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CHAPTER 17 PHYSIOLOGICAL AND BREEDING APPROACHES FOR ABIOTIC STRESS IN COTTON Ajay V. Narwade1*, Kiran Bhagat2, D.V. Patil2, Anjali Kumari2, Thakare Harish S1, Chandrakant Singh1 and Ban Yogesh G1
Abstract: Abiotic stresses are caused by environmental factors like water, temperature, radiation and chemicals. Among the four factors, water stress is caused by either excess loss of water and inadequate absorption or a combination of the two. The internal water stress in crop plants due to the variations in plant environment is generally considered as a drought. Low productivity, delay in prevention of crop establishment, weakening or destruction of established crops and alteration of physiological and biochemical metabolism in plants are caused due to drought. It appears that the osmotic adjustment behaves like a system and not a simple drought tolerant trait. Osmotic adjustment has inter linkages with several traditionally known dehydration avoidance and tolerance traits and thus leads to the differences in yield by modifying various morphological process involving in macro level under water deficit condition. Several naturally occurring lines and development of cultivars and hybrids through the conventional breeding approaches sowed significant enhancement in tolerance to abiotic stresses. However advancement in biotechnology research into a highly exciting phase improved techniques involved in making enhancement of database on novel stress responsive genes for raising the genetic level of stress tolerance in crop plants.
17.1 Introduction Prosperity of Indian economy wholesome depends upon positive strides in agricultural and industrial sectors. In the new millennium, 1
Department of Genetics and Plant Breeding, N.M. College of Agriculture, Navsari Agricultural University, Navsari- 396450, Gujarat, India. 2
National Institute of Abiotic Stress Management (NIASM), Baramati,- 413115, Pune, Maharashtra, India. *
Author for correspondence:
[email protected]
498 sustainable agriculture will be the need of the hour in the context of global and liberalization trends. Crop production encounters various biotic and abiotic stresses particularly in the arid and semiarid regions. The micro and macro crop environments pave way for the natural formation of different agro-climatic zones and the quantum of deleterious and adverse stress factors becomes realistic in limiting yield realization in crops. In this context, plant physiological and approaches in crop production assume importance to unravel the abiotic stress mechanisms and to identify explicit tolerance traits for countering the detrimental stress onslaught during ontogeny. Cotton is the most important fiber crop and is the basic input to the textile industry. In India, cotton is grown in about 9 million hectare of which more than 70 per cent area is rainfed. Maharashtra has an estimated area of 2.8 million ha under cotton, predominantly under rainfed cultivation. In world, Turkey produced 1170kgs of cotton per hectare and occupies the first rank while per hectare yield of cotton in India is as low as 333kg/ha. Rainfed cotton production has significant contribution towards productivity and an erratic cotton output trend may offset lint-fabric continuum and may upset our exim policies. It is obvious that growth and development of cotton has to face one or other stress entities under rainfed situation. Cotton physiology portrays unique indeterminate growth habits with longer crop duration which make cotton vulnerable to abiotic stress influences from emergence to senescence. The adverse effects on the ongoing physiological processes may affect yield projection trends leading to production lapses, inadequacies and may become the focal point of attention. The various abiotic stress factors affecting cotton growth, development and yield mostly originate due to weather and soil constraints. Their occurrence may be erratic or specific and the intensity may be varying in their adversity. Consequently, stress imprints are marked in the altered plant traits. The major abiotic stress factors affecting cotton production are discussed in the chapter. 17.2 Water Deficit Stress Water deficit is the major abiotic factor limiting plant growth and crop productivity around the world (Kramer, 1983). Approximately one third of the cultivated area of the world suffers from chronically inadequate supplies of water (Massacci et al., 2008). In all agricultural regions, yields of rain-fed crops are periodically reduced by drought (Kramer, 1983), and the severity of the problem may increase due to changing world climatic trends (Le Houerou, 1996). In India, cotton is grown in about 11.0 million
499 ha of which more than 70 per cent area is rainfed. Maharashtra has an estimated area of 3.503 million ha under cotton, predominantly under rainfed cultivation (97%). The rainfed tract of central India receives about 500-1200 mm rainfall annually. Prolonged dry spell due to uneven and erratic monsoon particularly under rainfed condition will lead to rapid depletion in soil moisture. Drought stress gradually develops and intensifies during the course of soil moisture decline resulting in restricted growth and development in cotton. Advances in irrigation technology have helped reduce the gap between potential and actual yield, but irrigation costs and limited water supplies constrain irrigation throughout the world. 17.2.1 Effects of Water Deficit Stress on Morphological Characteristics Cotton has an indeterminate growth habit (that is, it is a perennial that keeps growing), and therefore under favourable conditions the number of leaves, new nodes, fruiting branches and squares can increase rapidly, unlimited by a phenological time frame, and continue to be produced while conditions remain favourable. During the pre-flowering stages of growth, production of carbohydrates (through photosynthesis) is in excess of demands, and as a result vigorous vegetative growth occurs. As plant growth continues, the demands for carbohydrates by the component plant parts such as bolls increase, and production becomes limited by environmental conditions. Boll growth exerts large demands for carbohydrates and it is through the balance between boll demand and leaf production that vegetative growth is restricted. Water stress can restrict both vegetative and boll growth. It has been shown that no matter what degree of water stress is imposed on a crop, the proportionality between vegetative growth and boll development remains relatively constant. Similar results have been achieved with crops receiving different amounts of nitrogen. This implies that, independent of water or nutrient supply, the plant will always attempt to form a balance between vegetative growth and boll development. i) Root development Deep root system and higher root-shoot ratio are adaptive mechanisms in response to water deficit. Roots may penetrate deep into the soil in search of moisture and in this context desi (diploid) cotton exhibits better adaptation to drought due to deep and efficient root system. Some of the more important physiological process, such as nutrition uptake and assimilation, stress signals, and water movement occurs in the plant root, root characteristics logically play an important role in determining the response of plants to drought. Since cotton has taproot system, overall root density and exploring the available soil volume for water and nutrients
500 depend on the development of lateral roots. The number of lateral roots produced depends on the number of xylem poles in the taproots of cotton seedling (McMichael et al., 1999). Thus, modifying root system would be one of the ways to increase water use efficiency for drought resistance in cotton. ii) Leaf size Water-deficit stress reduces cell and leaf expansion, stem elongation, and leaf area index (Ball et al., 1994; Gerik et al., 1996). Leaf, stem and root growth rate are very sensitive to water stress because they are dependent on cell expansion. Water stress caused a reduction in the whole plant leaf area by decreasing the initiation of new leaves, with no significant changes in leaf size of leaf abscission. Both the main stem and sympodial branches developed significantly less leaves; however, the effect was less severe on the main-stem leaves. Pettigrew (2004) reported that water-deficit stress resulted in a decrease in leaf size, but noted that this decrease was accompanied by an increase in the specific leaf weight (SLW), a phenomenon also observed by Wilson et al. (1987). iii) Ultrastructural changes Water-deficit stress has also been shown to alter cell ultrastructure. Berlin et al. (1982) indicated that water stress caused significant changes in the grana and stroma lamellae, palisade cell walls, number and size of chloroplasts, and the structure of mitochondria. Loss of chloroplast membrane integrity accompanied by an increase in leaf wax production. Changes in the chemical composition of epicuticular wax and lipid content were also observed. The wax from water-stressed leaves contained more long-chain alkanes compared to the control (Oosterhuis et al., 1991; Bondada et al, 1996). Conversely, water-deficit stress decreased glycolipids and, to a lesser effect, phospholipids, while the triacylglycerols increased (Pham Thi et al., 1985; Wilson et al., 1987). Under water stress conditions, stomatal transpiration (TRst) is controlled by stomatal conductance, and cuticular transpiration (TRcu) is affected by the leaf surface characters such as the thickness of the wax layer and morphological structure iv) Boll development Squares exhibit little carbohydrate demand on the plant during early growth, with bracts supplying the majority of their requirements. A rapid increase in demand for carbohydrates occurs after flowering. This is the reason that the majority of fruit is shed as flowers or two- or three day old bolls. Shedding of bolls can occur up to an age of 10 to 14 days, after
