Shade Nets Modulate Photosynthetic Balance and Antioxidant Homeostasis to Improve Fruit Quality in Ponkan

To clarify the effects of shade-net covering on sunburn control and fruit quality of ponkan, white, gray, black, and blue nets with differing light transmittance were applied to assess the canopy microenvironment, leaf photosynthesis, peel antioxidant system, and fruit quality. All shading treatments significantly reduced fruit surface temperature and light intensity compared with the unshaded control. Light transmittance was significantly higher under white (26.49%) and blue (25.69%) nets than under gray (12.97%) and black (12.58%) nets. Consequently, sunburn incidence decreased significantly from 31.41% in the control to 5.47-11.13% under shading. Regarding leaf photosynthesis, white and blue nets maintained a significantly higher net photosynthetic rate (P_n) and PSII photochemical efficiency than gray and black nets, whereas gray and black nets showed no significant difference in P_n from the control. All shading treatments significantly lowered reactive oxygen species (ROS) and malondialdehyde (MDA) levels, alleviating peel oxidative injury. By contrast, the sunburn peel of the unshaded control accumulated high levels of ROS and MDA and activated SOD, POD, CAT, and the AsA-GSH cycle, thereby elevating total antioxidant capacity (T-AOC). At fruit maturity, shading generally improved juice yield and nutritional quality. White and gray nets enhanced soluble solids content (SSC), white and blue nets improved the sugar-acid ratio, and gray, black, and blue nets increased vitamin C content, while the white net showed the best performance in soluble sugar accumulation. These results indicate that white and blue shade nets are more effective in optimizing the canopy microenvironment of ponkan. Shading followed by post-shading light recovery alleviates fruit sunburn and enhances fruit quality.