Mineral Depletion and Leaf Senescence in Soya Bean as Influenced by Foliar Nutrient Application During Seed Filling*

Abstract

Senescence, as judged by the time courses of leaf lamina photosynthesis, soluble protein and chlorophyll contents, was studied in relation to mineral redistribution in field-grown soya beans [Glycine max (L.) Merr.] to investigate the hypothesis that the depletion of nutrients in the leaves by the developing seeds is the cause of soya bean senescence. A mineral nutrient solution was applied to the canopy during the seed-filling period, and the effects on senescence and mineral depletion of the leaves were determined in three cultivars, at two leaf positions, weekly from beginning of seed filling through physiological maturity. The onset of senescence occurred shortly after the beginning of rapid seed filling. Photosynthetic rate declined about 60 per cent within 3 weeks. Protein dropped by 52 per cent and chlorophyll by 48 per cent over the same period. Foliar nutrient application, at a rate previously shown to give significant yield increases in soya beans, increased the concentrations of N, P and K in the leaf laminae, but tended only to delay their decline and failed to either delay the onset or alter the course of senescence. The results of this experiment seem to indicate that, under normal growth conditions, the events of senescence in the soya bean are not causally related to the N, P or K concentrations of the leaf laminae.