501 which cell wall thickening between the boll and stem prevents an abscission layer from forming. In the case of a rapid onset of water stress, young bolls in which growth has stopped may be retained by the plant and appear as ‘mummified’ dry bolls. Boll development is affected by total carbohydrate supply and not by the rate of distribution from adjacent leaves. This is consistent with the fact that redistribution of carbohydrate can occur at stress levels beyond those that affect production. As boll demand exceeds supply from the adjacent leaf, inter-boll competition for further carbohydrate occurs. Older bolls compete more effectively than younger bolls and these results in the movement of carbohydrates away from the extremities of the main stem and individual fruiting branches. Those bolls unable to compete effectively are either shed by the plant or are reduced in size, hence the occurrence of smaller boll towards the top of the plant. It is for this reason that the majority of fruit, particularly secondary position bolls, is retained by lower fruiting branches. In non-stressed irrigated crops, increased early vegetative growth results in shading of lower leaves and this causes reduced retention and boll size on the first two or three fruiting branches. The final results of this combination of inter-boll competition and leaf shading in fully irrigated crops is the common bell-shaped distribution of bolls throughout the plant. In the case of crops under water stress, the same inter-boll competition occurs, but there is generally less total carbohydrate to be distributed amongst bolls. Reduced vegetative growth also minimizes shading of lower leaves, resulting in the higher boll retention and boll size occurring amongst the first fruiting branches. 17.2.2 Effects of Water-Deficit Stress on Physiological Characteristics The effects of water deficit on different plant physiological processes are complex and interrelated. Cellular water content largely controls stomatal aperture, and stomatal conductance directly affects CO2 diffusion and photosynthetic carbon fixation, which in turn affects metabolic functions such as respiration. i) Photosynthesis Photosynthesis plays a major role in determining crop productivity in all species and is directly affected by water stress. Photosynthetic rates of the leaves decrease as the relative water content and leaf water potential decrease (Lawlor and Cornic, 2002). The effects of water stress on photosynthesis are complex, and may include a combination of stomatal closure and the inhibition of metabolic processes, including ribulose
502 bisphosphate synthesis and adenosine triphosphate synthesis. In cotton, several reports have indicated that water stress causes a reduction in photosynthesis rates due to a combination of stomatal and non-stomatal limitations. However, there has been some controversy concerning the relative importance of these two processes responsible for photosynthetic impairment under water deficit. The relative contributions of stomatal opening and metabolic processes to the decrease of photosynthesis in drought-stressed plants are still being studied and debated. Decreased CO2 diffusion from outside the plant to the site of carboxylation is the main cause for reduced photosynthetic rates under most water-stress conditions. Reduced CO2 diffusion has been attributed to stomatal closure, reduced mesophyll conductance, or a combination of these factors (Flexas et al., 2002; Warren et al., 2004). Additionally, other factors, such as time of day, ambient CO2 concentrations, nutrient levels, leaf type, growth stage, genotypic differences and abscisic acid (ABA) concentrations may affect photosynthetic rate in drought-stressed plants. ii) Stomatal Factors Stomatal closure decreases water loss, but also decreases the movement of CO2 into the plant. Significant correlations have been reported between leaf water potential and stomatal conductance under conditions of water-deficit stress. Kanemasu and Tanner (1969) and Boyer (1970) quantified stomatal resistance on a variety of crops, including cotton, and found that stomatal resistance due to stomatal closure increased dramatically at between -0.8 and -1.2 MPa. Harris (1973) and Bielorai et al. (1975) also reported that in potted experiments stomatal conductance was significantly decreased under conditions of water-deficit stress. Stomatal opening and closing are modulated by guard cells. Stomatal closing are controlled by guard cells in two ways either direct water loss from guard cells, which is called hydropassive closure, or water loss from whole leaf, which is called hydroactive closure. Stomatal response to water stress is also controlled by messengers from the root system. Reports have noted that stomatal closure under drought stress is controlled essentially by the concentration of ABA transported in the xylem from the root to shoot and perceived at the guard cell apoplast (Ackerson, 1980; Hartung et al., 1998; Schroeder et al., 2001; Borel and Simonneau, 2002). In cotton (G. hirsutum L.), stomatal response to water stress is affected by ABA accumulation or ABA redistribution (Radin and Hendrix, 1988). iii) Other Factors Affecting Photosynthesis in Drought-Stressed Plants In addition to stomatal closure and changes in metabolic rates and
503 leaf photochemistry, several other factors have been linked to decreases in photosynthesis in drought-stressed cotton plants. Other factors such as abscisic acid (ABA) concentration, ambient CO 2 concentrations and nutrient deficiencies have been shown to have an effect on leaf stomatal conductance under limited water conditions. 17.2.3 Carbohydrate Production and Water Stress Leaf age is an important plant factor affecting daily photosynthesis. In non-stressed plants, peak carbohydrate production from an individual leaf occurs when the leaf is around 20 days old. Peak plant carbohydrate production will occur when the combination of photosynthesis per unit leaf area and leaf area is maximized. In non-stressed plants this usually occurs some 60 to 70 days from the unfolding of the first true leaf (75 to 85 days after planting). Decline in daily carbohydrate production after this date results from increasing canopy leaf age and increased self shading and the increase in boll demand for carbohydrates, which restricts any new leaf development. Water stress has been shown to reduce whole plant leaf area largely through reductions in total leaf numbers. However, the rate of leaf expansion is also reduced, which in turn reduces the size of individual leaves. Reduction in leaf area will obviously affect the level of total canopy photosynthesis. Photosynthesis is maintained in priority over leaf expansion and development. This allows the plant to maintain current photosynthetic capacity but limits future capacity. The value is that it also stops demand for water increasing when there is not enough to meet even current demands. In terms of plant growth, the maintenance of photosynthesis will enable boll and root growth to continue longer during periods of water stress than vegetative growth. 17.2.4 Plant Mechanisms Underlying Resilience to Water-Deficit Stress i) Antioxidants Drought stress has been reported to induce an oxidative stress due to inhibition of photosynthesis (Smirnoff, 1993) resulting from the production and accumulation of toxic oxygen species such as peroxide radicals, hydrogen peroxide and hydroxyl radicals (Foyer et al., 1997). The accumulation of reactive oxygen species (ROS) originates mainly from the decline in CO2 fixation which leads to higher leakage of electrons to O2 (Foyer et al., 1997), while other factors triggering formation of free radicals involve fatty acid â-oxidation (del Rio et al., 1998), membrane associated oxidases (Desikan et al., 1996) and photorespiration (Faria et al., 1997). These reactive oxygen species produced during water-deficit
