Physiological and ecological characteristics of high yielding varieties in rice plants. II. Leaf photosynthetic rates.

Abstract

This study was conducted to examine the hypothesis suggested in the previous paper that the difference in the net assimilation rate between the rice cultivars Nipponbare and Akenohoshi was probably caused by the difference in the photosynthetic rates of leaves in the canopy. The difference of the maximum photosynthetic rate of the uppermost expanded leaf in a day was very small between Nipponbare and Akenohoshi. However, the extent of the decrease in the photosynthetic rate during leaf senescence and in the afternoon with sufficient light intensity during diurnal course of the photosynthesis was smaller in Akenohoshi than that in Nipponbare. Thus the photosynthetic rates were maintained at higher level near the photosynthetic capacity in Akenohoshi compared with those in Nipponbare. Furthermore the cause why these differences existed in the photosynthesis between the two varieties was examined by measuring water absorption ability through roots. The resistance to water flow through plants was smaller and the exudation rate was larger in Akenohoshi than those in Nipponbare, which was probably related to smaller top-root and leaf area-root weight ratios in Akenohoshi. It can be concluded from these results that the higher photosynthetic rate in Akenohoshi was owing to its higher water absorption ability and physiological activity in roots.