Response Of Phenolic Metabolism And Other Photoprotection Mechanisms After Exposing Bagged Apple Fruit To Sunlight

The practice of fruit bagging can improve peel coloration significantly. Howerever，whenbagged apple fruits are re-exposed to sunlight after bag removal, they are vulnerable to stronglight stress and lead to the occurrence of sunburn. The present study focused on mechanismsof phenolic synthesis in apple peels regulated by sunlight, and the response of photoprotectionmechanism to sunburn after exposing the bagged apple to sunlight irradiation. These studiesmay provide an insight for improving apple color and preventing sunburn.Bagged ‘Golden Delicious’ and ‘Fuji’ apple fruits were used in this study. Phenolicsynthesis, xanthophyll cycle, the antioxidant system, and active oxygen species (AOS)production in apple peels were studied. The main results are as follows:1. During sunlight exposure, UV attenuation significantly lowered MdMYB10, MdPAL,MdCHS, MdF3H, MdDFR, MdANS and MdUFGT1expression, plasma membrane NADPHoxidase activity, superoxide anion concentrations, UDP-glycose:flavonoid3-O-glycosyltransferase (UFGT) activity, and concentrations of anthocyanin andquercetin-3-glycoside, but did not affect the expressions of MdHY5, MdCOP1and MdCRY2,and the concentration of other phenolic compounds. Exogenous application of hydrogenperoxide could enhance anthocyanin and quercetin-3-glycoside synthesis. Inhibition ofplasma membrane NADPH oxidase activity significantly increased the total production ofROS and inhibited the synthesis of anthocyanin and quercetin-3-glycoside in apple peel underdirect sunlight irradiation. In general, solar UV irradiation regulated anthocyanin andquercetin-3-glycoside synthesis in apple peel by modulating the production of ROS viaplasma membrane NADPH oxidase.2. After re-exposing bagged fruits to sunlight, the production of active oxygen speciesand the photoprotective capacity in apple peels were both significantly enhanced. As sunburnseverity increased, the concentration of hydrogen peroxide increased, while xanthophyll cyclepool size decreased. For the key genes involved in flavonoid synthesis, expressions ofMdMYB10and MdPAL were up-regulated, whereas the expressions of MdCHS, MdANS,MdFLS, and MdUFGT were down-regulated in sunburned fruit peel. Correspondingly,concentrations of quercetin-3-glycoside and cyanidin-3-galactoside decreased, while other phenolic compounds did not show any change in sunburned fruit peel. Total ascorbateconcentrations decreased as sunburn severity increased, accompanying a decrease in theactivity of L-galactono-1,4-lactone dehydrogenase, the key enzyme for ascorbate synthesis.Transcription of MdGalLDH did not change in sunburned peels. Although concentrations oftotal glutathione and activities of superoxide dismutase and ascorbate peroxidase increased,the activities of monodehydroascorbate reductase, dehydroascorbate reductase, andglutathione reductase decreased as sunburn severity increased. In general, the occurrence of‘photooxidative sunburn’ in ‘Fuji’ apples peel correlated with relatively lower xanthophyllcycle pool size, ascorbate synthesis and reduction, and flavonoid synthesis. Ascorbate mightplay a key role in the photoprotection when ‘photooxidative sunburn’ happened.