journal of Agricultural Research 54, 487497. Nuttall JG, Armstrong RD,Connor DJ (2006). Early growth of wheat is more sensitive to salinity than boron at levels encountered in alkaline soils of south-eastern Australia. Australian Journal of Experimental Agriculture 46, 1507-1514. Nuttall JG, Hobson KB, Materne M, Moody DB, Munns R,Armstrong RD (2010). Use of genetic tolerance in grain crops to overcome subsoil constraints in alkaline cropping soils. Australian Journal of Soil Research 48, 188-199. Öertli JJ (1968). Extracellular salt accumulation, a possible mechanism of salt injury in plants. Agrochimica 12, 461-469. Pessarakli M (1991). Formation of saline and sodic soils and their reclamation. J. Envriron. Sci. Health 26, 1303-1320. Pimentel D, Harman R, Pacenza M, Pecarsky J,Pimentel M (1994). Natural resources and an optimum human population. Population and Environment 15, 347-369. Pimentel D, Hurd LE, Bellotti AC, Forster MJ, Oka IN, Sholes OD,Whitman RJ (1973). Food production and the energy crisis. Science 182(4111), 443-449. Pooviah BW,Reddy ASN (1987). Calcium messenger systems in plants. CRC Critical reviews in Plant Science 6, 47-103. Qadir M,Schubert S (2002). Degradation processes and nutrient constraints in sodic soils. Land Degradation & Development 13, 275-294. Quirk JP (2001). The significance of the threshold and turbidity concentrations in relation to sodicity and microstructure. Australian Journal of Soil Research 39, 1185-1217. Quirk JP,Schofield RK (1955). The effect of electrolyte concentration on soil permeability. Journal of Soil Science 6, 163-178. Rajendran K, Tester M,Roy SJ (2009). Quantifying the three main components of salinity tolerance in cereals. Plant, Cell & Environment 32(32), 237.
99
Rawson HM,Munns R (1984). Leaf expansion in sunflower as influenced by salinity and short-term changes in carbon fixation. Plant Cell Environ 7, 207-213. Rawson HM, Richards RA,Munns R (1988). An examination of selection criteria for salt tolerance in wheat, barley and triticale genotypes. Australian Journal of Agricultural Research 39, 759-772. Reid JR,Smith FA (2000). The limits of sodium/calcium interactions in plant growth. Australian Journal of Plant Physiology 27, 709-715. Rengasamy P (2002). Transient salinity and subsoil constraints to dryland farming in Australian sodic soils: an overview. Australian Journal of Experimental Agriculture 42, 351-361. Rengasamy P (2006). World salinization with emphasis on Australia. Journal of Experimental Botany 57(5), 1017-1023. Rengasamy P (2010a). Osmotic and ionic effects of various electrolytes on the growth of wheat. Australian Journal of Soil Research 48 120-124. Rengasamy P (2010b). Soil processes affecting crop production in salt-affected soils. Functional Plant Biology 37, 613-620. Rengasamy P,Marchuk A (2010). Cation ratio of soil structural stability (CROSS). Australian Journal of Soil Research 49, 1-8. Rengasamy P,Olsson KA (1991). Sodicity and soil structure. Australian Journal of Soil Research 29, 935-952. Rengasamy P,Sumner ME (1998). Processes involved in sodic behaviour. Sodic soils: distribution, properties, management and environmental consequences. ME Sumner,R Naidu, Oxford University. Rhoades JD, Manteghi NA, Shouse PJ,Alves WJ (1989). Soil electrical conductivity and soil salinity: New formulations and calibrations. Soil Sci. Soc. Am. J. 53(2), 433-439. Richards RA (1983). Should selection for yield in saline regions be made on saline or non-saline soils? Euphytica 32(2), 431-438. Richards RA, Dennett CW, Qualset CO, Epstein E, Norlyn JD,Winslow MD (1987). Variation in yield of grain and biomass in wheat, barley, and triticale in a salt-affected field. Field Crops Research 15(3-4), 277-287. Rispail N, Kaló P, Kiss GB, Ellis THN, Gallardo K, Thompson RD, Prats E, Larrainzar E, Ladrera R, González EM, Arrese-Igor C, Ferguson BJ, Gresshoff PM,Rubiales D (2010). Model legumes contribute to faba bean breeding. Field Crops Research 15(3), 253-269. Rogers ME,Noble CL (1992). Variation in growth and ion accumulation between two selected populations of Trifolium repens L. differing in salt tolerance. Plant and soil 146(1), 131-136. Rogers ME, Noble CL, Halloran GM,Nicolas ME (1997). Selecting for salt tolerance in white clover (Trifolium repens): chloride ion exclusion and its heritability. New Phytologist 135, 645-654. Royo A,Aragüés R (1999). Salinity-yield response functions of barley genotypes assessed with a triple line source sprinkler system. Plant and soil 209(1), 9-20.
