Abstract: Apple fruits（Malus domestica‘Fuji’）bagged for three months were removed bags in vitro. Bag-removing included two ways, both outer bag and inner bag removed at a time, and outer bag removed, after 3 d then inner bag removed. The fruits were exposed to sunlight and the sunburn development was studied. The results showed that the fruits removed outer and inner bags, appeared the symptom of sunburn on the first day exposed to sunlight, and their epidemic cells were disorganized and chloroplasts were disaggregated. The fruits removed outer bag then inner bag after 3 d became red without sunburn, and some chloroplasts in epidemic cell were converted chromoplasts. The no-bag-removed fruits were heath, and their cells kept intact. It is suggested that retaining red bag could avoid sunburn from sudden strong sunlight which resulted in cell membrane injured.

Key words: apple, bag-removing, sunburn, fruit peel, ultra-microstructure