Coordinated photo-protective and antioxidant systems alleviate sunburn injury in ponkan fruit under intense light and high-temperature stress

Sunburn, a physiological disorder caused by intense light and high-temperature stress, significantly reduces the yield and quality of ponkan fruit. However, the coordinated roles of photo-protective mechanisms and antioxidative defense systems in mitigating sunburn damage under such stress remain poorly understood. In this study, transcriptome analysis revealed that in mild and moderate sunburn peels, the expression of carotenoid biosynthesis genes ( CsPSY , CsCRTISO , CsLCYB , CsHYB ), particularly those associated with the xanthophyll cycle ( CsVDE ), were up-regulated, while carotenoid degradation genes ( CsCCD , CsNCED ) were down-regulated. Consequently, 15 carotenoids (including 12 carotenoid fatty acid esters) showed significant accumulation. Notably, the xanthophyll cycle pool size increased dramatically in mild sunburn peel. Concurrently, reactive oxygen species (ROS), including superoxide anion (O 2 ·ˉ) and hydrogen peroxide (H 2 O 2 ), accumulated, accompanied by the up-regulation of antioxidative genes ( CsSOD , CsGPX , CsAPX , CsMDHAR ). This enhanced the activity of related antioxidative enzymes and the efficiency of the ascorbate to glutathione (AsA-GSH) cycle in sunburn peels. Furthermore, levels of ascorbate (AsA) and glutathione (GSH), and redox ratios [AsA/(AsA+DHA) and GSH/(GSH+GSSG)] increased substantially in sunburn peels. Interestingly, mild and moderate sunburn symptoms were reversible after 20 days of shading with a black net, whereas severe sunburn resulted in irreversible cell browning and necrosis. These findings suggest that the xanthophyll cycle, carotenoid biosynthesis, and AsA-GSH cycle act synergistically to alleviate oxidative stress in ponkan fruit peel under intense light and high-temperature conditions. This study offers critical insights into photo-protective and antioxidative defense mechanisms and provides a foundation for breeding sunburn-tolerant ponkan cultivars in the future.