504 stress can damage many cellular components including lipids, proteins, carbohydrates and nucleic acids. Efficient antioxidant systems in the plant can minimize the level of oxidative stress and protect the tissues. Such antioxidant systems can be enzymatic or non-enzymatic. The major antioxidant species in the plants are superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (AP), and glutathione reductase (GR), along with carotenoids and á-tocopherol (Gaspar et al., 2002). ii) Proteins Plants have been shown to accumulate specific stress-associated proteins in order to survive adverse environmental conditions (Vierling, 1991; Ingram and Bartels, 1996). Heat shock proteins (HSPs) and late embryogenesis abundant (LEA)-type proteins are two major types of stress induced proteins that are produced upon the induction of drought stress and are considered to play a role in cellular protection during the stress. Heat shock proteins have been observed to be produced at any stage of crop development and under different environmental factors such as waterdeficit stress (Bray, 1993), UV-radiation (Dohler et al., 1995), or heavy metal accumulation (Neumann et al., 1994). Their molecular weights and proportions differ among species, and they are considered as molecular chaperones essential for the maintenance of protein homeostasis and prevention of denaturation. Late embryogenesis abundant proteins, the second major type of stress-induced proteins, have been found in a wide range of plant species in response to desiccation or drought stress. Most LEA proteins exist as random coiled á-helices. They are characterized by their high hydrophilicity index and glycine content (Garay-Arroyo et al., 2000). They are considered to act as water-binding molecules, participate in ion sequestration, and contribute in membrane stabilization (Ingram and Bartels, 1996). iii) Osmotic Adjustment and Compatible Osmolytes Plants experiencing stressful conditions, such as drought, tend to actively accumulate highly soluble organic compounds of low molecular weight, called compatible solutes, as well as inorganic ions, i.e. K, in order to prevent water loss, maintain water potential gradients and re-establish cell turgor (Hsiao, 1973). This process is called osmotic adjustment and according to Boyer (1982) enables plants to: (1) continue normal leaf elongation but at a reduced rate, (2) adjust their stomatal and photosynthetic functions, (3) maintain the development of their roots and subsequently continue soil moisture extraction, (4) postpone leaf senescence, and (5)
505 achieve better dry matter accumulation and yield production under adverse conditions. Osmolytes are organic compounds that exist in a stable form inside the cells and are not easily metabolized. In general, they do not have an effect on cell functions, even when they have accumulated in high concentrations, i.e. more than 200mM (Hare et al., 1998; Sakamoto and Murata, 2002). Compatible solutes include sugars and sugar alcohols (polyols) (Yancey et al., 1982), amino acids such as proline (Aspinall and Paleg, 1981; Bonhert et al., 1995) and its analogues (Naidu et al., 1987), quaternary ammonium compounds (betaines) and tertiary sulfonium compounds (Rhodes and Hanson, 1993). Production of osmolytes is a general method in plants to maintain osmotic potential and cell turgor, as stated above; however, they also have secondary roles such as stabilization of membranes and maintenance of proper protein conformation at low leaf water potentials (Papageorgiou and Morata, 1995), protection of cells by scavenging for ROS (Pinhero et al., 2001), as well as regulation and integration in the metabolism of stressed photosynthetic tissues (Lawlor and Cornic, 2002). 17.2.5 Effect of Water-Deficit Stress on Yield and Fiber Quality Water deficit significantly compromises plant development and productivity around the world (Boyer, 1982). In many crops, reproductive development is most sensitive period to drought stress following seed germination and seedling establishment (Saini, 1997). In cotton, however, there is still debate about the most sensitive period to water-stress during development in relation to yield, even though water sensitivity during flowering and boll development has been well established (Constable and Hearn, 1981; Cull et al., 1981a,b; Turner et al., 1986).The degree of plant response to stress will vary depending on the level of stress which occurs and the timing at which the stress is imposed, relative to crop growth. Fiber development: Fiber development begins the day after flowering and is a twostage process with fiber elongation (length) preceding secondary wall development (thickening). In a non-moisture stressed situation, fiber length reaches a maximum between 20 and 30 days post flowering with fibre wall development being completed some 40 to 60 days post flowering, depending on temperature. Although temperature is the main determinant of the length of the period between flowering and boll opening, carbohydrate supply directly affects the degree of fiber development and final boll size. Under water stress younger bolls are shed to enable the development of older bolls. The plant has increased its adaptation for survival during
506 drought by placing priority on seed and fiber development over total fruit retention. This is demonstrated by the fact that young boll shedding can occur at lower moisture stress levels; while fiber development is not affected until higher stress levels are reached. The increase in micronaire generally associated with cotton suffering from water stress at the end of flowering is a good example of this plant adaptation. Increase in micronaire occurs because younger bolls are shed, and more carbohydrate becomes available to lower bolls. With fiber development continuing under higher stress levels, any extra carbohydrate available is allocated to increases in fiber cell wall thickening, leading to increases in micronaire. Moisture stress during peak flowering will tend to affect fiber length rather than fiber maturity, while stress later in the season will primarily affect fiber maturity. 17.3 Effect of High Temperature Stress The temperature range 26-32oC is desirable during day time but the night should be cool during flowering and fruiting in cotton. This crop is able to tolerate short periods of high temperature upto 43-45 oC if soil moisture condition is favorable. High day temperature coupled with high night temperature delay flowering. The associative trend of high temperature low relative humidity is more harmful in desiccating the leaf surface due to sharp increase in leaf temperature. High temperature regimes affect plant metabolism by impairing membrane thermostability and photosynthetic process. Enzyme activity is more sensitive and proteins may be denatured at elevated temperatures. i) Impact on cotton growth: Cotton can regulate its leaf temperature to maintain optimum growing temperatures. However when leaf temperatures rise too much during the day, this slows the function of plant enzymes for photosynthesis & growth. This can cause an increase in square and boll shedding and a reduction in seed number per boll. This can all lead to a decrease in yield. Warm nights (>26ºC minimum) mean that leaf temperature & respiration remain high, consuming energy that the plant would have normally used for additional growth. High temperature regimes affect plant metabolism by impairing membrane thermostability and photosynthetic process. Enzyme activity is more sensitive and proteins may be denatured at elevated temperatures. ii) Plant tissue damage High temperature stress can also damage plant tissue. This happens when the plants ability to cool evaporatively has been effected (at night or
507 during a hot humid day) increasing the plant tissue temperature to approach or exceed air temperature. -
Two known consequences of tissue damage from severe heat stress are:
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Parrot Beaked Bolls – Small bolls with uneven seed numbers between the locks caused by poor pollination/seed set particularly in one lock. High temperatures reduce the viability of the pollen at flowering. This reduces boll size & can reduce yield.
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Boll Freeze, Cavitation or Boll Dangle – Occurs when young bolls die before the abscission layer forms. So instead of dropping off the fruit hangs on the plant by a dangling piece of tissue.
