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Shinichi Sawada, Satoshi Enomoto, Takaaki Tozu, Minobu Kasai, Regulation of the Activity of Ribulose-1,5-Bisphosphate Carboxylase in Response to Changes in the Photosynthetic Source-Sink Balance in Intact Soybean Plants, Plant and Cell Physiology, Volume 36, Issue 4, June 1995, Pages 551–556, https://doi.org/10.1093/oxfordjournals.pcp.a078793
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Abstract
Sink-limited conditions cause a reduction in the rate of photosynthetic fixation of CO2 in single-rooted soybean leaves (Glycine max. Merr.). We suggested previously that this reduction is due to the deactivation of ribulose-1,5-bisphosphate carboxylase (RuBPcase; EC 4.1.1.39) that is caused by a decrease in the level of free Pi via a decrease in the rate of conversion of phosphorylated intermediates to the end-product (sucrose) in sink-limited leaves [Sawada et al. (1989) Plant Cell Physiol. 30: 691, Sawada et al. (1990) Plant Cell Physiol. 31: 697, Sawada et al. (1992) Plant Cell Physiol. 33: 943]. In the present study, we investigated whether, in intact soybean plants, sink-limited conditions would also cause a reduction in the rate of photosynthesis and whether such a reduction might be due to the deactivation of RuBPcase via the same regulatory mechanism as that suggested from previous studies with single-rooted leaves. Continuous removal of flower buds from intact plants brought a large decrease in ratio of the dry weight of sink organs (stem, roots, pods) to source organ (leaves) as a result of the absence of pod formation. Pods are likely to function as the major sink at the reproductive stage. Upon continuous removal of flower buds, the treated (sink-limited) plants showed a large decrease in the rate of photosynthetic fixation of CO2 as compared to control plants. RuBPcase in the leaves of treated plants was continuously inactivated with the decrease in photosynthetic activity. However, the inactivated enzyme was totally reactivated upon incubation in the presence of 10 mM NaHCO3 and 5 mM MgCl2. The levels of sucrose and ribulose-1,5-bisphosphate in leaves of the treated plants increased significantly. All these results coincide exactly with those obtained in previous studies of single-rooted leaves under the sink-limited conditions.
