PROTECTIVE ROLES OF D1 PROTEIN TURNOVER AND THE XANTHOPHYLL CYCLE IN TOMATO (SOLANUM LYCOPERSICUM) UNDER SUB-HIGH TEMPERATURE AND HIGH LIGHT STRESS

• D1 turnover plays a more important role than xanthophyll cycle in photoprotection under sub-high temperature and high light (HH) conditions in tomato. • D1 protein turnover and xanthophyll cycle avoided the accumulation of excess energy and photooxidative damage of photosystem through maintaining photosynthetic pigments content and inducing changes in NPQ core components. • D1 protein turnover and xanthophyll cycle can also avoided accumulation of ROS and membrane lipid peroxidation by maintaining the activity of the ROS scavenging system. D1 protein turnover and the xanthophyll cycle (XC) are important photoprotective mechanisms in plants that operate under adverse conditions. Here, streptomycin sulfate (SM) and dithiothreitol (DTT) were used in tomato plants as inhibitors of D1 protein turnover and XC to elucidate their photoprotective impacts under sub-high temperature and high light conditions (HH, 35°C, 1000 µmol·m^-2·s^-1). SM and DTT treatments significantly reduced the net photosynthetic rate, apparent quantum efficiency, maximum photochemical efficiency, and potential activity of photosystem II, leading to photoinhibition and a decline in plant biomass under HH. The increase in reactive oxygen species levels resulted in thylakoid membrane lipid peroxidation. In addition, there were increased non-photochemical quenching and decreased chlorophyll pigments in SM and DTT application, causing an inhibition of D1 protein production at both transcriptional and translational levels. Overall, inhibition of D1 turnover caused greater photoinhibition than XC inhibition. Additionally, the recovery levels of most photosynthesis indicators in DTT-treated plants were higher than in SM-treated plants. These findings support the view that D1 turnover has a more important role than XC in photoprotection in tomato under HH conditions.