[Changes in antioxidative system and cell ultrastructure in the fruit peels of apple during sunburn development]

Fruits from 6-year-old apple trees (Malus domestica Borkh cv. Fuji) were used as materials to test the changes of antioxidative system and cell ultrastructure of fruit peels during sunburn development. Fruit sunburn appeared in mid July, 2002. The process of apple fruit sunburn was divided into three phases (0 degrees : control): 1 degrees : bleaching, 2 degrees : brownness, and 3 degrees : necrosis. Fuji apple fruits in different sunburn state were picked. Phenolic compounds, membrane protective enzymes (SOD, POD, PPO, CAT), cell membrane lipid peroxidation and cell ultrastructure in fruit peel were studied. The result showed that the degree of cell membrane lipid peroxidation enhanced along with development of sunburn. The activity of membrane protective enzymes also increased remarkably. However, the cell structure kept its integrity, except some organelles which partly disassembled, and cytoplasm and vacuoles became enriched with electron-dense substances while fruit peels became pale. As peel became brown, chlorogenic acid, quercetin, rutin and myricetin accumulated, and cells of outer layers of the epidermis collapsed correspondingly, cell wall became thicker. It is suggested that changes in both cell ultrastructure and antioxidative system confirm that physiological state of fruit peels becomes disordered during sunburn development.