Controlled sand and solution culture methods (Hewitt, 1966) came handy in supplying the micronutrients at varying levels through defined changes in nutrient solution. However, the conventional nutrient solutions have a drawback in that these solutions include some nutrients, including the micronutrients, at levels far in excess of their concentrations normally found in soil solution. Several variations and improvements have been made to make the solution culture method more realistic by maintaining, throughout the growth period, the concentrations of the nutrient elements in the nutrient culture close to that encountered in the soil solution (Asher and Edwards, 1983; Parker and Norvell, 1999). The flowing solution technique developed by Edwards and Asher (1974) offers major advantages over the use of the static solution culture methods by providing the control of root temperature, pH and ion concentrations. But in spite of these added advantage; the flowing solution culture method has not found much use in working out the critical concentration limits of micronutrients because of practical difficulties in manipulating the supply of the micronutrients to desired levels, particularly in the deficiency range. The limitation of supplying nutrients at a relatively high concentration in the culture solution for maintaining their adequacy over a long periods of time may be largely overcome by the use of the buffered nutrient solutions (Parker and Norvell, 1999). The difficulty in case of the cationic micronutrients (Fe, Mn, Cu, Zn) has been overcome by the use of buffered solutions that contain synthetic metal chelators, such as DTPA, to 'buffer' the excess free cation activities in the nanomolar range. The formation constant of the metal chelate complex, the excess of the chelator in the nutrient solution, and the composition of the nutrient solution control the activity of the micronutrients in the nutrient solution to desired levels. Chelated buffered nutrient solutions have been developed for the cationic micronutrients—Fe, Mn, Cu, Zn (Chaney et al. 1989; Bell et al. 1991; Parker 1993,1997; Welch and Norvell, 1993; Yang et al. 1994). Asad et al. (1997) have developed a buffered nutrient solution for boron, wherein a boron specific resin is used for complexing with boric acid. Mixing appropriate quantities of boron-loaded and boron-free resin can supply boron at different levels. Advances in solution culture techniques, including micronutrient-buffered solution, have been reviewed by Parker and Norvell (1999). The buffered micronutrient solutions are currently preferred over the conventional culture solution for screening plant genotypes for tolerance to micronutrient deficiencies and toxicities.

The method of plant analysis for assessing nutrient status in terms of deficiency, sufficiency and toxicity is based on the well-established relationships between nutrient supply and uptake and between nutrient concentration and growth or dry matter production of plants. Within limits, there is a linear and/or exponential relationship between nutrient supply and nutrient accumulation in plants. A plot of nutrient concentration versus plant yield (dry matter production) shows two points of inflection. First, yield increases with nutrient concentration and then it slows down to form a plateau. Second, yield declines with further increase in nutrient concentration. The former corresponds to critical deficiency limit and the latter to critical toxicity limit. These critical limits are best worked out by growing plants under controlled culture conditions in greenhouses at known levels of micronutrients, ensuring that no other nutrient becomes limiting. Plants are grown over a wide range of nutrient supply, increasing several folds starting from a minimum, sustaining early vegetative growth, to excess. At well-identified stages of active growth, samples are drawn for determining biomass and tissue concentration of nutrients in selected plant parts, generally the leaves that develop the first symptoms of the deficiency. The biomass yield, relative to the maximum obtained under the experimental conditions (relative yield) is plotted against the nutrient concentration in plants grown with varying levels of nutrient supply. Generally, increase in the nutrient supply is associated with increase in leaf tissue concentration and biomass yield until the yield reaches a maximum. Increase in nutrient supply beyond this leads to increment in tissue concentration of the nutrient but not the plant yield. Beyond a certain limit, increase in nutrient supply causes increase in nutrient concentration concomitant with yield decrement.

The nutrient concentration in plants corresponding to optimal biomass yield ±10% is taken as the nutrient sufficiency range (NSR). In this range, yield response to change in nutrient supply/concentration is limited to <10%. Nutrient concentrations corresponding to 90% of the optimal yield in the sub-and supra-optimal ranges are taken as the critical concentration for deficiency (CCD) and toxicity (CCT), respectively. Extent of the decrease in the tissue concentration in the sub-optimal range and increase in the concentration in the supra-optimal range denotes the severity of deficiency

Table 10,3. Dry matter yield arid leaf tissue concentration of manganese in maize (Zea mays L.) grown with varying levels of manganese supply1

Mn supply (fi moles L"1)

Dry weight yield (g plant1)

Relative yield (% maximum)

Mn-concentration (fig g'dry weight)

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