Abstract
Soon after attaining full expansion, soybean (Glycine max [L.] Merr.) leaves enter a senescence phase marked by decline in photosynthetic rate and the progressive loss of chloroplast activity and composition. Our primary goal was to determine if this loss could be accounted for by sequential degradation of whole chloroplasts or by simultaneous degeneration of all chloroplasts. Total photosynthesis (TPs) measured as (14)CO(2) uptake, chloroplast number, ribulose 1,5-bisphosphate carboxylase activity, uncoupled photosynthetic electron transport activity, soluble protein content, and chlorophyll content declined progressively during the 37 days after full leaf expansion. During this period, chloroplast number per unit leaf area was constant for all genotypes studied. We conclude that leaf senescence may be a two-stage process wherein the first stage chloroplast activity and composition declines, but chloroplast numbers do not change. During a brief terminal stage (11 days in our experiment), whole chloroplasts may be lost as well. As a second objective we wished to determine if variation in single-leaf total photosynthetic rate among soybean cultivars is related to corresponding variation in chloroplast number and/or chloroplast activity/composition. By comparing the means for three cultivars known to have rapid leaf TPs and for the three known to have slow TPs, we found the former group to be superior to the latter for all the previously mentioned leaf physiological traits. This superiority was related primarily to differences in chloroplast number and only secondarily to differences in activity and composition per chloroplast.