Remobilization of mineral nutrients from leaves to reproductive struct
ures is a possible regulatory factor in leaf senescence. The relations
hip between P remobilization from leaves of soybean (Glycine max [L.]
Merr. cv McCall) during reproductive development and leaf senescence w
as determined by utilizing soil P treatments that supplied deficient,
optimum, and supraoptimum soil P levels. The soil P treatments simulat
ed field conditions, being initiated at the time of planting with no s
ubsequent addition or removal of P. It was hypothesized that P deficie
ncy 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 inc
rease in leaf P concentration compared with optimum P, and during sene
scence there was no net P remobilization from leaves for this treatmen
t. Leaf P concentration was similar for plants grown at optimum or def
icient soil P, and there was significant net P remobilization from lea
ves of both treatments in one of the two experiments. As indicated by
changes in leaf N, carbon dioxide exchange rate, ribulose 1,5-bisphosp
hate carboxylase/oxygenase activity, and chlorophyll concentration, le
af senescence patterns were similar for all soil P treatments. Thus, i
t can be concluded that leaf senescence was not affected by either P d
eficiency or enhanced leaf P concentration resulting from supraoptimum
soil P. The results suggest that P nutrition in general, and specific
ally P remobilization from leaves, does not exert any regulatory contr
ol on the process of leaf senescence.