Abstract: Mechanical damage is the main reason for the occurrence of oleocellosis in citrus fruit because it can cause the rupture of rind oil glands and the consequent symptoms of oleocellosis. Nevertheless, mechanical damage also favors the development of other rind disorders. More complicated physiological disorder symptoms appear in the orange rind after mechanical damage, which cannot be simply attributed to the development of oleocellosis. To better elucidate the initiation and progression of mechanical damage-induced orange rind disorders, the ultrastructural changes occurring in ‘Fengjie’ navel oranges during these processes were investigated by transmission electron microscopy and scanning electron microscopy. The contents of pectin, cellulose and lignin, the rate of generation of superoxide anion radicals and lipoxygenase activity were also measured. Results indicated that after mechanical damage, the integrity of the cell membrane was lost and the cells of oil glands in the flavedo underwent an apparent degradation, resulting in the rupture of oil glands, and consequently the release of orange oil and the degradation of both the flavedo and albedo. During the early stage of storage after mechanical damage, the activity of lipoxygenase and the generation rate of superoxide anion increased steeply, and the content of protopectin reduced sharply. The contents of water-soluble pectin significantly increased from Day 7 to 11 relative to the control (P < 0.05). On the whole, the results from this study can provide some insights into the cause of the ultrastructural changes occurring in navel orange rind after mechanical damage.

Key words: mechanical damage, ‘Fengjie’ navel orange, cell ultrastructure, physiological rind disorder, oil gland