Acclimation to Midday Photoinhibition of Rice Grown Under Upland Condition in Relation to Drought Stress.

Drought stress with high radiation enhance excess light energy in thylakoid membrane and cause generation of reactive oxygen species (ROS). Oxidative damage induced by ROS in photosynthetic cells promote water leakage and leaf desiccation. The amount of excess light energy are well associated with the degree of photoinhibition which also induced by ROS in photosystemII. Then, photoprotective mechanisms which dissipate excess light energy are thought to contribute to alleviate photoinhibition or oxidative damage under over-reductive conditions. In this study, long term changes of midday photoinhibition of rice grown under upland condition was monitored by chlorophyll fluorescence measurement, and compared with that under lowland condition. Midday depletion of maximum quantum yield of photosystemII(Fv/Fm) was highly related to received radiation in a day. In upland, the slope of regression line between Fv/Fm and radiation was changed through the experimental period that the slope was moderated, and this trend was seen in all the nine varieties used. On the other hand, the relation between Fv/Fm and radiation was almost constant in lowland. The major photoprotective mechanism of non photochemical quenching (NPQ) measured in upland did not increase although photoinhibition was mitigated. In addition to that, effective quantum yield of photosystemII (Φ_PSII) also did not changed. These results suggested that alternative energy flow was up regulated in upland to reduce excess light energy. Malondialdehyde (MDA) content in leaves of varieties which showed lower midday depletion of Fv/Fm was low under upland condition. Acclimation to photoinhibition seems to decrease generation of ROS, then the differences of Fv/Fm among varieties represented the differences of oxidative damage under drought condition.