Effects of exogenous melatonin treatment on storage quality and physiological characteristics of table grape at room temperature

Abstract:【Objective】Melatonin is a newly found plant hormone and multifunctional signal regulator in various organs, and has been extensively studied related to postharvest storage and preservation of horticultural crops in the recent years. Water loss, decay, browning or abscission may occur during storage of table grapes, bringing great losses to grape production. This study aimed to explore the effects of exogenous melatonin treatment on quality and physiological characteristics of table grape at room temperature during postharvest storage.【Methods】Red Globe grape berries were taken as the material. The berry samples were first sterilized for 5 min with 0.5% sodium hypochlorite solution, and then washed for three times with tap-water and then with distilled. They were then soaked in solutions of 0, 50, 100, 200 and 500 μmol·L^-1. After drying naturally, the samples were packed with PE bags and stored at（25±0.5）℃ with a relative humidity of 75%±5% for 28 days in the incubation chamber to determine the effects of melatonin treatment on the postharvest quality and physiological characteristics including rates of grape weight loss, abscission and decay, firmness, contents of soluble solids, titratable acid, and vitamin C as well as malondialdehyde, superoxide anion, hydrogen peroxide, and activities of antioxidant enzymes (superoxide dismutase, peroxidase, catalase, and ascorbate peroxidase). Except for  fruit weight loss rate, abscission rate and decay rate, all the parameters were measured every 7 days. Weight loss rate (% ) was calculated by the difference method. The abscission rate (% ) was calculated according to the ratio between the weight of abscission and the total weight. The decay rate (%) was calculated according to the percentage of decay weight against total weight. The firmness (kg·cm^-2 ) was determined at two opposite points on fruit equator with a GY-4 handheld digital hardness tester. The content of soluble solids (%) was determined with a PAL-Fu handheld digital sugar meter. The titratable acid content (%) was determined using the sodium hydroxide titration method. The content of vitamin C (mg·kg^-1) was determined with the phenanthroline colorimetric method; the malondialdehyde content (mmol·g^-1) with the thiobarbituric acid method. The activities of superoxide dismutase (U·g^-1·h^-1), peroxidase (μg·g^-1·min^-1), catalase (U·g^-1·min^-1) ascorbate oxidase (U·g^-1·min^-1 ) were determined with nitroblue tetrazolium photoreduction method, the guaiacol method, ultraviolet absorption method at 240 nm and . spectrophotometry at 290 nm, respectively. The production rate of superoxide anion (nmol·g^-1·min^-1 ) was determined with the hydroxylamine method, and hydrogen peroxide content (μmol·g^-1 ) with the spectrophotometry method at 417 nm.【Results】Melatonin treatments at all concentrations could inhibit the abscission and decay of the berries and delay the water loss of fruit clusters due to lowed weight loss rate. There was significant difference among the five treatments. Compared with the control, the weight loss rate, abscission rate and decay rate were decreased by more than 20%, 10% and 25%, respectively by melatonin treatments. Among them, 200 μmol·L^-1 melatonin treatment had the best effect on postharvest quality of table grape fruit. With the extension of storage time, the soluble solid content, and the activities of superoxide dismutase, catalase and ascorbate peroxidase first increased and then decreased in general, and the soluble solid content, superoxide dismutase and ascorbate peroxidase activities reached their maximum values on the 14th day but the peak of catalase activity was on the 7th day. Malondialdehyde content, peroxidase activity, hydrogen peroxide content and superoxide anion production rate showed a gradual increase trend, while titratable acid content, hardness and vitamin C content showed a gradual decrease trend. There was significant difference between 200 μmol·L^-1 melatonin treatment and the control. The peak values of soluble solid content, superoxide dismutase and the activities catalase and ascorbate peroxidase activities of grape fruit in the 200 μmol·L^-1 melatonin treatment were 1.07-fold, 1.38-fold, 1.26-fold and 1.15-fold than the control, respectively. The titratable acid content, hardness vitamin C content, and peroxidase activity in the 200 μmol·L^-1 melatonin treatment were 22.09%, 19.07%, 30.21% and 19.34% higher on the 28th day, while the contents of malondialdehyde, hydrogen peroxide and superoxide anion production rate were 39.51%, 26.61% and 25.10% lower than the control, respectively.【Conclusion】200 μmol·L^{- 1} melatonin treatment could slow down the decline of fruit firmness, inhibit the decrease of soluble solids and titratable acid contents, maintain the content of vitamin C, and thus is effective to maintain quality of grape during storage. Meanwhile, melatonin can delay the senescence of grape fruit by strengthening the activities of superoxide dismutase, peroxidase, catalase and ascorbate peroxidase, inhibiting the increase of superoxide anion production rate, and delaying the accumulation of malondialdehyde and hydrogen peroxide. Therefore, exogenous application of melatonin is effective to delay postharvest senescence of table grape and improve grape postharvest storage qualities at room temperature.