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Changes in Photosynthesis, Ribulose Bisphosphate Carboxylase, Proteolytic Activity, and Ultrastructure of Soybean Leaves during Senescence¹

Changes in photosynthesis, rlbulose bisphosphate carboxylase (RuBPCase), and proteolytic activity were followed in the leaves of field-grown soybeans [Glycine max (L.) Merr. cv. Kent] from flowering through senescence. These parameters were followed in relation to changes in leaf resistance, chlorophyll, protein, starch, total N levels, and seed development. In addition, changes in leaf ultrastructure were observed. The initial symptoms of senescence (evident 3 to 4 weeks after flowering) were a decline in photosynthesis, chlorophyll, and total leaf N and an increase in proteolytic activity. Preceding these changes there was a swelling of the chloroplasts and a disorientation of the chloroplast lamellae, possibly resulting from the apparent increase in starch deposition. Also, large numbers of osmiophilic granules appeared within the chloroplasts. These changes were evident prior to the time the seed entered its most rapid period of growth which was 4 to 7 weeks after flowering, The initial decline in photosynthesis did not appear to be due to an increase in leaf resistance or a decline in RuBPCase activity or level. The decline in protein levels began between 5 and 6 weeks after flowering and was paralleled by the decline in carboxylase activity and level. Associated with these changes were an increase in the size of the osmiophilic granules within the chloroplasts, a decrease in the number of chloroplasts with a corresponding increase in the apparent cellular breakdown products, and a dissolution of the vacuoles. No large increase in leaf resistance or change in specific activity of carboxylase was observed until late in senescence.

Key Words: Glycine max L. Merr. • Leaf resistance • Nitrogen • Protein • Chlorophyll • Starch

² Research scientists, E. I. du Pont de Nemours and Co., Inc., Wilmington, DE 19898.

Received for publication September 4, 1979.

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