Responses of photosystem II and antioxidative systems to high light and high temperature co-stress in wheat

In the past few decades, extreme weather conditions with high temperature and long light exposure increased dramatically, which significantly affect the photosynthesis of plants. In this study, the impacts of heat and high-light co-stress on Photosystem II (PSII) and antioxidant defense system were investigated in wheat (Triticum aestivum L.). Under heat and high-light co-stress, chlorophyll content, relative water content, and photochemical activity of PSII decreased. For rapid responses (1 h of the co-stress), levels of reactive oxygen species (ROS), activities of six antioxidant enzymes, and protein levels of Lhcb5 (CP26) and Lhcb6 (CP24) increased significantly. However, the rapid induction of CP26 and CP24 did not be found under single high light or heat stress. In addition, the strong phosphorylation of Lhcb4 (CP29) and D2 proteins and an obvious disassembly of the PSII-LHCII supercomplexes were found under the cross-stress for 1 h. However for the late responses (3 h of co-stress), among the six antioxidant enzymes, only the activities of APX and GPX significantly increased. Meanwhile, the photosynthetic activity and the level of D1 and PsbQ significantly decreased after 3 h of co-stress. The remarkable decrease in PsbQ was unique under high light and heat co-stress. Therefore, we concluded that high light and heat co-stress had synergetic effect and PSII had the rapid and strong changes under co-stress. Taken together, the results show that regulations to PSII-LHCII complex structures and their phosphorylation statuses may be important for alleviating photo-damages to chloroplasts under high light and heat co-stress in wheat.