100
Sanders D (1980). The mechanism of Cl- transport at the plasma-membrane of characorallina. 1. Cotransport with H+. J. Membr. Biol. 53, 129-141. Schachtman DP, Munns R,Whitecross MI (1991). Variation in sodium exclusion and salt tolerance in Triticum tauschii. Crop Sci 31(4), 992-997. Serrano R, Mulet J, Rios G, Marquez J, de Larrinoa I, Leube M, Mendizabal I, Pascual-Ahuir A, Proft M, Ros R,Montesinos C (1999). A glimpse of the mechanisms of ion homeostasis during salt stress. J. Exp. Bot. 50(suppl_1), 1023-1036. Shabala S, Shabala L,Volkenburgh EV (2003). Effect of calcium on root development and root ion fluxes in salinised barley seedlings. Functional Plant Biology 30(5), 507-514. Shainberg I,Caiserman A (1971). Studies on Na/Ca montmorillonite systems. II. The hydraulic conductivity. Soil Sci. 11, 111: 276. Shalhevet J (1993). Plants under water and salt stress. Plant adaptation to environmental stress. L Fowden, T Mansfield,J Stoddart. New York, Chapman and Hall: 133-154. Shannon MC (1997). Adaptation of plants to salinity. Adv. Agron. 60, 75-120. Sharkey TD (1990). Water stress effects on photosynthesis. Photosynthetica 24, 651-658. Shavrukov Y, Gupta N, Miyazaki J, Baho M, Chalmers K, Tester M, Langridge P,Collins N (2010). HvNax3—a locus controlling shoot sodium exclusion derived from wild barley (Hordeum vulgare ssp. spontaneum). Functional & Integrative Genomics 10(2), 277-291. Shomer-Ilan a, Jones GP,Paleg LG (1991). In vitro Thermal and Salt Stability of Pyruvate Kinase Are Increased by Proline Analogues and Trigonelline. Australian Journal of Plant Physiology 18, 279286. Singh S,Singh M (2000). Genotypic basis of response to saliniity stress in some crosses of spring wheat Triticum aestivum L. Euphytica 115(3), 209-214. Skerrett M,Tyerman SD (1994). A channel that allows inwardly directed fluxes of anions in protoplasts derived from wheat roots. Planta 192, 295-305. Slabu C, Zörb C, Steffens D,Schubert S (2009). Is salt stress of faba bean (Vicia faba) caused by Na+ or Cl- toxicity? J. Plant Nutr. Soil Sci. 172, 644-650. Sohan D, Jasoni R,Zajicek J (1999). Plant-water relations of NaCl and calcium-treated sunflower plants. Envriron. Exp. Bot 42, 105-111. Srivastava JP,Jana S (1984). Screening wheat and barley germplasm for salt tolerance. Salinity tolerance in plants: strategies for crop improvement. RC Staples,GH Toenniessen. New York, Wiley: 273-283. Stoddard FL, Balko C, Erskin W, Khan HR, Link W,Sarkar A (2006). Screening techniques and sources of resistance to abiotic stresses in cool-season food legumes. Euphytica 147, 167-186. Storey R,Walker RR (1999). Citrus and salinity. Sci. Hort. 78, 39-81. Sumner ME (1993). Sodic soils - new perspectives. Australian Journal of Soil Research 31(6), 683750.
