Plant and Cell Physiology, 1999, Vol. 40, No. 5 496-503
© 1999
Loss of the Photosynthetic Capacity and Proteins in Senescing Leaves at Top Positions of Two Cultivars of Rice in Relation to the Source Capacities of the Leaves for Carbon and Nitrogen
1 Faculty of Agriculture, Tokyo University of Agriculture and Technology Fuchu, Tokyo, 183 Japan
2 Department of Life Science, Faculty of Science, Himeji Institute of Technology Harima Science Garden City, Hyougo, 678-12 Japan
3 Department of Biology, Faculty of Science, Toho University Miyama 2-2-1, Funabashi, 274-8510 Japan
4Author for correspondence
Senescing leaves are a source organ of both carbon and nitrogen but, because degradation of chloroplast proteins and export of their degradation products to sink organs give rise to loss of the photosynthetic capacity, the leaves serve as the source of nitrogen only at the cost of their source capacity for carbon. Changes in the photosynthetic capacity and levels of proteins in leaves at the top four positions of two cultivars of rice, Nipponbare and Akenohoshi, during the ripening stage were investigated taking the trade-off relationship between the two source capacities into consideration. Rate of light-saturated photosynthesis (Pmax) in leaves decreased more rapidly in Nipponbare than in Akenohoshi after heading. Various proteins were also degraded during senescence, with Nipponbare leaves showing faster loss of proteins than the corresponding leaves of Akenohoshi. Decline in Pmax was correlated, similarly in the two cultivars, with losses of ribulose-l,5-bisphosphate carboxylase/oxygenase, soluble proteins, chlorophyll a that bound to the reaction center complexes of the two photosystems, the activity of whole-chain electron transport, and a major part of insoluble proteins during senescence. The results suggest that degradation of proteins during senescence of rice leaves is coordinated so as to enable the leaves to perform photosynthesis with a high use efficiency of protein and export nitrogen at a low or nearly minimum cost of the source capacity for carbon.
5 Present address: Section of Biology, University of California, Davis, California 95616, U.S.A.
6 Present address: Faculty of Agriculture, Utsunomiya University, 350 Mine-machi, Utsunomiya, 321 Japan.
(Received November 27, 1998; Accepted February 23, 1999)
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