Effects of drought stress on the oxidation of the reaction center chlorophyll of photosystem I and grain yield in paddy-field grown rice plants

Drought is a serious abiotic stress that leads to decreased yield. In pot experiments, the reaction center chlorophyll of photosystem I (PSI) (P700) has been reported to be oxidized in response to drought stress. In the present study, we examined whether drought stress that decreases grain yield was detected by P700 oxidation in paddy-field-grown rice (Oryza sativa L.) plants. The P700 reduction and SPAD values were also evaluated. Drought stress was imposed by the prolonged mid-season drainage in 2019 and 2020. P700 oxidation was measured at saturated CO2 levels before heading. P700 oxidation was not stimulated in the 2019-drought plots. In contrast, P700 oxidation was stimulated in the 2020-drought plot. However, this phenomenon in 2020 was primarily accounted for by decreases in total leaf-N levels, as previously observed in the rice plants grown under low N availability. Rough rice yields and ratios of filled spikelets tended to decrease in the drought plots. Therefore, drought stress responsible for a decrease in grain yields could not be directly detected by P700 oxidation in paddy-field-grown rice plants, whereas the P700 oxidation responded to changes in total leaf-N levels due to the drought stress treatment. The same conclusion was drawn for the P700 reduction and SPAD value.