when a plant enters into reproductive phase following the ...... Future Strategies ..... Sims. JR and Bingham. FT (1968). Soil Sci. Soc. Am. Proc. 32(3): 364-369.
Agric. Rev.. 26 (3) • 173 - 186. 2005
BORON NUTRITION IN CITRUS - CURRENT STATUS AND FUTURE STRATEGIES - A REVIEW A.K. Srivastava and Shyam Singh National Research Centre for Citrus. Amravati Road. Nagpur - 440010. India
ABSTRACT Growing scale of B-deficiency, second to Zn-deficiency, has imparted a greater significance to B-amendment. An adequate B-amendment ensures not only ample fruit set, but guarantees optimum fruit yield with excellent fruit quality in terms of juice content, ratio between total soluble solids and acidity, and fruit peel colour. A critical analysis of work accrued in the past from worldover using citrus as a test crop suggested, has revealed a variety of soil-plant test values, fit to diagnose Bconstraints in citrus orchards established under varying edaphological conditions. Large variation in B-application through soil and foliar spray showed that both the methods of fertilization are equally efficient in meeting crop B-requirement. However, the combination of two methods offers a better scope to raise a nutrient constraint free production.
Boron is one of the most important micronutrients, requires to be researched more elaborately to have answer to many evasive issues associated with B-nutrition. This becomes even more imperative following emerging B- deficiency from both acidic as well as alkaline soils irrespective of their mineralogy. Citrus trees in this regard, is no exception. The resultant widening gap between B-containing fertilizers applied and rpmoved by the plant and fruits annuaily takes an alarming shape when a plant enters into reproductive phase following the long juvenility period of citrus which varies from 4-5 years in budded plants to as high as 8-9 years in seedling plants. Next to zinc, boron deficiency is wide spread in many soils, especially in northeast India and to a lesser extent in other citrus belts like Marathawada and Vidarbha region of Maharashra, northwest and south India leading to low crop yields. Of the 36825 surface samples analyzed from length and breadth of India, 33% soil samples were found to be deficient in available boron. Extensive deficiency (39-68%) has been recorded in red and lateritic soils and leached acidic soils of hot semi-arid ecoregion with shallow and medium black soils of, hot sub-humid ecoregion with alluvium derived soils, hot sub-humid to
humid (inclusion of perhumid) ecoregion with alluvium derived soils, warm perhumid ecoregion with brown and red hill soils and warm perhumid ecoregion with red and lateritic soils and highly calcareous soils of hot subhumid ecoregion with alluvium derived soils and warm sub-humid to humid (Sakal. 2001; Tiwari, 2002a). These are the combinations of soil and climate where best quality citrus is commercially produced in other countries like Brazil, China, Japan etc. as well. Role of B-nutrition in maintaining sustained fruit yield and quality are now increasingly observed. The essentiality of boron for higher plants was demonstrated as early as in 1910 (Agulhon, 1910). Since then, a number of reports have appeared ascribing numerous plant physiological roles to boron. The earlier literature reviewed by Gauch and Dugger (1953) and interpreted in the light of some of the more evidences. Boron for1']1_s complexes with certain carbohydrates blIt natural sugar-borate complexes in plants yet to be identified. However, a role of B- in flowering, pollen tube growth, N-metabolism, hormonal activity. and maintenance of Ca in soluble form is well established (Srivastava and Singh, 2003a). It is one such essential nutrient which occurs as a non-ionized molecule in soil
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solution and plant root absorbs B as boric acid most efficiently. Alt and Schwarz (1973) observed that B is..§bsorbed as a molecule\and is passively distributed with transpiration stream to different plant organs. An analysis on chronological developments took place in various aspects of B-nutrition with citrus as perennial test crop, was systematically reviewed to harness the potential benefits towards quality citrus production accruing out of B-fertilization. Diagnosis of B-Constraints A number of methods are frequently used to identify B-constraints in citrus orchards. These are deficiency symptoms developed on foliage or fruit, leaf analysis, soil analysis and biochemical markers. Each one of them has its own merits or demerits. Morphological deficiency symptor.1s As early as Haas and Klotz (1931) described the boron deficiency symptoms exhibited by experimen,al citrus plants growing under controlled nutrient conditions, characterised by the mature leaves turning thickened and brittle, with downward curling at right angles to the midrib and are yellowish to bronze in colour. The veins are enlarged and corky: and split on the upper surface. Shoots from multiple buds produce a cabbage head type of growth. Splits developed in the bark of the internodes through which amber coloured gum oozed and woody tissues are exposed. A marked reduction of growth and in a few instances, death of the tree may occur. In most instances. the symptoms disappeared and the trees recovered fully in two years' time follOWing the B-fertilization. Morris (1937. 1938) later reported for the first occurrence of hard fruit disease in citrus in addition of splitting and corking of leaf veins. In citrus, boron deficiency usually results in gum-soaked spots in the albedo of the fruit, lumpiness and a hard, dry fruit, prone to drop off V0maturely (Smith and Reuther. 1949).
