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1 35 DAS, top 3 leaves

1 35 DAS, top 3 leaves

Fig. 10.1. Growth response of maize to varying levels of manganese supply: Arranged

Fig. 10.2. ['lot of relative yield of maize (Zcti iruiys L.} plants grown at varying levels of manganese supply against [issue concentration« in young (top three) leaves, CCD and CCT denote critical concentration for deficiency and toxicity, respectively.

Mn Concentration (pg g' dry weight)

Fig. 10.2. ['lot of relative yield of maize (Zcti iruiys L.} plants grown at varying levels of manganese supply against [issue concentration« in young (top three) leaves, CCD and CCT denote critical concentration for deficiency and toxicity, respectively.

or toxicity. Plant tissue analysis, thus, helps to quantify the extent of deficiency or toxicity and predicted response to fertilizer amendment.

10.3.1 Iron Deficiency

Plant analysis forms a poor basis for evaluation and management of iron deficiency. Plants with about the same or even higher concentration of iron as in apparently healthy plants may exhibit chlorosis, a feature referred to as 'Fe-chlorosis paradox' (Romheld, 2000). There may be more than one reason for this discrepancy. The field-drawn samples may contain substantial quantities of dust adhering to the leaf surfaces and this may, if not removed prior to determination of tissue iron, contribute varying amounts of iron to the leaf tissue concentration, (Jones, 1992). Removal of surface contamination of dust has been a subject of several investigations. Washing of leaf material with water is of little advantage. A quick wash with a dilute solution of a detergent (0.1%) along with or followed by dilute hydrochloric acid (0.1 m HC1) may prove effective in removal of surface contamination of iron. It has, however, been pointed out that washing of samples may remove a part of endogenous iron, manganese and zinc (Moraghan, 1991).

Even in plants grown under controlled conditions, where dust is not a major problem, iron concentration of plants may show a poor correlation with the level of iron supply (Agarwala and Sharma, 1961). Notwithstanding this, several workers have reported critical concentration values for iron deficiency. These values range from 50 to 150 ¡ig Fe g"1 dry weight. In general, critical limits are not of much use in categorizing plants in terms of deficiency and sufficiency or responsiveness to iron amendments. Smith et al. (1984) made a comparative study of iron requirement of some C3 and C4 plants and found that even though the C4 plants have a higher requirement of iron than the C. plants, the critical deficiency concentration of iron in the two types of plants is about the some 72 jig g1 dry weight). Tissue concentration of iron in plants receiving iron at different levels of supply may show dissimilar diurnal variations (Sharma and Mehrotra, 1998), which may limit the value of iron concentration as a dependable measure of iron status of plants. Differences in within- the-plant distribution and compartmentalization of iron in normal and iron-deficient plants also limits the suitability of tissue concentration of iron as a measure of iron status of plants.

Over 70 years ago, Oserkowsky (1933) studied the relationship between chlorophyll content and iron concentration in green and chlorotic pear leaves and suggested that the chlorotic leaves accumulate a large proportion of iron that is physiologically 'inactive'. Ever since then, efforts have been on for developing methods for determining what Somers and Shive (1942) termed 'active iron'. These methods involve extraction of iron from leaf tissues in dilute acids or iron-chelates (Katyal and Sharma, 1980; Mehrotra et al. 1985) and finding one which correlates best with chlorophyll concentration. Some workers suggested quantification of Fe(II) iron in plant extracts as a measure of active iron (Katyal and Sharma, 1980; Olsen et al. 1982), but this could be misleading because, during extraction, varying amounts of Fe(III) may be reduced to Fe(II), both photochemically (Krizek et al. 1982) and chemically (Mehrotra and Gupta, 1990). Mohammad et al. (1998) described nitric acid and o-phenanthroline extractable iron in citrus lemon leaves as a measure of active iron that could be used for diagnosis of iron chlorosis. The presently known methods for quantification of active iron have, however, not found to be of much use because of limited information on what constitutes this fraction in terms of the oxidation state of iron and its compartmentalization,

10.3.2 Manganese Deficiency

Manganese concentration of plants forms a suitable basis for evaluation of manganese deficiency and prediction of plant responsiveness to manganese fertilization. Compared to other micronutrients, the range of critical deficiency concentration of manganese in plants is narrow. For most crops, the critical deficiency concentrations range between 10 and 20 fig Mn g1 dry weight.

