Diagnostic indices of zinc deficiency in tropical legumes

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Abstract
The effect of 0.005 ppm, 0.01 ppm, 0.05 ppm and 0.10 ppm zinc on the growth and chemical composition of Desmodium uncinatum Jacq. cv. Silverleaf; Macroptilium lathyroides L.; Lablab purpureus; and Glycine max. L. cv. Wills were studied in solution culture in a controlled environment room. Dry matter production of tops of all species was reduced at the lower zinc treatments while that of roots was unaffected by zinc supply. Zinc concentrations of leaves from three plant positions did not provide a reliable index of zinc nutrition. The lowest zinc treatments often produced greater leaf concentrations of zinc than did higher zinc treatments. The cytoplasmic fractions of the leaf tissue contained the highest zinc concentrations. Cell walls, nuclei, chloroplasts and mitochondria contained extremely low concentrations, and the data were generally variable, reflecting the poor reproducibility of the techniques used. Phosphorus concentrations in leaves were generally increased at the low zinc treatments, an effect not entirely attributable to the reduction in dry matter production. The proportion of inorganic phosphorus increased and that of organic phosphorus decreased at the lower zinc treatments. Lipoid and residual phosphorus fractions were not affected consistently by zinc treatments. Leaves of plants receiving the lowest zinc treatment generally had significantly lower nitrogen concentrations than those receiving the higher zinc treatments. In another experiment, in which Glycine wightii was substituted for G. max., RN‐ase activity increased as the zinc supply was reduced and this was accompanied by a decrease in protein concentration. The data show that RN‐ase activity could be a useful diagnostic index of incipient zinc deficiency.