Metabolic changes in senescing soybean leaves
Ontogenetic patterns of several metabolic parameters were studied in leaves of outdoor-grown soybean Glycine max (L.) Merr. cv. 'Amsoy 71' plants to determine which parameters were associated with the onset of leaf senescence, and to compare parameters among leaves at different nodes; i.e., of different plant ontogeny. The onset of leaf senescence was defined as the time that leaf photosynthesis began to decline irreversibly;Changes in dark respiration and leaf diffusive resistance to water were not associated with the onset and subsequent decline in leaf photosynthesis (CER), as measured in situ by infrared gas analysis, in any of the leaves sampled. Chlorophyll content (CHL) and, more so, ribulose 1,5-bisphosphate carboxylase (E.C. 220.127.116.11) (RuBPCase) activity were related well with leaf senescence in all leaves sampled, but that the decline in RuBPCase was caused by proteolysis in some leaves and by both proteolysis and kinetic changes in others;Leaves that emerged during reproduction had similar relationships of CER with CHL and RuBPCase, whereas the relationships differed among leaves of different plant ontogeny. The relationships differed in three ways. First, leaves that emerged and abscised before pod-filling senesced more slowly than leaves that emerged later. Second, in leaves that emerged after pod-filling had begun, the decline in RuBPCase was attributable to decline in both amount and kinetics, but in leaves that emerged earlier, the decline was attributable solely to a decline in amount of RuBPCase. Third, the relationships of CER with CHL and RuBPCase activity on a leaf area basis were different in leaves that emerged before flowering compared to leaves that emerged after plant flowering had begun;There was evidence that the appearance of pods delayed the rapid decline of CER;Because similar parameters are associated with leaf senescence in leaves of different plant ontogeny, but that their relationships with leaf senescence differ, we conclude that a common mechanism exists for leaf senescence, but that it may be regulated differently among leaves of different plant ontogeny.