Effect of vapor pressure deficit on photosynthesis of rice leaves with reference to light and CO2 utilization efficiency.

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In our previous studies on diurnal courses of photosynthesis, transpiration and diffusive conductance of the leaf in the rice plants grown in the paddy field, it was recoginized that the photosynthetic rate decreased with the decreasse of diffusive conductance due to high vapor pressure deficit under sufficient light intensity in the fine midday. It has been pointed out that water stress affects photosynthetic rate through the decrease of both CO2 supply to mesophyll through stomata and photosynthetic activity in mesophyll. This study was conducted to examine whether high vapor pressure deficit reduced photosynthetic activity by measuring light and CO2 utilization efficiencies at low quantum flux density and CO2 concentration under different vapor pressure deficit, respectively. The results were as follows : 1) Transpiration rate increased rapidly, and photosynthetic rate, diffusive conductance and intercellular CO2 concentration (Ci) decreased gradually with changing vapor pressure deficit from low to high (Figs. 1 and 2). 2) Light utilization efficiency, which was shown by the degree of inclination of light dependent straight line of photosynthesis under low quantum flux density, decreased under high vapor pressure deficit (Figs. 3 and 4). And also CO2 utilization efficiency, shown by the degree of inclination of the straight line expressing the relation between photosynthetic rate and low intercellular CO2 concentration, decreased under high vapor pressure deficit (Figs. 5, 6 and 7). As light and CO2 utilization efficiencies were thought to indicate photochemical and carboxylation activity of photosynthesis, respectively, it was suggested that not only diffusive conductance but also photosynthetic activity related to photochemical and carboxylation activities were affected by high vapor pressure deficit. 3) Intercellular CO2 concentration decreased with the decrease of photosynthetic rate and diffusive conductance under high vapor pressure deficit (Fig. 1). Furthermore, it was found that the degree of inclination of the straight line, expressing the relation between ambient CO2 and intercellular CO2 concentrations, was smaller under high vapor pressure deficit than that under low vapor pressure deficit (Fig. 9). These facts indicated that the decreasing of diffusive conductance was more effective on the decrease of photosynthesis under high vapor pressure deficit than the decreasing of photosynthetic activity. 4) The three following equations were obtained on the basis of the relation between intercellular CO2 concentration and photosynthetic rate (Fig. 6) and on the fact that transpiration rate was proportional to diffusive conductance under constant vapor pressure deficit. In this case, GAASTRA's concept was also considered. P=(44.99×C)/(C+0.377) Ci=(350×C+18.86)/(C+0.377) T=3.43×C The figure (Fig.8) drawn by using these equations showed that the relations between diffusive conductance (C) and photosynthetic rate (P) and between diffusive conductance and intercellular CO2 concentration (Ci) were hyperbollic, while the relation between diffusive conductance and transpiration rate (T) was linear. Thus, if diffusive conductance were decreased without decreasing the photosynthetic activity in mesophyll, water use efficiency (photosynthetic rate to transpiration rate ratio) could be increased.