PHOSPHORUS-NUTRITION INFLUENCE ON LEAF SENESCENCE IN SOYBEAN

Remobilization of mineral nutrients from leaves to reproductive structures is a possible regulatory factor in leaf senescence. The relationship between P remobilization from leaves of soybean (Glycine max [L.] Merr. cv McCall) during reproductive development and leaf senescence was determined by utilizing soil P treatments that supplied deficient, optimum, and supraoptimum soil P levels. The soil P treatments simulated field conditions, being initiated at the time of planting with no subsequent addition or removal of P. It was hypothesized that P deficiency would accelerate leaf senescence and that supraoptimum P nutrition would delay the timing or rate of leaf senescence relative to plants grown with optimum P. Supraoptimum soil P led to a two-to fourfold increase in leaf P concentration compared with optimum P, and during senescence there was no net P remobilization from leaves for this treatment. Leaf P concentration was similar for plants grown at optimum or deficient soil P, and there was significant net P remobilization from leaves of both treatments in one of the two experiments. As indicated by changes in leaf N, carbon dioxide exchange rate, ribulose 1,5-bisphosphate carboxylase/oxygenase activity, and chlorophyll concentration, leaf senescence patterns were similar for all soil P treatments. Thus, it can be concluded that leaf senescence was not affected by either P deficiency or enhanced leaf P concentration resulting from supraoptimum soil P. The results suggest that P nutrition in general, and specifically P remobilization from leaves, does not exert any regulatory control on the process of leaf senescence.