101
Sumner ME (1995). Sodic soils: new perspectives. Australian Sodic Soils: Distribution, Properties and Management. ME Naidu,P Rengasamy. Australia, CSIRO. Szabolcs I (1994). Soils and salinization. Handbook of Plant and Crop stress. M Pessarakli. New York, Marcel Dekker: 3-11. Tanji KK (1990). Nature and extent of agriculture salinity. Agricultural salinity assessment and management, Manual and Report on Engineering Practice. KK Tanji. New York, American Society of Civil Engineers: 1-17. Teakle NL,Tyerman SD (2010). Mechanisms of Cl- transport contributing to salt tolerance. Plant, Cell & Environment 33, 566-589. Tester M,Davenport R (2003). Na+ tolerance and Na+ transport in higher plants. Annals of Botany 91, 503-527. Tsai YC, Hong C-Y, Liu L-F,Kao H (2004). Relative importnace of Na+ and Cl- in NaCl-induced antioxidant systems in roots of rice seedlings. Physiologia Plantarum 122, 86-94. Turner NC, Wright GC,Siddique KHM (2001). Adaptation of grain legumes (pulses) to water-limited environments. Advances in Agronomy, Academic Press. Volume 71: 193-231. Ueda A, Kathiresan A, Inada M, Narita Y, Nakamura T, Shi W, Takabe T,Bennett J (2004). Osmotic stress in barley regulates expression of a different set of genes than salt stress does. J. Exp. Bot. 55(406), 2213-2218. USSL (1954). Diagnosis and improvement of saline and alkali soils. Washington DC, USDA. Vetterlein D, Kuhn K, Schubert S,Jahn R (2004). Consequences of sodium exclusion on the osmotic potential in the rhizosphere- Comparing of two maize cultivars differing in Na+ uptake. Plant Nutrition and Soil Science 167, 337-344. Weimberg R (1983). Induction of solute release from nicotiana tabacum tissue cell suspensions by polymyxin and EDTA. Journal of Experimental Botany 34(147), 1333-1346. Westgate ME,Boyer JS (1985). Osmotic adjustment and the inhibition of leaf, root, stem and silk growth at low water potentials in maize. Planta 164, 540-549. Wetson AM,Flowers TJ (2010). The effect of saline hypoxia on growth and ion uptake in Suaeda maritima. Functional Plant Biology 37, 646-655. White PJ,Broadley MR (2001). Chloride in Soils and its Uptake and Movement within the Plant: A Review. Annals of Botany 88(6), 967-988. Wieneke J,Läuchli A (1979). Short-term studies on the uptake and transport of Cl- by soybean cultivars differing in salt tolerance. Plant Nutrition and Soil Science 142(6), 799-814. Wright D,Raiper I (2000). An assessment of the relative effects of adverse physical and chemical properties of sodic soil on the growth and yield of wheat (Triticum aestivum L.). Plant and soil 223, 277-285. Xu G, Magen H, Tarchitzky J,Kafkafi U (2000). Advances in chloride nutrition of plants. Advances in Agronomy 68, 97-150.
102
Yamashita K, Kasai M, Yamamoto Y,Matsumoto H (1994). Stimulation of plasmamembrane H+transport activity in barley roots by salt stress – possible role of increase in chloride permeability. Soil Sci. Plant Nutr. 40, 555-563. Yeo A (1998). Molecular biology of salt tolerance in the context of whole-plant physiology. J. Exp. Bot. 49(323), 915-929. Yeo AR, Lee S, Izard P, Boursier PJ,Flowers TJ (1991). Short- and Long-Term Effects of Salinity on Leaf Growth in Rice (Oryza sativa L.). J. Exp. Bot. 42(7), 881-889. Yermiyahu U, Nir S, Ben-Hayyim G,Kafkafi U (1994). Quantitative competition of calcium with sodium or magnesium for sorption sites on plasma membrane vesicles of melon (Cucumis melo L.) root cells. Journal of Membrane Biology 138, 55-63. Yermiyahu U, Nir S, Ben-Hayyim G, Kafkafi U,Kinraide TB (1997). Root elongation in saline solution related to calcium binding to root cell plasma membranes Plant and soil 191(10), 67-76. Ziska LH, Seemann JR,DeJong TM (1990). Salinity induced limitations on photosynthesis in Prunus salicina, a deciduous tree species. Plant Physiology 93(3), 864-870.
103
