Photosystem II photochemistry, photoinhibition, and the xanthophyll cycle in heat-stressed rice leaves

To investigate how high light affects the responses of photosynthesis to heat stress, the effects of high temperature (25–42.5 °C) either in the dark or in the light (1000 μmol m−2 s−1) on photosystem II (PSII) photochemistry and the xanthophyll cycle were investigated in rice plants. At temperatures higher than 35 °C, there was a decrease in the CO2 assimilation rate, and this decrease was greater in the light than in the dark. The maximal efficiency of PSII photochemistry (Fv/Fm) showed no significant change in the dark, but did show a significant decrease in the light. In addition, there was an increase in non-photochemical quenching (NPQ) and this increase was greater in the light than in the dark. Furthermore, the de-epoxidation status of the xanthophyll cycle increased significantly with increasing temperature in the light. Compared to the control leaves, the dithiothreitol-fed leaves showed a greater decrease in Fv/Fm but a very small increase in NPQ and de-epoxidation status of the xanthophyll cycle at temperatures higher than 35 °C. On the other hand, the ascorbate-fed leaves showed less of a decrease in Fv/Fm but a greater increase in NPQ and the de-epoxidation status of the xanthophyll cycle. Ascorbate peroxidase and glutathione reductase activities in leaves and chloroplasts were enhanced and this enhancement was greater in the light than in the dark. Heat stress had no significant effect on the contents of ascorbate and glutathione in leaves and chloroplasts in the dark, but led to an increase in the contents of reduced ascorbate and glutathione in leaves and chloroplasts in the light at the temperatures higher than 35 °C. Our results suggest that the xanthophyll cycle plays an important role in protecting PSII against heat-induced photoinhibition by an increase in the ascorbate pool in the chloroplast.