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.
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