a b s t r a c t
Mild intermittent drought is one of the major factors of yield reduction in rainfed rice. To minimize the
yield reduction, the mechanisms that maintain photosynthetic activity play a crucial role for sufficient
plant growth. The photoinhibition of photosystem II (PSII) depends on the balance between the rate
of energy absorption and the rate of energy dissipation, including the energy flux for photosynthesis.
Photoinhibition decreases the photosynthetic rate and restricts the primary production of the plant. In
contrast, PSII photoinhibition can be regarded as a redox control mechanism that alleviates the excess
light energy. In this study, the genotypic variation of photoinhibition in rice was examined in relation to
the adaptability of photosynthesis to the fluctuating soil water conditions. 20 rice genotypes were grown
under flooded and aerobic conditions, and the midday photoinhibition was evaluated by measuring the
maximum quantum yield of PSII (Fv/Fm, a parameter of chlorophyll fluorescence). The larger decrease in
Fv/Fm under aerobic condition than flooded condition was thought to be caused by the photosynthetic
depletion derived from stomatal closure. The decrease of Fv/Fm occurred concomitantly with the increase
of non-photochemical quenching (NPQ). Enhanced photoinhibition and NPQ might oxidize the redox state
of the plastoquinone (PQ) pool under the aerobic condition to the same level as that under the flooded
condition. The genotypic variation of the Fv/Fm and energy partitioning were affected little by the mild soil
water deficiency of the aerobic condition. The genotypic variation of the midday Fv/Fm was dependent on
the decreasing rate of Fv/Fm against an increase in midday light intensity. Some genotypes that showed
more severe midday photoinhibition tended to have a lower rate of reduction of the shoot dry weight
under the aerobic condition. These findings suggest that photoinhibition is a preferable trait for energy
control in PSII and for photosynthesis under mild soil water deficiency.