| Apple fruits (Malus domestica cv.Fuji) were used as material in this study. Firstly, the changes of antioxidative system and cell ultra-structural of apple peel during sunburn development were studied. According to the symptom of sunburn development, the process of apple fruit sunburn was divided into three phases: 1.bleaching, 2.browness, and 3.necrosis. Sunburn was induced by lipid peroxidation of cell membrane and further was aggravated. In the phase of pale, the activities of protective enzymes (SOD, POD, PPO and CAT) of apple peel were increased remarkably. Correspondingly, the cell structure kept its integrity, except some organelles partly disassembled and much vesicles enriched in cytoplasm. As peel became brown, the contents of chlorogenic acid, quercetin, rutin and myricetin were increased, and the outer layer cells of epidermis were collapsed and filled up by electron dense substances, and the cell wall was thickened.Secondly, bagged fruits, which were prone to sunburn when exposed to strong sunlight, were used as material and the peel adaptation to light was studied after fruits were removed bags. The contents of UV absorbing compounds, rutin, anthocyanins and quercetin glucoside were decreased in the fruits after bagged for three months. Clorogenic acid, UV absorbing compounds, anthocyanins and rutin were accumulated quickly after bag-removed in the field. Correspondingly, electron dense substances was accumulated in the vacuole of the 1st or 2nd in epidermis and some chloroplasts were turning or turned to chromoplasts. In another trial, Detached bagged fruits were removed bags at 6:30 a.m. The activities of SOD, POD, CAT were reduced at noon in the peel of once bag-removed fruits, but UV absorbing compounds, chlorogenic acid were increased. The symptom of sunburn was observed at about 16:00 p.m..Lastly, the heat acclimation which could prevent sunburn development was adopted in the study. The peel response to natural strong sunlight was studied in the once bag-removed fruits which had been pretreated by high temperatures of 38℃ and 48℃ for two hours. The results indicated that: the activities of POD and PPO of peel were increased by high temperature, and the total antioxidant activity (TAA) of methanol extraction in the systems of scavenging DPPH radical and the system of inhibiting linoleic acid oxidant was increased. Simultaneityly, UV absorbing compounds and some phenolic compounds, such as chlorogenic acid, epicatechin and SA, were enriched in methanol extraction, but rutin was decreased. In a word, free phenolics was accumulated after high temperature treatment. When fruits pretreated by high temperature were exposed to natural strong sunlight. Higher TAA could be persisted for a short time which would make the development of fruit sunburn delayed.
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