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Metabolic Changes in Senescing Soybean Leaves of Similar Plant Ontogeny¹

Delaying the onset or reducing the rate of leaf senescence may be one means of increasing crop productivity. But before senescence can be controlled, it must first be characterized. We compared the onset of leaf senescence, defined as the time at which apparent photosynthesis began to decline irreversibly, with ribulose-l,5-bisphosphate carboxylase (RuBPCase) activity, chlorophyll content, total protein content, leaf diffusive resistance, and dark respiration in outdoorgrown soybean [Glycine max (L.) Merr. ‘Amsoy 71’] leaves that had emerged during plant reproduction. We sought to determine which parameter(s) was associated with the onset of senescence. Apparent photosynthesis, as estimated by infrared gas analysis, began to decline about 85 days after planting in leaves at both nodes 12 and 15. There was no change in diffusive resistance, protein content, or dark respiration until about 3 weeks later. On the other hand, RuBPCase activity and chlorophyll content declined concomitantly with photosynthesis. This pattern was similar in both leaves. Thus, we conclude that RuBPCase activity and chlorophyll were best associated with the onset of senescence and that a similar mechanism seems to be operating in both leaves.

Key Words: Photosynthesis • Aging • Ribulosebisposphate carboxylase • Chlorophyll • Leaf resistance • Leaf protein • Respiration • Glycine max

² Graduate research assistant (currently research associate, Dep. of Agronomy, Univ. of Wisconsin, Madison, WI 53706), professor, Dep. of Agronomy, and professor, Dep. of Botany, respectively, Iowa State Univ., Ames, IA 50011.

Received for publication April 23, 1982.