A metabolic comparison between progressive and monocarpic senescence of soybean

The onset of senescence, defined as the time that photosynthesis began to decline irreversibly, was compared with ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39) (RuBPCase) activity, chlorophyll content, protein content, and leaf diffusive resistance in outdoor-grown soybean (Glycine max (L.) Merr. cv. Amsoy 71) leaves that had emerged and senesced during different stages of plant ontogeny. The purpose was to compare metabolic events in soybean leaves associated with progressive and monocarpic senescence. Soluble protein, chlorophyll, and RuBPCase activity declined coincidently with photosynthesis in all leaves. In leaves that emerged before flowering, the decline in RuBPCase activity could be accounted for by protein degradation, whereas in a leaf that emerged after flowering had begun, the decline also was attributable to, but to a much lesser extent, a change in specific activity. All parameters declined more rapidly in leaves at nodes higher on the plant. There was an indication that developing pods temporarily retarded the rate of decline in photosynthesis. Regression of photosynthesis on chlorophyll, protein, and RuBPCase activity yielded coefficients that were different in a leaf that developed during the vegetative period from those of leaves that developed later, whereas no differences were detected among the later leaves. We conclude from our results that the factors associated with senescence are similar among leaves that senesce during different stages of plant development. But there is evidence that the rate of senescence may be differentially regulated.