Metabolic Changes in Senescing Soybean Leaves of Similar Plant Ontogeny¹

Abstract

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.

Photosynthesis and Other Traits in Relation to Chloroplast Number during Soybean Leaf Senescence

“…Most closely and consistently associated with this decline is the loss of RuBP carboxylase protein and activity (1 1, 14, 15, 20, 21). Uncoupled electron transport activity (5,8) and Chl content (14,15,21) also diminish during senescence, but the onset and/or rate of decline may or may not be coupled with the decline in photosynthetic rate. Two hypotheses have been advanced to explain the loss of chloroplast composition and function.…”

Changes in Photosynthesis and Other Chloroplast Traits in Lanceolate Leaflet Isoline of Soybean

“…Ontogenic and genotypic changes of leaf photosynthetic rate in soybean (Glycine max L. Merrill) have been studied, and the results indicate that photosynthetic rate is related to physiological traits such as Chl content, soluble protein content, and Rubisco2 activity of soybean leaves (2,5,9,12). Recent studies (3) have further shown that genotypic differences in leaf photosynthetic rate of soybean were mainly related to differences in chloroplast number per unit leaf area.…”

Leaf physiology and anatomy of soybean genotypes differing in leaf photosynthetic ability

“…Others have also found that PRO (Mondai et al, 1978;Secor et al, 1983) or CHL (Wittenbach, 1983) may decline later in the senescence process than photosynthesis. However, losses of RuBPCase usually coincide with decline in photosynthesis (Mondai et al, 1978;Wittenbach, 1983;Secor et al, 1983).…”

“…The filter paper and cell residue was quantitatively Ribulose 1.5-bisphosphate carboxylase Also, on the second day after TPs sampling, the remain ing discs that had been frozen in liquid N were ground in 5 mL of ice-cold, grinding buffer (Secor et al,, 1983) with a small mortar and pestle. The extract was centrifuged at 4200 X g for 20 minutes.…”

“…The extract was centrifuged at 4200 X g for 20 minutes. Aliquots were removed from the supernatant for determination of soluble protein (by the modified Bradford method) and RuBP carboxylase activity (Secor et al, 1983). RuBP carboxylase activity per unit leaf area (RuBPCase) was calculated as RuBP carboxylase activity per unit soluble protein multiplied by PRO,…”