Changes in photosynthetic carbon fixation processes were comparatively studied in soybean leaves and pods harvested at different growth stages. Newly fully expanded leaves exhibited both the highest 14CO2 assimilation rate and the maximum ribulose bisphosphate carboxylase levels. Amino acids biosynthesis was more important in young tissues and this result agreed with the evolution of nitrate reductase activities. The radiocarbon distribution in glycine and serine suggested that photorespiratory metabolism increases with leaf age; the activity of glycolate oxidase was found to be significantly lower in younger leaves than in mature ones. Net photosynthesis, chlorophyll content, and ribulose bisphosphate carboxylase activity were low in isolated pods compared to leaves. However, the study of photosynthate translocations within the pod revealed that the pod wall could contribute to the carbon nutrition of the seeds. Soluble compounds labelled after 14CO2 incorporation and glycolate oxidase activity measurement indicated that organic acids biosynthesis and photorespiratory metabolism are relatively higher in pods than in leaves.

Changes in soluble proteins and Rubisco (E.C.4.1.1.39) contents were examined in leaves of nitrogen-deprived and nitrogen-sufficient soybeans. Rubisco content was very responsive to nitrogen stress, and this protein appeared to be the largest source of mobilizable nitrogen in the senescent leaf. Loss of soluble proteins and Rubisco was associated with a decrease in the activities of several proteolytic enzymes measured using artificial substrates: carboxypeptidase, aminopeptidase and haemoglobinase.The in vitro activity of enzyme(s) which can degrade Rubisco was investigated using endogenous Rubisco and in vitro radiolabelled Rubisco as substrates. Highest endopeptidic cleavage of endogenous Rubisco occured at pH 4; the enzyme responsible for this breakdown appeared to be a sulfhydryl-dependent proteinase. In contrast, [(14)C] Rubisco was attacked preferentially at pH 9, by a peptide hydrolase sensitive to EDTA. No increase in Rubisco-degrading activities was detected in nitrogen-deficient soybean leaves compared to control plant leaves.

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