Abstract
The effect of pod and seed development on leaf chlorophyll concentration, and on activities of leaf ribulose bisphosphate carboxylase, leaf nitrate reductase, and root nodule acetylene reduction were studied in field-grown soybean (G. max [L.] Merr.). Two genetic male-sterile lines and their fertile counterparts (Williams and Clark 63) were compared in both 1978 and 1979. Two additional lines (Wells .times. Beeson and Wells .times. Corsoy) were compared in 1979. The expression of male-sterile character was nearly complete as very little outcrossing due to insect pollinators was observed. Male-sterile plants showed a delayed late season decline in leaf chlorophyll content and ribulose bisphosphate carboxylase activity when compared with fertile plants. A slight delay in the loss of in vivo leaf nitrate reductase activity was also observed for male-sterile plants. Root nodule fresh weight and acetylene reduction activity declined slightly more rapidly for fertile lines than for male-sterile lines in both years with differences significant on the last 2 to 3 sampling dates as leaf loss occurred in the control plants. Seed development increased slightly the rate of decline of metabolic activity in fertile lines compared with that of male-sterile lines. Pod development was not an a priori requirement for leaf and root nodule senescence. Male-sterile plants also lost photosynthetic and N metabolic competence, but at a slower rate. Pod and seed development apparently does not signal monocarpic senescence per se but rather affects the rate at which senescence occurs after flowering.