Abstract
The effect of leaf water potential on the photosynthetic rate and diffusive conductance in the leaf of rice plants was studied at different levels of leaf-air vapor pressure difference. The photosynthetic rate and diffusive conductance decreased rapidly to 50-60% and 25-40% of its maximum with decreasing leaf water potential from -1.5 to -5 bar, respectively. However, they slowly declined when leaf water potential decreased to lower than -5 bar (Fig. 1). Furthermore, the photosynthetic rate and diffusive conductance in leaves with high leaf water potential decreased with increasing leaf-air vapor pressure difference (Fig. 3) ; air humidity had a direct effect on the photosynthetic rate and diffusive conductance, independent of leaf water potential when leaf water potential was high (Figs. 3 and 4, Table 1). When leaf water potential decreased to lower than -5 bar, the photosynthetic rate and diffusive conductance were not significantly affected by the increase in leaf-air vapor pressure difference (Fig. 3). These results and the relation of diffusive conductance to photosynthetic rate at different leaf water potentials and leaf-air vapor pressure difference (Figs. 2 and 5) suggested that the cause for the decrease in photosynthetic rate differed in leaves with high and low leaf water potentials. In leaves with high leaf water potential where air humidity had a direct effect on the photosynthetic rate and diffusive conductance, the photosynthetic rate decreased due to the decrease of CO2 supply through stomata to mesophyll. On the other hand, in leaves with low leaf water potential where the diffusive conductance was considerably low, the photosynthetic rate decreased due to the decrease of photosynthetic activity in mesophyll in addition to the decrease in the CO2 supply (Fig.6).
