Poster: Water Relations

Abs # 218: Causal factors of the reduction in the rate of photosynthesis during senescence in rice plants under water stress

Presenter:	Hirasawa, Tadashi , hirasawa@cc.tuat.ac.jp
Authors	Hirasawa, Tadashi  (A)   Ozaki, Koichi  (A)   Fujiki, Hiroko  (A)   Ookawa, Taiichiro  (A)
Affiliations:	(A): Faculty of Agriculture, Tokyo University of Agriculture and Technology

Decrease in soil moisture accelerates the reduction in the rate of leaf photosynthesis during senescence. We investigated causal factors of the reduction in the rate of photosynthesis in rice plants (Oryza sativa L. cv. Nipponbare). Plants were grown in vermiculite under submerged conditions with a culture solution in an environment-controlled chamber. At the early ripening stage, plants were grown in vermiculite with the moisture content of about 150% (control) and the content at which the rate of evapotranspiration decreased to about 40 % of the control (water stress). Water potential of a leaf was kept lower by 0.3 to 0.6 MPa in the water-stressed plants than in the control plants during the treatments. The reductions in leaf nitrogen content and the rate of leaf photosynthesis were remarkable in the water stressed plants compared with the control plants. There was a close correlation between the content of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and the rate of photosynthesis in plants subjected to both treatments. Nitrogen derived from Rubisco accounted for the amount of the reduction in leaf nitrogen. The nitrogen content of roots increased and it was estimated that the increased nitrogen in roots was mainly from leaves. Cytokinin activity in root exudates decreased significantly in the water-stressed plants compared with the control plants. When a 10^-4M solution of 6-benzylaminopurine (BA) was sprayed on the water-stressed plants, leaf nitrogen contents were kept high, compared with the plants sprayed with water. The reduction of cytokinins transported from root to shoot might be one of the causal factors of the reduction in the rate of photosynthesis during senescence in the plants under water stress.