A Short UV-B Exposure Stimulated Enzymatic And Nonenzymatic Antioxidants And Reduced Oxidative Stress Of Cold-stored Mangoes

Cold storage is widely used with fruit to suppress quality deterioration and extend postharvest life. However, many tropical and subtropical fruit such as mangoes are extremely sensitive to chilling injury, and this limits the maintenance of postharvest quality and reduces consumer acceptance. Many methods have been studied to reduce chilling-induced deterioration of cold-stored fruit, among which the induction of chilling tolerance is a great potential approach for protecting harvested fruit from chilling injury. In previous studies, we reported that a short exposure of UV-B before storage effectively reduced chilling injury of cold-stored mangoes, which suggests an easily applicable strategy to maintain postharvest quality of mango fruit. However, the mechanisms involved in UV-B-induced cold tolerance of mangoes are still largely unknown.The objective of this work is to assay the effects of UV-B irradiation on reactive oxygen species (ROS) levels, antioxidant compound contents, antioxidative enzyme activities, and oxidative damage of cold-stored mangoes were examined. The main mechanism of UV-B treatment in reducing chilling injury in mango. The main contents and results are as follows:Effects of UV-B irradiation reduced ROS levels and alleviated oxidative damage of the cold-stored mangoes. The effects of pre-radiated with UV-B on superoxide anion production rate, hydrogen peroxide concentration, ion leakage level and malondialdehyde content of the cold-stored fruit are examined. The results show:ion leakage levels and MDA contents of the cold-stored mangoes pre-irradiated with UV-B for were significantly lower than those of the cold-stored fruit without UV-B irradiation during ripening period,which showed that pre-exposure of mangoes with UV-B suppressed both membrane peroxidation and structural damage of the fruit. The results also indicated that H2O2 levels and O2·- production rate of the cold-stored mangoes pre-irradiated with UV-B were significantly lower than those of the cold-stored fruit without UV-B irradiation during cold storage and ripening period. The data of this work strongly suggests that a short period of UV-B irradiation before cold storage may alleviate cold-induced oxidative damage by reducing ROS levels in the fruit.Effects of UV-B on ROS generating systems of cold-stored mangoes. The data suggested that the cold-induced plasma membrane-localized NADPH oxidases and cell wall-localized peroxidases might be responsible for over-production of excess ROS in cold-stored mangoes. As shown in resuits, the activities of both enzymes in UV-B-irradiated mangoes were not significantly decreased as compared with those of the fruit without UV-B exposure during cold-storage and ripening period, which indicated that UV-B irradiation did not influence ROS generating enzymes in cold-stored mangoes. Therefore, the data demonstrated that the.UV-B-reduced ROS levels in cold-stored mangoes were not due to the suppression of ROS generation.Effects of UV-B on enzymatic antioxidants. Examined in UV-B radiation on cryopreserved fruit SOD, CAT, GPX and APX. Our results indicated that UV-B irradiation enhanced SOD and CAT activities in cold-stored mangoes. But no significant differences were observed in GPX and APX activities between UV-B treatment and control fruit. It is presumed that the UV-B-reduced ROS levels in cold-stored mangoes are at least partially due to the enhancement of SOD and CAT activities, but GPX and APX did not participate in UV-B-reduced ROS levels in cold-stored mangoes.Effects of UV-B on nonenzymatic antioxidants. Our results showed that ascorbate and polyphenols contents in UV-B-irradiated mangoes were significantly increased during cold storage. And the highly positive correlation between the decreases of ROS levels and the increases of the contents of polyphenols and ascorbate in UV-B-irradiated mangoes are examined.Effects of UV-B on total anti-oxidative capacity of cold-stored mangoes. The results indicated that UV-B radiation enhanced reducing powers of the cold-stored mangoes, and the UV-B-stimulated polyphenols and ascorbate contributed to the enhancement of reducing power in UV-B-irradiated mangoes. It also indicated that UV-B irradiation enhanced the scavenging effects of the fruit on DPPH radical, and the UV-B-stimulated polyphenols and ascorbate contributed significantly to the enhancement of DPPH scavenging capacities in the fruit.In conclusion, The UV-B-enhanced antioxidant compounds and antioxidative enzymes were highly correlated with the reduced-ROS levels in UV-B-irradiated mangoes. The data indicated that a short UV-B exposure reduced oxidative stress and alleviated oxidative damage of the cold-stored mangoes by triggering both enzymatic and nonenzymatic antioxidant systems although ROS generation in the fruit was not affected. Studies illustrate UV-B to improve the cold resistance of mango part of the mechanism, so as to provide useful information for future reference UV-B applications in the field of fresh mango, but also improve a reduction of chilling injury loss techniques.