Antioxidant enzymes activities and rindstaining in ‘Navelina’ oranges as affected by storage relative humidity and ethylene conditioning

Abstract

Abstract The involvement of active oxygen detoxifying enzymes on postharvest rindstaining occurring in citrus fruit at non-chilling temperature has been investigated. Changes in the activities of superoxide reductase (SOD, EC 1.15.1.1), catalase (CAT, EC 1.11.1.6), ascorbate peroxidase (APX, EC 1.11.1.11) and glutathione reductase (GR, EC 1.6.4.2) in the flavedo of ‘Navelina’ (Citrus sinensis L. Osbeck) oranges stored at 22 °C under high (85–90%) and low (55–60%) relative humidity (RH) have been examined. In addition, the effect of conditioning the fruit for 4 days with 10 μl l−1 ethylene at 22 °C and 85–90% RH before being transferred to air at the same temperature and 65–70% RH was studied. The SOD activity increased during storage of fruit at 22 °C under low (55–60% RH) or high (85–90% RH) humidity, being the rate of increase higher in fruit kept under low humidity. By contrast GR decreased and a slower decline rate was observed in fruit stored at low RH. A significant reduction in the activities of CAT and APX also occurred in ‘Navelina’ oranges maintained at 85–90% RH, but not at 55–60% RH. Thus, fruit kept under high RH, which showed higher rindstaining, presented lower SOD, CAT, GR and APX activities than fruit stored under low RH. The incidence of this physiological disorder was reduced by the ethylene pre-treatment. No significant differences in the activities of these enzymes were found in fruit examined after 4 days of ethylene or air treatment. GR activity of fruit pre-treated with ethylene and then held under air was significantly higher than that of their respective control air-treated fruit. Conversely, CAT was higher in fruit continuously held under air and no significant differences in APX and SOD were found. These results indicate that SOD, CAT, GR and APX may play a role in the lower rindstaining incidence observed in ‘Navelina’ fruit continuously exposed to low RH, as compared with fruit held under high RH, while GR may be involved in the beneficial effect of ethylene reducing this non-chilling physiological disorder.