iii) Effect on fiber quality Heat stress can also reduce the interval between flowering & boll opening, shortening the time to maturity & reducing yield. This may increase final micronaire by limiting the number late set bolls that can have lower micronaire. Fibre length can also affected by sustained periods of high temperatures as the time required for fibre elongation is reduced, not allowing for genetic potential for fibre length to be reached. Plants tolerate high temperature by the accumulation of low molecular weight 70 kda heat shock proteins. The other heat avoidable mechanisms include light reflectance and transmittance to reduce the radiation load and to maintain active transpiration cooling. Thick cuticle and hairiness are desirable characters to minimize the heat stress effects. iv) Management implications Where possible it is important to ensure that heat stress is not accompanied by water stress. Cotton plant responses to water stress vary depending on the stage of growth at which the stress occurs, the degree of stress, & the length of time the stress is imposed. Both yield & fibre quality can be impacted. Where irrigation is available, decreasing the interval between irrigations maybe necessary to avoid water stress. Simultaneously, identifying the plants with relatively higher tolerance or acclimatization to high temperature growing conditions is crucial. In this context, cotton possesses near xerophytic characters and wild cotton species have relatively more heat tolerance. High temperature effect may considerably be reduced by providing adequate irrigation at critical stages of growth which may lead to leaf cooling effects. Use of light reflectant material and soil moisture conservation at appropriate times may be useful in reducing the ill effects
508 of elevated temperature. Such measures may enhance water use efficiency. Fruit loss (through shedding, boll freeze, or parrot beaking) can cause the crop to grow rank following heat stress. Management during & after periods of high temperatures involves closely monitoring vegetative growth rate (VGR), fruit retention & boll size. If excessive vegetative growth is detected, the use of mepiquat choride (PIX®) is recommended. Using a growth regulator to limit vegetative growth is preferred rather than limiting water as this may increase fruit shedding. 17.4 Effect of Salinity Stress Salinity is one of the most serious abiotic stresses (Rahman et al., 2004). It is a constraint to plant growth and development and causes low productivity in crop species in agriculture worldwide (Hameed et al., 2008). Saline soils adversely affect plant growth and lead to plant death due to the effects of water stress, ion toxicity, ion imbalance, or a combination of these factors (Bor et al., 2003). Plants are immobile organisms and constantly exposed to changes in the environment. In saline soils, roots are the primary point of contact with salt. To some extent, plants could overcome osmotic and ionic stress caused by high salinity via developing efficient and specific mechanisms (Mahmood et al., 2010). But several cellular stress responses are induced by excess of salt causing damage to different cellular components (Dolek et al., 2001). Salinity alters almost every aspect of crops including their morphological, physiological and biochemical aspects. In addition, salinity markedly reduces water potential in cells leading to closure of stomata and limits net CO2 assimilation rate (Munns et al., 2006). Electron transport is a primary phenomenon of mechanism of photosynthesis, comprises a variety of components, the most distinctive ones are photosystem I and II. As a consequence of osmotic and ionic stresses, secondary stresses such as oxidative damage often occur. Reactive oxygen species generated by salt stress are highly reactive and alter normal cellular metabolism causing lipid peroxidation, protein denaturing and DNA mutation (Dat et al., 2000; Implay, 2003). Salt stress causes nuclear deformation and subsequent nuclear degradation (Katsuhara & Kawasaki, 1996). Structural changes of nuclei caused by salt stress have been reported (Werker et al., 1983). Salt affected soils are found in almost all agro-ecological regions. In India, approximately 12m ha of cultivable land is under salty affect of which 3.6 m ha land is under alkali and rest is under saline soils. The rainfed tract of Gujarat, Maharashtra, Karnataka and Andhra Pradesh has approximately 3m ha of salt affected soils of which nearly ¼ th of area is
509 under alkali condition. Most of these areas belong to the cotton growing districts of Karnataka (Bijapur, Dharwad, Gulberga, Raichur), Maharastra (Akola, Amaravati, Buldana, Solapur, Dhule, Ahmednagar, Sangli), Andhra Pradesh (Guntur, Bapatla and other cotton growing district), and Gujarat (Some of the coastal districts). 17.4.1 Causes of Salinity Stress Soil degradation through salinisation and alkalization is seriously affecting the productivity of large areas of cultivated soils. Salinisation and alkalization causes poor plant growth and yield due to three major effects i) Water stress caused by salt acting as an osmoticum ii) Specific ion toxicity iii) Nutrient imbalances In saline soils and saline sodic soils, all the 3 factors may contribute to reduced growth, whereas in alkaline soils ion toxicity and nutrient imbalances contribute to reduced growth. 17.4.2 Effects of Salinity on Growth and Yield A major difficulty associated with assessment and measurement of salt tolerance in cotton is variation in tolerance with growth stages or ontogeny. i) Germination Cotton is very sensitive to salinity germination and emergence. The germination percentage of most of the genotypes is not affected by salinity upto 10 dS m-1.Beyond 10 dS m-1, however germination is drastically reduced. In one of the experiments, seed treatment with 15 and 20 dS m-1 NaCl reduced the germination percentage by 45 and 65 per cent of control plants respectively. More than the germination, cotton seeds are more sensitive to the emergence duration. The seedling emergence from the soil took 8 and 9 days at salinity concentrations of 15 and 20 dS m-1respectively in comparison to 4 days in control plants. By the time the seedling emerge from the soil all the nutrients are exhausted and resulted in poor crop stand at higher salinity levels. ii) Plant growth and yield Cotton is relatively a salt tolerant crop however species as well as varietal variation exists for salt tolerance. Amongst the different crops viz.
510 wheat, soybean, cotton and maize, cotton is the most tolerant and soybean is the most sensitive crop to salinity. Salinity stress reduces the vegetative growth of cotton to a great degree. Salinity decreases shoot/root ratio, shoots being sensitive to salt stress more than roots. Salinity causes the induction of phytotoxicity, including necrosis of apices and leaf margins, stunted growth, and leaf chlorosis. It also found that increasing salinity causes a reduction in the number of bolls due to both decreased fruiting positions and an increase in the percentage of bolls shedding. Hydroponic experiments revealed that plant height, leaf area, square and boll number and plant biomass were not affected by salinity upto10 dS m-1 beyond which i.e., at 15 and 20 dS m-1 they were drastically reduced. Early seedling stage is more sensitive to saline condition than later stages of growth. G.herbaceum and G.barbadense were found to be more tolerant than G.arboreum and G.hirsutum. One of the important problems due to salinity is the reduction of the fiber quality of cotton (Chowdhry, 1972). The fiber length, strength, and micronaire values of G. hirsutum and G. barbadense were reduced when grown under saline conditions (Longenecker, 1974). Increased salinity was found to decrease seed index and increase fiber fineness (Korkor et al., 1974), and at higher salinity levels lint percentage, staple length, fiber fineness, and seed weight were reduced, while fiber strength was increased (Latif and Khan, 1975). Increasing levels of salinity (5.42 to 20.31 dS m– 1 ) made the fibers course and decreased bundle length, but fiber length and maturity percentage were not affected (Ray et al., 1989). 17.4.3 Mechanism of Salinity Tolerance Although the mechanism of adaptation of plants salinity is not clear, many basic physiological attributes including direct modification of the influx and/ or efflux of ions such as K and Na across the plasma membrane and tonoplast, synthesis of compatible osmotica such as proline, other amino acids, soluble carbohydrates, and glycine betaines, and modification of membrane composition have been found to be important components of a salt tolerant phenotype. An important consequence of salinity stress in plants is the excessive generation of reactive oxygen species (ROS) such as superoxide anion (O-2), hydrogen peroxide (H2O2) and the hydroxyl radicals (OHÏ%) particularly in chloroplasts and mitochondria. Plants possess a number of antioxidant enzymes like superoxide dismutase (SOD), ascorbate peroxidase (APX) and glutathione reductase (GR) for protection against the damaging effects of ROS (Asada, 1992; Prochazkova and Wilhelmova, 2007). Membrane disorganization, metabolic toxicity due
511 to ROS and attenuated nutrients are the factors, which initiate more catastrophic events in plants subjected to salinity stress. Accumulation of metabolites that act as compatible solutes is one of the probable universal responses of plants to changes in the external osmotic potential. Metabolites with osmolyte function like sugar alcohols, complex sugars and charged metabolites are frequently observed in plants under unfavorable conditions (Hasegawa et al., 2000; Satiropoulos, 2007). Proline and glycine betaine are known to serve as compatible osmolytes, protectants of macromolecules and also as scavengers of ROS under stressful conditions. 17.4.4 Management against Salinity Conditions Two broad approaches viz. reclamation and adoption of appropriate management practices can be utilized to overcome the salt effect. Under rainfed condition however, reclamation of the soil is very difficult mainly because of the scarcity of water to leach out the salts. Management practices that can aid in obtaining better crop production include choice of crops that are more tolerant to salt affected condition and other practices that minimize the salt concentration in the root zone of growing crops. Amongst the various amendments used for improvement for alkali soils gypsum along with improved package of practices altered the physico- chemical characteristics of soil. Application of gypsum decreased pH, ECe and ESP and also enhanced the yield. Organic residue incorporation improved the physical conditions of the soil. A close relationship between aliphatic component of soil organic matter and microbial biomass and soil aggregation has been reported. Various cultural and mechanical measures like tillage operations, opening of furrows can help in in situ moisture conservation and improve the soil moisture storage by way of reducing the surface run off. Improved physical properties of soil increased yield of salt affected soils. 17.5 Effect of Water-logging Cotton in India is grown in different agroclimatic zones. In North India more than 95 per cent of the crop is irrigated. Due to excessive canal irrigation, the water table in parts of Haryana, Rajasthan and Punjab had risen to such a high level that it is becoming difficult to grow cotton crop. In due course, rice or other crops may replace cotton in these areas. In Central India, more than 70 per cent of the crop is grown under rainfed conditions and often it suffers from waterlogging duration early and mid vegetative growth stages. Even in South India where part of the crop is cultivated under irrigated condition and rest under rainfed condition cotton experiences waterlogging at one or other stages of its life cycle.