10.3.3 Copper Deficiency

Copper concentration of plants is lower than for the other cationic micronutrients and difference between its deficiency and sufficiency values is narrow. The critical concentration for copper deficiency is within 1 to 5 ¡1 g g"1 dry weight (Robson and Reuter, 1981). The copper concentration of young leaves is a more sensitive indicator of copper status than the old leaves (Hill et al. 1979; Loneregan et al. 1981). The critical deficiency limits of copper may also be influenced by the level of nitrogen supply (Thiel and Fink, 1973).

10.3.4 Zinc Deficiency

Zinc concentration in leaves has been widely used as a measure of zinc-nutrient status of plants. Several workers have worked out critical zinc concentrations in a wide variety of crops. In most plants, the critical deficiency concentration of zinc ranges between 15 and 20 pig g"1 dry weight. (Robson and Reuter, 1981, Sharma, 1996). However, reports suggest poor correlation between the total concentration of zinc and its physiological availability (Leece, 1978; Ghoneim and Bussler, 1980; Gibson and Leece, 1981; Cakmak and Marschner, 1987, 1993). Rahimi and Shropp, 1984, showed that in several plants of different taxa, water-soluble zinc in leaves served a better indicator of zinc nutrient status than its total concentration.

Similar observations were made by Cakmak and Marsehner (1987), particularly in the case of phosphorus-induced zinc deficiency. Limitation in use of total zinc concentration as a measure of functionaly available zinc may be caused by binding of free ionic zinc to cell walls (Youngdahl et al. 1977), an organic ligand or change in apoplast-symplast distribution of zinc.

10.3.5 Molybdenum Deficiency

Deficiency values and critical concentrations for molybdenum deficiency for a wide variety of crop plants have been presented by Gupta (1997c) and Johnson et al. (1997). Generally, the deficiency values range between 0.01 and 0.5 ng Mo g"1 dry weight. The critical concentration for molybdenum deficiency shows wide variations. Apart from genotypic differences in molybdenum efficiency, these differences may be caused by differences In the functional requirement of molybdenum. For example, plants that are supplied nitrogen as nitrate require molybdenum for its reduction, which is not the case when nitrogen is supplied in the reduced form (NH4). Likewise, nodulating legumes require more molybdenum for N2 fixation than the non-nodulating plants. In nodulating legumes, accumulation of relatively large proportion of molybdenum in root nodules may also affect its concentration in top parts. Reported differences in the deficiency limits of molybdenum for some plants can possibly be attributed to differences in the stage of growth or plant part sampled, and even to the different methods used for quantification of molybdenum (Gupta, 1997c). Presently, there is lack of an accurate, easy-to-follow method for measurement of molybdenum concentration at levels, which may be critical for deficiency.

10.3.6 Boron Deficiency

In keeping with large differences in boron requirement of plants, the critical concentrations for boron deficiency also show a wide range. In graminaceous plants, the critical deficiency values generally range from 5 to 10 //g B g"1 dry weight. In the gum-bearing plants such as poppy, the critical deficiency values of boron may be higher (Bergmann, 1992). Kirk and Loneragan (1988) suggested that the relationship between tissue concentration of boron and rate of elongation of the youngest leaf may serve a better basis for working out the critical limits for boron deficiency than the dry matter yield of plants.

10.3.7 Chlorine Deficiency

Chloride content of plants usually varies between 2 and 20 mg g4 dry weight. This is many times higher than for any other micronutrient. Tissue concentration of chlorine at which deficiency symptoms are observed varies between 0,1 and 5 mg g"' dry weight (Xu et al. 2000). Such low concentrations of chlorine can be supplied through rainfall. Since tissue concentration of chlorine in field-grown crops is far in excess of this, deficiency concentrations of chlorine have little relevance.

10,3,8 Tissue Tests

Tissue testing is a rapid method for determining tissue concentration of micronutrients in a field or in laboratories. It involves a semi-quantitative test of plant sap for one or two nutrients. Fresh tissue samples drawn from a selected plant part are put to a semi-quantitative test for a nutrient using rapid colorimetric methods. Such tests have a limited value for evaluating micronutrient status of plants, possibly excepting iron (Chaney, 1984). Studies described under Part I show a relation between accumulation of the micronutrients in the seeds and their concentration in the vegetative parts (leaves) of the mother plants. This relationship forms the basis of the evaluation of plant micronutrient status on the basis of seed analysis. Rashid and Fox (1992) have found zinc analyses of seeds as a suitable method for evaluation of zinc requirement of grain crops. Information about the seed content of micronutrients is of much use in predicting plant performance during the seedling growth, particularly on marginal soils, but its use as a measure of plant nutrient status for diagnostic purpose has limited value.

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