Response of leaf photosynthesis to vapor pressure difference in rice (Oryza sativa L) varieties in relation to stomatal and leaf internal conductance

In the afternoon when air humidity decreases, leaf photosynthetic rate (P_n) often declines in rice grown under irrigated conditions. To clarify the genotypic difference of P_n in response to humidity, we measured P_n and stomatal conductance (g_s) for nine rice varieties with diverse genetic backgrounds, at various vapor pressure differences (VPD) and developmental stages. P_n and g_s of all the varieties decreased with VPD increase from 1.0 to 2.3 kPa of VPD. The variety with high g^s at low VPD exhibited a greater decline of g ^s with VPD increase than the variety with low g^s, but cv. Takanari showed the highest g^s under altered VPD conditions. Significant logarithmic relations were found between the decreased P_n and g_s at the respective developmental stages, suggesting that g_s is the dominant factor determining P_n and its response to VPD change. To explicate the effect of decreased g_s on P _n, we analyzed the relations by using the model that accurately estimated the genotypic difference in P_n at a low VPD with g _s and leaf nitrogen content per unit leaf area in the previous study. The model assuming that leaf internal conductance (g_w) remains unchanged well explained the decreased P_n at high VPDs by g _s change alone. The analysis also suggested the constancy of g _w and carboxylation capacity at high VPD. It is concluded that the genotypic difference in the decrease of P_n at a high VPD is brought mainly by that in decreased g_s, and the varieties with a high g _s always exhibit a high P_n owing to their relatively high g_s at either high or low VPD environments.