512 Waterlogging has a number of impacts on cotton plants and soils. One of the most immediate is the adverse effect on soil oxygen content. Waterlogging of soils displaces oxygen held in soil aggregates thus halting the exchange of oxygen to cotton roots. Oxygen exchange in roots drives respiration in plants which provides free energy used for the maintenance and development of the plant. Oxygen is the ultimate electron acceptor in the electron transport train, when oxygen is unavailable oxidative respiration stops and plant respiration becomes limited to glycolytic and fermentative metabolism which provides only 4% of the energy that the complete oxidation process provides. In addition to the physiological aspects of plant growth, waterlogging may also have a significant impact on nutrient availability and uptake. Access to nutrients such as nitrogen, iron and zinc become limited due to reduced oxygen availability. Additionally, some bacteria present in soils that usually utilize oxygen as the electron acceptor during respiration are able substitute nitrate nitrogen for oxygen. This results in the loss of nitrogen from anaerobic soils (denitrification) in the form of nitrogen gas (N2). Recovery of flood affected plants is complex. Thus, growth and productivity of the plant will be affected. Cotton has been classified as a susceptible crop for waterlogging. However, the occurrence and extent of any particular response depends on many interrelated factors such as the species or the cultivar, its age and stage of development, duration depth and timing of flooding, soil type. i) Effect on root growth The imposition of excess water around roots affected its development. Root elongation has been inhibited under anaerobic condition. The deeper roots die and quite often there is proliferation of surface roots and hence, waterlogged plants will have a smaller and more superficial root system. The severity of the effect of transient flooding on root system depends on growth stage of the plant. Those roots that became submerged during the early seedling growth immediately ceased extension. With flooding immediately after flowering, roots below the water table ceased extension and new roots compensated for this by growing from roots located in the upper part of the soil. ii) Effect on shoot growth Waterlogged plants had a stunted growth compared to normal well aerated plants. Plants are more sensitive at early seedling stage and waterlogging the plants at or after flowering had no significant effect on plant height. Leaf growth is extremely sensitive to flooding and root anoxia. Leaf area per plant has been reduced by inhibiting leaf initiation and
513 expansion as well as by inducing leaf abscission. The above ground plant growths as well as root growth were very sensitive to waterlogging at early growth stages. Through, withdrawing the waterlogging treatment after 45 days resulted in a gradual recovery of plant growth in 45 day old waterlogged plants, but it failed to produce any yield. On the country, there was only a marginal reduction in yield when plants were waterlogged at 90 days after sowing. This suggests that plants subjected to waterlogging at flowering or later stages are getting adapted and maintain their growth. iii) Adaptation to waterlogging Species as well as varieties vary in their tolerance to flooding. Cotton plants exhibit metabolic adaptation to tolerant anoxia (truly anoxia tolerant plant), as well as morphological and physiological adaptation to avoid anoxia (apparently anoxia tolerant) and few do not adapt and succumb very quickly to anoxia (anoxia intolerant). iv) Morphological adaptation Waterlogged cotton plants at the zone of submergence produced specialized cells called as ‘lenticels’. Lenticels were formed few centimeters above and at the zone of submergence within 3 days of waterlogging. Further it was observed that when only bottom half of the roots were submerged, there was no formation of lenticels and all the roots became apparently negatively geotropic. Similarly, once the waterlogging treatment was withdrawn the lenticels gradually disappeared. This indirectly suggested the involvement of lenticel in oxygen uptake and its transfer from shoot to roots and thus helped the waterlogged plants to maintain stable root activity. Since the stem girth was very less at early growth stages, waterlogging of these plants produced only few lenticels and thus they were very sensitive to waterlogging. v) Metabolic adaptation Cotton plats exhibit metabolic adaptation in addition to morphological adaptation to withstand adverse affect of waterlogging. Anaerobiosis induced alteration in protein synthesis has been reported in cotton. Fourteen major polypeptides are shown to be selectively synthesized under anaerobiosis in cotton and designated as cotton anaerobic polypeptides (ANPs), of which 3 of the major ANPs are alcohol dehydrogenase (ADH) enzymes. Alcohol dehydrogenase plays a major physiological role plants during anaerobic stress when carbohydrate metabolism must switch from an oxidative to a fermentative pathway. ADH is a terminal enzyme in the ethanolic fermentation pathway converting
514 acetaldehyde to ethanol and regenerating NAD+ in the process. Removal of acetaldehyde is important because of its phytotoxic effects and the regeneration of NAD+ is essential for the continuation of anaerobic glycolysis, which is the major source of cellular ATP during periods of anaerobiosis. The ongoing work at our laboratory showed that waterlogged cotton plants had higher ADH activity in the roots compared to leaves and shoot thus enabling the plant to continue anaerobic glycolysis and the ATP synthesis. 17.6 Breeding Approaches for Abiotic Stress Management Development of varieties involves simultaneous improvement for abiotic stresses and high yield. Water deficit is one of the major factor adversely affected the plant growth resulted low yield. Certain mechanisms like drought avoidance (escape), drought resistance and drought tolerance through which plants are able to improve their performance under water deficit condition. Drought resistance is characterized by the ability of plants for better exploitation of soil water and ability of restricting water loss through extensive root system, stomatal control on transpiration, cuticular transpiration, smaller leaves and high root / shoot ratio etc. Drought tolerance is the ability of plants to maintain high plant water potential at a given level of water stress through osmo-regulation. Escape and avoidance are important for drought resistance. Tolerance is more important in low temperature stress. The difference between stress avoidance and tolerance may be best visualized by considering the cellular state which plants are exposed to various stresses. Avoidance mechanisms maintain a constant cellular state at the expense of energy which could have contributed to growth. Consequently, stress avoiding but intolerant plants are generally lower yielder under optimum environments. 17.6.1 Sources of Drought Resistance and Breeding Methods For developing the superior cultivars of crops for abiotic, it is imperative that the nature of stress condition(s) is defined and diverse range of germplasm needs to be tested for response to stress. The stress (es) may be governed by more than one character which may include the morphological, anatomical, physiological and chemical characters. The choice of breeding approaches and methods may depend on the method of reproduction, mode of gene action, source of tolerance available and the priority assigned to goal in relation to the other agronomic traits. The direct approach based on the absolute performance under actual stress or selection for only small reduction in performance under stress and indirect approach is based on screening and selection for morphological or
515 physiological characteristics that may be correlated with or that contribute to stress tolerance. The identified drought resistance traits can be introduced into superior genetic background. In upland cotton, exotic lines have been found to be good source of drought resistance (Quisenbarry et al. 1981). Asiatic cotton viz. Gossypium arboreum and G. herbaceum are very good source of drought resistance (tetraploid, 2n = 52). Species and races like G. hirsutum (Punctatum), G. arboreum (Sudananse), G. herbaceum (Acerfolium, Kuljanum, Persicium and Weightianum) are the source of drought resistance. Certain elite germplasm of G. hirsutum (EL 592, JK 259, Lankart 57, Acala SJ 5, Cocker 100 staple, Meade 9030D, Deviraj, JK 266, DS 59) and G. arboreum (Chandroda, Rozi 6, AC 27, New Million Dollar, Cocanada 5, Vira 6, AC 28, Naked seed) are source for abiotic stress resistance. Various wild species are good source of drought resistance (Sikka and Joshi, 1960). Wild species
Genome
Source of resistance
G. aridum
D4
Drought
G. davidsonii
D3.D
Salinity
G. thurberi
D1
Frost
G. darwinii
AD
Drought
G. tomentosum
AD3
Drought
G. austrate
C
Drought
G. sturtianum
C1
Frost and Cold
G. africanum
A
Drought
G. stocksii
E1
Drought
In India two drought resistance varieties (MA 9 and MCU 10) of upland cotton have been released (Singh et al.1996). The varieties developed from G. arboreum and G. herbaceum are the source of drought resistance or tolerance because of their deep root system. Various morphological traits such as earliness, stomatal features (sunken type, small size and number per unit size), leaf character (thick cuticle, and waxiness of leaf), rooting pattern (deep root system) and growth habit (indeterminate genotype) are associated with drought resistance. When resistance is found in an exotic variety it can be introduced. If resistance is available in the land races or n mixed population either pure line selection or mass selection is adapted. When resistance genes are available in the unadapted genotype, back crossing method is used. Whereas resistant gene is found in adapted genotype pedigree breeding is adopted. Backcross method is also used for
