The long-term changes in midday photoinhibition in rice (Oryza sativa L.) growing under fluctuating soil water conditions

Rice crops growing under fluctuating soil water conditions in a rainfed field frequently experience severe photoinhibition at midday, potentially decreasing their biomass production. In this study, the long-term changes in midday photoinhibition in five rice cultivars growing under variable soil water conditions in a rainfed field were evaluated by determining the maximum quantum yield of photosystem II (F_v/F_m). F_v/F_m was generally lower under rainfed conditions than under flooded conditions at 65 - 75 days after sowing (DAS), but was similar under both conditions at 109 - 124 DAS. This mitigation of photoinhibition over time is likely an up-regulation of mechanisms to dissipate excess electrons, and an analysis of covariance showed that the degree of mitigation under the rainfed condition varied among the cultivars. Such genotypic differences in the long-term changes in F_v/F_m might be determined by the capacity of the cultivar to adapt to drought conditions.