Many authors have concluded that the high concentration of sugars in the leaves of boron B-deficient plants, is due to a breakdown of the conduction tissues with a concomitant reduction in translocation. Most of these studies were conducted on plants showing morphological symptoms. There is some evidence (Gauch and Dugger. 1953; Mitchell et ai., 1953) that boron deficiency has an effect on the translocation of sugars, and possibly other organic compounds, before vascular dearrangement occurs. It is reported that boron deficient plants loose their capacity to respond to gravity. This would indicate that there is some relation between boron and the production. translocation. or action of hormones. Mitchell et al. (1953) found that 2, 4-D and other growth modifying substances are more rapidly translocated when boron and sugars are applied. A decrease in the translocation of various compounds could be involved in the appearance of boron deficiency , symptoms but, as with hormones. the effect of boron on the translocation of carbohydrates may be the main effect. Opitz and Platt (1967) described symptoms of B-deficiency in citrus orchards of central California, USA. Symptoms of B deficiency were more prominent during fruit maturation. when fruit colour changes from green to orange which is delayed in the areas having gummy granules (Primo et al.. 1969). The symptoms included development of gummy granules in the fruit albedo and exceptionally coarse leaf venation. Due to Bdeficiency, fruits are deshaped. with rough and thick peel and often brown spots are observed in the pulp (Feroughi et al.. 1974). Marsh GrapefrUit leaves affected by B-toxicity on the other hand are characterized by scattered yellow spots on the upper surfaces. brownish resinous gummy spots on the lower surface and edge or tip-burn (Cardenas et al.. 1971). Boron toxicity .n lemon leaves is characterized
Vol. 26, No.3, 2005
by the typical tip burn. The base of the burned leaf tip is in a direction at right angles to the midrib. The yellowing that precedes the burning, progresses from the leaf margins down toward the midrib. The green area along the lateral and midrib vems is the most resistant to the increasing B- concentration (Haas, 1950). Chiu and Chang (1985) described Bdeficiency on fruits which consists of smaller fruit size, thicker peel, less juice, and harder thanrqature fruits. Lumpiness in fruits is considered a primary symptom of B-deficiency. Sometimes, the fruits may become hard, abnormal in shape and small in size. Older fruits would be abnormal in shape with very thick albedo containing gum deposits. Seeds fail to develop and gum deposits are also found around the axis of fruits. Affected fruits are hard, dry and low in sugar and their quality is poor. The symptoms on fruits are more reliable as hard fruit and dry due to lumps in the rind caused by gum impregnations (Chiu and Chang, 1986; Zekri, 1995). In the orchard, premature wilting of the trees occur in spite of sufficient moisture in the soil. Subsequently, small water soaked spots or flecks appear on the leaves which become translucent as the leaves mature. Premature shedding results in severe defoliation, dieback of trees which assume a bushy upright growth similar to that of Zn-deficiency. The conspicuous curling of leaves at right angle to the midrib occurs due to enlargement, splitting and curling of the veins. The fruits develop gum spots both the albedo and flesh. These spots give rise to lumps under the peel which can usually be felt with hand (Randhawa and Srivastava, 1986).
in
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dwarfed and stunted plants. The flower development and seed production are usually impaired or lacking. The death of terminal growing points of the mc.in stem coupled with thickened leaves with tendency of leaves to curl downward are noticeable features of Bdeficiency (Zekri, 1995). Leaf analysis According to Bould (1984), the leaf analysis is based on four factors viz., i. leaf is the principal site for the plant metabolism; ii. changes in the nutrient supply are reflected in the composition of leaf; iii. the changes are more pronounced at certain stages of development than at the others; and iv. the concentrations of the nutrient in leaf at a specific growth stage are related to the performance of the crop. Leaf analysis is of the major importance in a number of ways which include: i. an aid to understand the internal functions of nutrients in the crop, ii. confirms the deficiency detected by visual symptoms, iii. distinguishes between two nutrients which cause similar deficiency or toxicity symptoms, iv. identifies the mineral imbalance in the absence of visible symptoms. which is not correctable with the addition of a simple nutrient, v. helps in investigating the toxicity of the elements, vi. identify the interactions or antagonism between the nutrients, vii. used for the diagnostic purposes in cases of the simultaneous multiple nutrient deficiencies and/or toxicities, viii .. ascertains. whether the applied nutrients have entered in the plant system or not, ix. identifies, whether the hidden hunger is causing suboptimal plant performance, slow growth, and lower yield and/or quality or not, x. prevents the deficiencies rather than correcting them after they develop, and xi. helps in locating the areas of the incipient deficiencies during the nutritional survey.