516 transfer of drought resistant genes from wild species. Mutation breeding is used when resistant genes are not found in the germplasm. Mutants tolerant to soil salinity caused by chlorides and sulphates have been obtained through gamma irradiation of relatively intolerant genotypes (Nazirov et al, 1979). 17.6.2 Drought Escape and Early Maturity The concept of early maturity was indentified during 1970’s and addressed by Robert Bridge at Stoneville, Mississipi with his release of ‘DES 24’ and ‘DES 56’ (Bridge and Chism,1978). DES 24 and DES 56 matured 10 days earlier than ‘ Deltapine 16’ which was most popular cultivar in the USA in 1970. DES 56 was used as a parent of Deltapine 20 and Deltapine50 are early maturing and high yielding cultivars. Deltapine 51 a reselection of Deltapine 50. Earliness continued to be a high breeding priority in 1979 and it was predicted a large number of early maturing cultivars (Deltapine 55, Deltapine 56 and MC T8-27) till 1995. Another early cultivar in the 1990’s was ‘Stoneville 132’ which was a selection out of MC-T8-27-8c. The prominent abiotic responsive genes like genes for biosynthesis of osmo-protectant like sorbitol, pinitol, proline, betaine genes for membrane protein, LEA gene proteins and alcohol dehydrogenease etc., have immense potential in development of newer cotton plant suitable under water stress, water logging and salinity stress. Scope of developing new cotton plant types (ideotype) with enhanced protective enzymes like superoxide dismutase, peroxidase, glutathione reductase during biochemical response to stress has become brighter. Introduction of bacterial genes into plants for sequestering heavy metal and toxic ions provided new scopes for growing cotton in underutilized and polluted soils. 17.7 Recent Approaches to Study Abiotic Stress in Cotton Environmental factors that impose water-deficit stress, such as drought, salinity and temperature extremes, place major limits on plant productivity. However, many abiotic stresses are complex in nature, controlled by networks of genetic and environmental factors that hamper breeding strategies. As traditional approaches for crop improvement reach their limits, agriculture has to adopt novel approaches to meet the demands of an ever growing world population. Recent technological advances and the aforementioned agricultural challenges have led to the emergence of high throughput tools to explore and exploit plant genomes for crop improvement. These genomics-based approaches aim to decipher the entire genome, including genic and intergenic regions, to gain insights into plant molecular responses which will in turn provide specific strategies for crop
517 improvement. 17.7.1 Functional Genomics Genomics research is frequently realized by functional studies, which produce perhaps the most readily applicable information for crop improvement. Functional genomics techniques have long been adopted to unravel gene functions and the interactions between genes in regulatory networks, which can be exploited to generate improved varieties. Functional genomics approaches predominantly employ sequence or hybridization based methodologies. Large-scale EST sequencing has been one of the earliest strategies for gene discovery and genome annotation. cDNA libraries from various tissues, developmental stages, or treatments generally serve as the sources for EST sequencing to reveal differentially expressed genes. These approaches can successfully identify tissue or developmental stage-specific and treatment responsive transcripts. However, such cDNA libraries may under represent rare transcripts or transcripts that are not expressed under certain conditions. In addition, ESTs are usually much shorter in length than the cDNAs from which they are obtained. Assembly of overlapping EST sequences into consensus contigs is likely to be more informative on the structure of the parental cDNA, which may reveal polymorphisms. EST sequencing is still a valid approach, and a recent study has demonstrated its potential in gene discovery via the comparison of different genotypes under control and stress conditions. Serial Analysis of Gene Expression (SAGE) has been developed to quantitate the abundance of thousands of transcripts simultaneously. In this approach, short sequence tags from transcripts are concatenated and sequenced, giving an absolute measure of gene expression. SAGE has also been used to investigate stressresponsive genes. A similar tag-based approach, Massively Parallel Signature Sequencing (MPSS), where longer sequence tags are ligated to microbeads and sequenced in parallel, enables analysis of millions of transcripts simultaneously. Due to longer tags and high-throughput analysis, MPSS is likely to identify genes with greater specificity and sensitivity. The ability of MPSS to capture rare transcripts is particularly beneficial in species that lack a whole genome sequence. Genome wide expression profiles are most useful in the detection of candidate genes for desired traits, such as stress tolerance. Targeting Induced Local Lesions IN Genomes (TILLING) enables high-throughput analysis of large number of mutants. Importantly, a modified strategy, called EcoTILLING, has been developed to identify natural polymorphisms, analogous to TILLINGassisted identification of induced mutations. Polymorphisms demonstrating natural variation in germplasms are valuable tools in genetic mapping.
518 These above-mentioned genomic approaches are use to identify genes accurately and unambiguously. 17.7.2 Structural Genomics Structural genomics focus on the physical structure of the genome, aiming to identify, locate, and order genomic features along chromosomes. Different DNA sequencing technologies have enabled the generation of a wealth of sequence information including whole genome sequences. Nextgeneration sequencing (NGS) platforms such as Roche 454GS FLX Titanium or Illumina Solexa Genome Analyzer can carry out high capacity sequencing at reduced costs and increased rates compared to conventional Sanger sequencing. Whole genome sequences provide remarkably detailed information on genomic features including coding and non coding genes, regulatory sequences, repetitive elements, and GC content which can be exploited in functional studies such as microarray or tiling arrays. Several types of molecular markers that have been developed and are being used in plants include restriction fragment-length polymorphisms (RFLPs), Amplified fragment-length polymoiphisin (AFLP), Random amplification of polymorphic DNA (RAPD), cleavable amplified polymorphic sequences (CAPS), single strand conformation polymorphisms (SSCP), sequence-tagged sites (STS), simple sequence repeats (SSRs) or microsatellites and single-imcleotide polymorphisms (SNPs) (Gosal et al.2009). Such markers, closely linked to genes of interest can be used to select indirectly for the desirable allele, which represents the simplest form of marker-assisted selection (MAS), now being exploited to accelerate the backcross breeding and to pyramid several desirable alleles. Selection of a marker flanking a gene of interest allows selection for the presence (or absence) of a gene in progeny. Thus molecular markers can be used to follow any number of genes during the breeding program. The discovery of molecular markers has enabled dissection of quantitative traits into their single genetic components and helped in the selection and pyramiding of QIL alleles through MAS. A fundamental step in any functional genomics study is the analysis of gene expression. One of the greatest strengths of genomics compared to other disciplines is the prospect of analyzing the expression of thousands of genes simultaneously; resulting in a more comprehensive picture of changes occurring in the transcriptome across different conditions. The technology available for the analysis of gene expression can be divided into two categories: closed and open systems. Closed systems are characterized by a finite number of genes that can be assessed by virtue of their inclusion by selection. Therefore, the coverage of genes will be related
519 to the completeness of the knowledge of the genome being studied, limiting this kind of analysis to the most well characterized species or system. Typically, closed systems such as microarrays and real-time polymerase chain reaction (RT-PCR) have been extensively used in gene expression analysis in plants. On the other hand, with open systems there is no need for previous knowledge of the genome or transcriptome of the organism. 17.8 Future Prospective In recent years, the necessity for a change in focus in plant abiotic stress research has become apparent. As plants have finite resources which must be balanced between growth and defence against stresses, often both natural and induced stress tolerance comes with a growth or yield penalty, making it agriculturally disadvantageous. Rather than developing crops that can survive extreme stress events, it may therefore be more beneficial to focus on producing crops which are stress tolerant but which maintain high photosynthesis, growth rates, and yield. New technologies are providing opportunities to generate transgenic crops able to maintain high yields under stressful and changing environments. Many genes associated with plant response(s) to abiotic stresses have been identified and used to generate stress tolerant plants. Most of these studies were conducted under laboratory conditions applying artificial stress conditions, using model plants and focusing on recovery from a stress episode as the main trait. The use of hybrids to introduce new genetic material into crop plants is a standard breeding strategy. Newer mechanisms or concepts are needed to comprehensively tackle salinity and moisture stress. The single or few gene-based approaches has to be perfected for specific stress for meaningful performance. Multiple gene encoding sequential steps in a stress protective path way require mutagenesis and enzyme redesign to allow effective accumulation of protective compounds in transgenic host plant. The advance microarray technology consist of nucleic acids chips containing other types of bio-molecules allowing increased high target density will open up avenues for the identification of desirable genes in a very rapid way and further in depth study on the gene of interest. Also the expression of the gene of various functions can be quantified in different genotypes which will eventually help in selection much easier and faster. References: Ackerson R C (1980). Stomatal response of cotton to water stress and abscisic acid as affected by water stress history. Plant Physiology, 69: 455-459. Asada K (1992). Ascorbate peroxidase-a hydrogen peroxide scavenging enzyme in plants. Physiologia Plantarum, 85: 235-241.
