Effects of Air Humidity on the Photosynthetic Rate in the Leaf of the Rice Plant

Abstract

It was suggested in the previous paper that the increase of photosynthetic rate owing to higher nitrogen content appears remarkably in case with little water stress, and that the decrease of photosynthetic rate due to the decrease of water absorption appears remarkably under the condition which brings on intense transpiration. The present study was conducted to ascertain the suggestion by investigating the effects of humidity or leaf-air vapour pressure deficit (LAVPD) on the photosynthetic rate of leaves in rice plants under different conditions. Transpiration rate increased with the increase of LAVPD and was practically constant under the condition of more than 11 mmHg LAVPD, while photosynthetic rate, diffusive conductance and photosynthetic rate/transpiration rate ratio (water use efficiency) decreased with the increase of LAVPD (Fig. 3). Photosynthetic rate of leaves with higher nitrogen was much higher in smaller LAVPD, but it was not so much different due to nitrogen content in larger LAVPD because the depression rate of photosynthetic rate was higher in leaves with higher nitrogen content (Figs. 1 and 4). Leaf nitrogen content was more effective for increasing photosynthetic rate in smaller LAVPD (Fig. 7). In rice plants with low root activity induced by application of soluble starch with additional ammonium sulfate to soil, and with low root-top ratio induced by shading and high humidity, the photosynthetic rate decreased severely with the increase of LAVPD (Figs. 5 and 6). From these results it can be considered that the increase of leaf nitrogen content is not sufficient for increasing photosynthetic rate in a day, and that in addition to the increase of leaf nitrogen content the increase of root activity and the promotion of root system development by improving soil condition of root zone are essential for increasing the daily total of photosynthesis (Fig. 8).