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

Senescing leaves are a source organ of both carbon and nitrogen but, becaus e degradation of chloroplast proteins and export of their degradation produ cts to sink organs give rise to loss of the photosynthetic capacity, the le aves serve as the source of nitrogen only at the cost of their source capac ity for carbon. Changes in the photosynthetic capacity and levels of protei ns in leaves at the top four positions of two cultivars of rice, Nipponbare and Akenohoshi, during the ripening stage were investigated taking the tra de-off relationship between the two source capacities into consideration. R ate of light-saturated photosynthesis (Pmax) in leaves decreased more rapid ly in Nipponbare than in Akenohoshi after heading. Various proteins were al so degraded during senescence, with Nipponbare leaves showing faster loss o f proteins than the corresponding leaves of Akenohoshi, Decline in Pmax was correlated, similarly in the two cultivars, with losses of ribulose-1,5-bi sphosphate carboxylase/oxygenase, soluble proteins, chlorophyll a that boun d to the reaction center complexes of the two photosystems, the activity of whole-chain electron transport, and a major part of insoluble proteins dur ing senescence. The results suggest that degradation of proteins during sen escence of rice leaves is coordinated so as to enable the leaves to perform photosynthesis with a high use efficiency of protein and export nitrogen a t a low or nearly minimum cost of the source capacity for carbon.