520 Aspinall D and Paleg L G (1981). Proline accumulation: physiological aspects. In: L.G. Paleg and D. Aspinall (eds.).The Physiology and Biochemistry of Drought Resistance in Plants. Academic Press: Sydney, Australia, pp. 205-241. Ball R A, Oosterhuis D M and Maromoustakos A (1994). Growth dynamics of the cotton plant during water-deficit stress. Agronomy Journal, 86: 788-795. Berlin J, Quisenberry J E, Bailey F, Woodworth M and McMichael B L (1982). Effect of water stress on cotton leaves: I. an electron microscopic stereological study of the palisade cells. Plant Physiology, 70: 238-243. Berlin J, Quisenberry J E, Bailey F, Woodworth M and McMichael B L (1982). Effect of water stress on cotton leaves: I. an electron microscopic stereological study of the palisade cells. Plant Physiol. 70:238-243. Bielorai H and Hopmans P A M (1975). Recovery of leaf water potential, transpiration, and photosynthesis of cotton during irrigation cycles. Agronomy Journal, 67: 629632. Bondada B R, Oosterhuis D M, Murphy J B and Kim K S (1996). Effect of water stress on the epicuticular wax composition and ultrastructure of cotton (Gossypium hirsutum L.) leaf, bract, and boll. Environmental and Experimental Botany, 36: 61-69. Bonhert H J, Nelson D E and Jensen R G (1995). Adaptations to environmental stresses. Plant Cell, 7: 1099-1111. Bor M, Ozdemir F and Turkan I (2003). The effect of salt stress on lipid peroxidation and antioxidants in leaves of sugar beet (Beta vulgaris L.) and wild beet (Beta maritima L.). Plant Science, 164(1): 77-84. Borel C and Simonneau T (2002). Is the ABA concentration in the sap collected by pressurizing leaves relevant for analyzing drought effects on stomata? Evidence from ABA-fed leaves of transgenic plants with modified capacities to synthesize ABA. Journal of Experimental Botany, 53 (367): 287-296. Boyer J S (1970). Leaf enlargement and metabolic rates in corn, soybean, and sunflower at various leaf water potentials. Plant Physiology, 46: 233-235. Boyer J S (1982). Plant productivity and environment. Science, 218: 443-448. Bray E A (1993). Molecular responses to water deficit. Plant Physiology, 103: 10351040. Bridge R R and Chism J F (1978). Registration of DES 24 cotton. Crop Science, 18:523. Chowdhry T M (1972). Cotton soils of Pakistan. Cotton in Pakistan. Pak. Cent. Cott. Committee, Karachi, pp: 275-305. Constable G A and Hearn A B (1981). Irrigation of crops in a subhumid climate, 6: effects of irrigation and nitrogen fertilizer on growth, yield and quality of cotton. Irrigation Science, 2: 17-28. Cull P O, Hearn A B and Smith R C G (1981a). Irrigation scheduling of cotton in a climate with uncertain rainfall. I. Crop water requirements and response to irrigation. Irrigation Science, 2: 127-140.
521 Cull P O, Smith R G C and McCaffery K (1981b). Irrigation scheduling of cotton in a climate with uncertain rainfall. II. Development and application of a model for irrigation scheduling. Irrigation Science, 2: 141-154. Dat J, Vandenabeele S, Vranova E, Van Montagu M, Inze D and Van Breusegem F (2000). Dual action of the active oxygen species during plant stress responses. Cellular and Molecular Life Sciences, 57: 779-795. Del Rio LA (1998). The activated oxygen role of peroxisomes in senescence. Plant Physiology, 116: 1195-1200. Desikan R, Hancock J T, Coffey M J and Neill S J (1996). Generation of active oxygen in elicited cells of Arabidopsis thaliana is mediated by an NADPH-oxidase like enzyme. FEBS Letter, 382: 213-217. Dohler G, Hoffmann M and Stappel U (1995). Pattern of proteins after heat-shock and UV-B radiation of some temperature marine diatoms and the Antarctic odontella. Botanica Acta, 108: 93-98. Dolek B, Bajrovic K and Gozukirmizi N (2001). Salinity effects on plant tissue culture of common bean (Phaseolus vulgaris L.). Biotechnology & Biotechnological Equipment, 15(2): 97-100. Faria T, Vaz M, Schwanz P, Polle A, Pereira J S and Chaves M M (1997). Responses of photosynthetic and defense systems to high temperature stress in Quercus suber seedlings grown under elevated CO2. Plant Biology, 1: 365-371. Flexas J and Medrano H (2002). Drought-inhibition of photosynthesis in C3 plants: Stomatal and non-stomatal limitations revisited. Annals of Botany, 89: 183-189. Foyer C H, Lopez-Delgado H, Dat J F and Scott I M (1997). Hydrogen peroxide and glutathione-associated mechanisms of acclamatory stress tolerance and signaling. Annual Plant Reviews, 22: 325-346. Garray-Arroyo A, Colmenero J M, Garciarrubio A and Covarrubias A A (2000). Highly hydrophilic proteins in prokaryotes and eukaryotes are common during conditions of water deficit. Journal of Biological Chemistry, 275: 5668-5674. Gaspar T, Franck T, Bisbis B, Kevers C, Jouve L, Hausman J F and Dommes J (2002). Concepts in plant stress physiology. Application to plant tissue cultures. Plant Growth Regulators, 37: 263-285. Gerik T J, Gannaway J R, El-Zik K M, Faver K M and Thaxton P M (1996). Identifying high yield cotton varieties with carbon isotope analysis. In: Proc. Beltwide Cotton Prod. Res. Conf. Natl. Cotton Counc. Am. Memphis, Tenn, pp. 1297-1300. Gosal S S, Wani S H and Kang M S (2009). Biotechnology and drought tolerance. Journal of Crop Improvement, 23: 19-24. Hameed A, Naseer S, Iqbal T, Syed H and Haq M A (2008). Effects of NaCl salinity on seedling growth, senescence, catalase and protease activities in two wheat genotypes differing in salt tolerance. Pakistan Journal of Botany, 40(3): 1043-1051. Hare P D, Cress W A and Staden J V (1998). Dissecting the roles of osmolyte accumulation during stress. Plant and Cell Environment, 21: 535-553.
