Effects Of Melatonin On Antioxidant And Bacteriostatic Activities Of Fresh-Cut Pears During Storage

‘Crown'pear is rich in nutrients,but operations such as peeling and cutting can lead to softening,browning and spoilage,which shortens the shelf life of fresh-cut pears.Therefore,it is urgent to find a convenient,safe and effective treatment method to maintain the fresh-cut quality of pears.Melatonin is a natural plant indoleamine hormone,which has extensive biological functions such as growth regulation and oxidation resistance.In this study,fresh-cut‘Crown'pear slices were treated with different concentrations(0,50,100,150?mol·L^-1)of exogenous melatonin.The effects of exogenous melatonin treatment on the antioxidant system,nitric oxide?NO?synthesis pathway and antibacterial activity of fresh-cut pears were investigated in order to find the optimum melatonin concentration,and also to provide a theoretical basis for the application of melatonin in fresh-cut pears.Plants maintain the balance of reactive oxygen species?ROS?through the redox system in vivo,thereby avoiding oxidative damage of plants by excessive reactive oxygen species.In this study,the effects of different treatments on antioxidant enzyme activity,ROS content and related metabolites of fresh-cut pears were detected.The results showed that exogenous melatonin inhibited the activities of peroxidase?POD?and polyphenol oxidase?PPO?,increased the L value of chromatic aberration,and reduced the degree of fruit browning of fresh-cut pears.Moreover,exogenous melatonin treatment effectively reduced the peaks of respiration and ethylene release,reduced the content of soluble solids?SSC?,delayed the senescence of fruit,and better maintained the fruit quality.Melatonin treatment increased the activities of superoxide dismutase?SOD?,catalase?CAT?and ascorbate peroxidase?APX?and enhanced the scavenging ability of fruit to ROS,thus reducing the contents of hydrogen peroxide?H₂O₂?,superoxide anion radical?O₂^-?and hydroxyl radical?·OH?in fresh-cut pear fruit.Furthermore,exogenous melatonin treatment increased the activities of glutathione reductase?GR?,dehydroascorbate reductase?DHAR?and monodehydroascorbate reductase?MDHAR?,as well as the ratio of ascorbic acid?AsA?/dehydroascorbic acid?DHA?and reduced glutathione?GSH?/oxidized glutathione?GSSG?,kept the cells in a high reductive state and enhanced the antioxidant capacity of fruits.NO is an important signaling molecule in plants and plays an important role in plant growth and stress tolerance.Therefore,we investigated the effects of melatonin treatment on the activity of nitric oxide reductase?NR?,nitric oxide synthase?NOS?,as well as the contents of nitrite and L-arginine.The results showed that there was no significant difference in the NR activity and nitrite content between melatonin treated pears and the control.However,the NOS activity and L-arginine content of melatonin treated fruits were significantly increased,which increased the endogenous NO content of fresh-cut pear.These results suggest that melatonin probably regulates the synthesis of NO mainly through the NOS pathway.A series of operations such as peeling and cutting of fruits lead to direct contact with air and juice outflow,which causes the increase of pathogenic microorganisms and the decay of fruit.Therefore,the numbers of total colonies,molds and yeasts and the resistance to Monilinia fructicola of various treated fresh-cut pears were detected.The results showed that compared with the control,melatonin treatment effectively reduced the number of total colonies,molds and yeasts in fresh-cut pear,improved the resistance of fresh-cut pear to Monilinia fructicola,and better maintained the quality of fresh-cut pears.In the above studies,100?mol·L^-1 melatonin treatment showed better effects on the preservation and bacteriostasis of fresh-cut pears than other treatments,which indicates that100?mol·L^-1 melatonin treatment is a feasible method to inhibit browning and maintain the quality of fresh-cut pears.