Effect Of Postharvest Melatonin Treatment On Softening Of Mango Fruit At Low Temperature

Mango（Mangifera indica）is an important tropical fruit with a delicate taste,pleasant aroma,and high nutritional value.In addition,mango has become the world’s second most planted tropical fruit,with a large consumer market.However,mangoes are climacteric fruits prone to postharvest decay and deterioration,which has become a key factor limiting the development of the mango industry and postharvest storage.In this study,two mango cultivars（’Guiqi’and’Tainong 1’）from Guizhou Province were treated with 1000μmol·L^-1 exogenous melatonin（MT）to investigate the effects of MT on fruit quality,cell wall metabolism,and reactive oxygen metabolism of mangoes under low-temperature conditions.In addition,the effects of MT on these parameters of the two cultivars of mangoes were investigated under low-temperature conditions to provide a theoretical basis and technical reference for the storage and preservation of mangoes.The main results obtained were as follows:1.Melatonin treatment can effectively delay fruit weight loss,inhibit respiration rate,maintain high fruit firmness,and reduce fruit decay rate compared to control treatment.However,melatonin does not significantly affect both mango cultivars’total soluble solids,titratable acidity,and sugar:acidity ratio.Moreover,melatonin treatment had a significant cultivar effect under low temperatures,and’Guiqi’shows better storability than’Tainong 1’.2.Melatonin treatment was found to significantly increase the content of cell wall polysaccharides（pectin,cellulose,and hemicellulose）in mango peel and inhibit the activity of cell wall degrading enzymes（polygalacturonase（PG）,pectin methyl esterase（PME）,cellulase（Cx）andβ-glucosidase（β-glu））.Moreover,the transcriptional expression of cell wall degrading enzyme genes（Mi PG14,Mi PME,Mi Cel,and Miβ-glu）in the pericarp of the two mango cultivars was significantly downregulated in a variety-dependent manner.Therefore,melatonin could maintain cell wall metabolic stability and delay fruit softening.3.Melatonin maintained the reactive oxygen species（ROS）metabolism balance in two mango cultivars.The results demonstrated that melatonin treatment reduced the contents of hydrogen peroxide,superoxide anion,and malondialdehyde（MDA）.In addition,melatonin upregulated the transcriptional expression of the catalase gene（Mi CAT1）,superoxide dismutase gene（Mi Mn SOD）,peroxidase（Mi POD20）,and ascorbate peroxidase（Mi APX）and enhances the activities of CAT,SOD,POD,APX and glutathione reductase（GR）,thereby reducing oxidative damage and prolonging shelf life.4.Melatonin treatment upregulated phenylalanine lyase gene（Mi PAL）expression and increased PAL enzyme activity.The activity of polyphenol oxidase（PPO）and the expression of the Mi PPO gene in the two mango cultivars were significantly inhibited.Moreover,melatonin significantly increased the content of non-enzymatic antioxidants（ascorbic acid,reduced glutathione,total phenol,and total flavonoids）,and the DPPH antioxidant scavenging capacity was maintained compared to the control treatment.These results（point 4）indicated that MT helped to maintain the storage quality and nutritional values of the mangoes and delayed the oxidative aging of the fruit by increasing the non-enzymatic antioxidants.