522 Harris D G (1973). Photosynthesis, diffusion resistance and relative plant water content of cotton as influenced by induced water stress. Crop Science, 13: 570-572. Hartung W, Wilkinson S and Davis W J (1998). Factors that regulate abscisic acid concentrations at the primary site of action at the guard cell. Journal of Experimental Botany, 49: 361-367. Hasegawa P M, Bressan R A, Zhu J K and Bohnert H J (2000). Plant cellular and molecular responses to high salinity. Annual Review of Plant Physiology and Plant Molecular Biology, 51: 463-499. Hsiao T C (1973). Plant responses to water stress. Annual Review of Plant Physiology, 24: 519-570. Implay J A (2003). Pathways of oxidative damage. Annual Review of Microbiology, 57: 395-418. Ingram J and Bartels D (1996). The molecular basis of dehydration tolerance in plants. Annual Review of Plant Physiology and Plant Molecular Biology, 47: 377-403. Kanemasu E T and Tanner C B (1969). Stomatal diffusion resistance of snap beans. I. Influence of leaf water potential. Plant Physiol. 44:1547-1552. Katsuhara M and Kawasaki T (1996). Salt stress induced nuclear and DNA degradation in meristematic cells of barley roots. Plant and Cell Physiology, 37(2): 169-173. Korkor S, Tayel M Y and Anter F (1974). The effect of salinity on cotton yield and quality. Egyptian Journal of Soil Science, 14: 137-148. Kramer P J (1983). Water deficits and plant growth. In: P.J. Kramer (ed.). Water relations of plants. Academic Press, New York, pp: 342-389. Latif A and Khan M A (1975). The effect of soil salinity on cotton (G. hirsutum L.) at different stages of growth. In: Field Crop Abstracts, 26 (8): Abstract 3766. Lawlor M M and Cornic G (2002). Photosynthetic carbon assimilation and associated metabolism in relation to water deficits in higher plants. Plant and Cell Environment, 25: 275-294. Le Houerou H N (1996). Climate changes, drought and desertification. Journal of Arid Environments, 34: 133-185. Longenecker D E (1974). The influemce of high Na+ in salts upon fruiting and shedding boll characteristics, fibre properties and yield of two cotton species. Soil Science, 118: 387-396. Mahmood T, Iqbal N, Raza H, Qasim M and Ashraf M (2010). Growth modulation and ion partitioning in salt stressed Sorghum (Sorghum bicolor L.) by exogenous supply of salicylic acid. Pakistan Journal Botany, 42(5): 3047-3054. Massacci A, Nabiev S M, Petrosanti L, Nematov S K, Chernikova T N , Thor K and Leipner J (2008). Response of the photosynthetic apparatus of cotton (Gossypium hirsutum L.) to the onset of drought stress under field conditions studied by gasexchange analysis and chlorophyll fluorescence imaging. Plant Physiology and Biochemistry, 46: 189-195.
523 McMichael B L, Oosterhuis D M, Zack J C and Beyrouty C A (1999). Growth and development of root system, in J.M. Stewart, D.M. Oosterhuis, and J. Heitholt (eds.), Cotton Physiology. Book II. National Cotton Council, Memphis, TN. Munns R, James R A and Lauchli A (2006). Approaches to increasing the salt tolerance of wheat and other cereals. Journal of Experimental Botany, 5: 1025-1043. Naidu B P, Cameron D F and Konduri S V (1998). Improving drought tolerance of cotton by glycinebetaine application and selection. Proceedings of the 9th Australian Agronomy Conference, Wagga. Nazirov N N, Tasmator N T and Vakhabor A (1979). Cotton mutants with better salinity tolerance. Mutation breeding Newsletter, 14:1. Neumann D, Licthenberger O, Gunther D, Tschiersch K and Nover L (1994). Heat-shock proteins induce heavy-metal tolerance in higher plants. Planta, 194: 360-367. Oosterhuis D M, Hampton R E and Wullschleger S D (1991). Water deficit effects on cotton leaf cuticle and the efficiency of defoliants. International Journal of Agronomy and Plant Production, 4: 260-265. Papageorgiou G G and Murata N (1995). The unusually strong stabilizing effects on glycine betaine on the structure and function in the oxygen evolving photosystem II complex. Photosynthetic Research, 44: 243-252. Pettigrew W T (2004). Moisture deficit effect on cotton lint yield, yield components, and boll distribution. Agronomy Journal, 96: 377-383. Pham Thi, A T, Borrel-Flood C, DaSilva J V, Justin A M and Mazliak P (1985). Effects of water stress on lipid metabolism in cotton leaves. Phytochemistry, 24: 723-727. Pinheiro C, Chaves M M and Ricardo C P P (2001). Alterations in carbon and nitrogen metabolism induced by water deficit in the stem and leaves of Lupinus albus L. Journal of Experimental Botany, 52: 1063-1070. Prochazkova D and Wilhelmova N (2007). Leaf senescence and activities of the antioxidant enzymes. Biologia Plantarum, 51: 401-406. Quisenbarry J E, Jordan W R, Roark B A and Fryear DW (1981). Exotic cotton as genetic sources for drought resistance. Crop Science, 21: 889-895. Radin J W and Hendrix D L (1988). The apoplastic pool of abscisic acid in cotton leaves in relation to stomatal closure. Planta, 174: 180-186. Rahman M U, Malik T A, Chowdhary M A, Iqbal M J and Zafar Y (2004). Application of random amplified polymorphic DNA (RAPD) technique for the identification of markers linked to salinity tolerance in wheat (Triticum aestivum). Pakistan Journal Botany, 36(3): 595-602. Ray N, Yadave S B and Khaddar V K (1989). Effect of graded salinity levels on the lint quality of hirsutum cotton cultivars. Indian Journal of Agricultural Research, 21: 127-132. Rhodes D and Hanson A D (1993). Quaternary ammonium and tertiary ammonium compounds in higher plants. Annual Review of Plant Physiology and Plant
524 Molecular Biology, 44: 357-384. Saini H S (1997). Injuries to reproductive development under water stress, and their consequences for crop productivity. Journal of Crop Production, 1:137-144. Sakamoto A and Murata N (2002). The role of glycine betaine in the protection of plants from stress: clues from transgenic plants. Plant and Cell Environment, 25: 163171. Schroeder J I, Allen G J, Hugouvieux V, Kwak J M and Waner D (2001). Guard cell signal transduction. Annual Review of Plant Physiology and Plant Molecular Biology, 52: 627-658. Sikka S M and Joshi A B (1960). Breeding In: Cotton in India – A monograph. ICCC Publications, pp. 137- 335. Singh S B, Singh D and Narayanan S S (1996). Variation in Physio-morphological characters related to drought tolerance in cotton. Indian Journal of Agricultural Sciences, 66 (6): 357-359. Smirnoff N (1993). The role of active oxygen in the responses of plants to water deficit and desiccation. New Phytologist, 125: 27-58. Sotiropoulos T F (2007). Effect of NaCl and CaCl2 on growth and contents of minerals, chlorophyll, proline and sugars in the apple rootstock M4 cultured in vitro. Biologia Plantarum, 51: 177-180. Turner N C, Hearn A B, Begg J E and Constable G A (1986). Cotton (Gossypium hirsutum L.) physiological and morphological responses to water deficits and their relationship to yield. Field Crops Research, 14: 153-170. Warren C R, Livingston N J and Turpin D H (2004). Water stress decreases the transfer conductance of Douglas-fir (Pseudotsuga menziensii). Tree Physiology, 24: 971979. Werker E, Lerner H R, Weimberg R and Poljakoff Mayber A (1983). Structural changes occurring in nuclei of barley root cells in response to a combined effect of salinity and ageing. American Journal of Botany, 70: 222-225. Wilson R F, Burke J J and Quisenberry J E (1987). Plant morphological and biochemical responses to field water deficits. II. Responses of leaf glycerolipid composition in cotton. Plant Physiology, 84: 251-254. Yancey P H, Clark M E, Hand S C, Bowlus R D and Somero G N (1982). Living with water stress: evolution of osmolyte systems. Science, 217: 1214-1222.
