Communication mechanisms between root and shoot in the reduction of photosynthesis during senescence in crop plants under water stress

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In order to clarify the mechanism of the reduction in photosynthesis during senescence under water stress with reference to root-shoot relations, we investigated photosynthetic rate, content of ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco), which is, a key enzyme of the rate of photosynthesis, and the expression of Rubisco genes during senescence under various conditions.1. There were close correlations among Rubisco content and the abundances of rbcL and rbcS (genes encoding the large subunit and small subunit of Rubisco, respectively) mRNA of a leaf during the reduction of photosynthetic rate due to senescence.2. Partitioning of nitrogen decreased to leaves and increased to roots under deficient soil moisture conditions, causing the reduction of leaf nitrogen content and, therefore, photosynthetic rate.3. There were close correlations among nitrogen content and the abundances of rbcL and rbcS mRNA of a leaf during senescence and the nitrogen topdressing increased their abundances.4. When plants were treated with exogenous cytokinin, nitrogen content and the abundances of rbcL and rbcS mRNA of a leaf increased without the increase in total nitrogen content of a plant, suggesting that cytokinin might increase the partitioning of nitrogen to leaves.5. The activity of cytokinin transported from root to shoot decreased in plants grown under deficient soil moisture conditions.From these results, it was suggested that one of the causal factors in the significant reduction of photosynthetic rate under deficient soil moisture conditions might be the reduction of leaf nitrogen content due to smaller partitioning of leaf nitrogen to leaves and that the levels of cytokinin transported from root to shoot might affect the leaf nitrogen content by changing the nitrogen partitioning.