Terminal necrotic leaves shed prematurely with internodes of terminal shoots shortened, usually rosetting with apical meristems blacken and die, coupled with general breakdown of meristimatic tissue, and With reference to plant analysis, when roots turning short, stubby, resulting in for the purpose of diagnosis a static criterion
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is applied, this cannot be final as the level of the element varies throughout the cycle and even during the same day, according to the time when the sample is taken. Moreover, the philosophy of conventional leaf analysis was originally based on two starting points. It is based on the concept of the total content of a nutrient at a given moment, as an integral value in the feeding of the said element closely linked with the law of the minimum, and from which the existence of a positive link between the said content and the yield is deduced (nutrition production curve). It is also based on the concept of equilibrium, according to which nutrition is not the result of individual actions of nutrients but of their total quantity and state of equilibrium. A number of methods are being employed to develop leaf nutrient gUide which include the sand culture technique, crop modelling after generating the data through an extensive survey of citrus orchards and field experimentation. However, all these methods are under constant scrutiny, test, and recurrent use. Leaf B standards: A number of methods are being employed to develop leaf nutrient guide, which principally include the sand culture technique. crop modelling after generating the data through an extensive survey of citrus orchards and field experimentation. The first step to develop a comprehensive leaf nutrient guide is the categorisation of leaf analysis data. A number of recommendations from across the various citrus belts of world have been suggested for various diagnostic value of boron (Table 1). Soil analysis Soil tests are based on the approach which describes the amount of nutrient in a definite chemical form viz., i. water soluble, ii. exchangeable, iii. chelated or complexed form. iv. secondary clay minerals or oxides, and v. primary minerals. The available B. therefore.
does not reflect its total content in soil. Chapman (1968) described a number of soil conditions leading to possible B-deficiency which consist of i. acid soils regardless of parent material having B excessively leached out. ii. )soils inherently low in B, especially those derived from igneous and metamorphic sandstone rocks, iii. alkaline soils though potentially rich in total B but low in available B, iv. laterite soils with lower silica and high Fe and Mn, v. acid muck or peat soils, and vi. soils with low clay content. The soil analysis method rests on the assumptions that roots would extract nutrients from the soil in a manner comparable to chemical soil extractants, and that there is a simple direct relation between the extractable concentration of nutrients in the soil and uptake by plants. (Jones et al., 1955). Optimum value of boron are in citrus orchards from across the continent has been suggested to be 0-50-75 mg kg 1 hot water soluble B in USA. China and Japan (Bingham and Martin, 1956; Sato et al., 1962; Bingham 1973; Quyang et al., 1984; Swietlik, 1996), 1.0 mg kg'! in Egypt (Elseewi arid ElMalky, 1977) and 0.32-0.48 mg kg'! in India (Srivastava and Singh, 2003b). Chapman and Kelly (1943) observed that granitic soil containing upto 10mg kg'! total B showed Bdeficiency. Biochemical analysis The variable activity of phenylalanine ammonia lyase in different plant species was variable, thus, indicated differential susceptibility of the test plants subjected to boron deficiency (Shkol'nik et al.. 1980). Sakal and Singh (1995) explained the role of B in cell differentiation and development, germination and growth of pollen grains. sugar translocation through formation of sugarborate complex, movement of growth regulators. and lignin synthesis through polymerisation of phenolic compounds., Boron acts by forming a strong.
Country USA South Africa USA Australia Brazil Brazil China Italy Italy Italy USA USA USA USA India
Citrus sinensis Osbeck cv.Navel Citrus sinensis Osbeck cv. Navel Citrus sinensis Osbeck cv. Valencia Sweet orange (Citrus sinensis Osbeck) Citrus sinensis Osbeck cv. Valencia Sweet orange (Citrus sinensis Osbeck) Sweet orange (Citrus sinensis Osbeck) Citrus reticulata Blanco cv. Clementine Citrus sinensis Osbeck cv. Sanguine Citrus sinensis Osbeck cv. Maoroclat Citrus sinensis Osbeck cv. Valencia Citrus sinensis Osbeck cv. Valencia Citrus sinensis Osbeck cv. Valencia Citrus sinensis Osbeck cv. Valencia Citrus sinensis Osbeck Mosambi
160-260 100-250 160-260 130-250 >150
50-150 30-100 50-150 31-129 36-100 35-68 20-59 60-85 37-55 60-85 50-200 36-100 31-100 36-100 24.9-356
21-40 15-30 21-40 20-30
