Cytokinin effects on ribulose-1,5-bisphosphate carboxylase/oxygenase and nitrogen partitioning in rice during ripening

High yield rice (Oryza sativa L.) cultivar Akenohoshi can realize a higher rate of photosynthesis during ripening by maintaining a higher level of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) than cv. Nipponbare. To clarify the causal factors for the difference in the levels of Rubisco in leaves between rice cultivars and the mechanisms of action of cytokinins on the maintenance of high levels of Rubisco, in leaves, the level of rbcL and rbcS transcripts and the nitrogen content of leaves were investigated. The Rubisco content remained high not only in plants given additional nitrogen fertilizer (NF) but also in plants sprayed with 6-benzylaminopurine (BA). There was a close correlation between the Rubisco, content and the level of rbcL and rbcS mRNAs. The maintenance of high leaf levels of Rubisco and of rbcL and rbcS mRNAs was related to the high nitrogen content in leaves, irrespective of the treatments and cultivars. The maintenance of high leaf nitrogen content was caused by an increase in nitrogen partitioning to leaves and not by an increase in nitrogen absorption by plants treated with BA. These results indicate that there are two probable effects of BA on the leaf level of Rubisco. One is the direct effect of BA inducing the synthesis of Rubisco. The other is the indirect effect of BA inducing the effective partitioning of nitrogen to leaves. Cytokinins could account for the difference in the reduction in leaf level of Rubisco, during senescence between the rice cultivars.