Melatonin Regulates Reactive Oxygen Species Metabolism to Induce Resistance to Black Spot of Postharvest Apricot Fruit

Xinjiang 'Saimaiti' apricot fruit were used as the material to study the inhibitory effects of postharvest treatment with melatonin on black spot and its influence on reactive oxygen species metabolism. Fruit were vacuum impregnated into different concentrations of melatonin (50, 100, 200 μmol/L) under pressure of 0.05 MPa for 2 min, then atmospheric pressure was restored, and the fruit was maintained in the same solution for 8 min. The distilled water treatment was used as blank control. After naturally drying, the fruit were refrigerated (0±1℃, RH 90%~95%) for 48 h. After that, apricot fruits were inoculated with Alternaria alternata and stored at the same conditions.The lesion diameter and disease incidence of apricot fruit inoculated with A.alternata, the activities of superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), and superoxide anion radical (O₂⁻·) production rate, cell membrane permeability, hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) contents were measured regularly. Results showed that compared with the control group, different concentrations of exogenous melatonin treatment significantly delayed the increase of apricot fruit morbidity and inhibited the increase of apricot fruit the lesion diameter. At the end of storage, the spot diameters of apricot fruit were significantly lower in the 50, 100, and 200 μmol/L melatonin-treated groups by 11.92%, 28.49%, and 19.67% (P<0.05) than in the control group. And 100 μmol/L melatonin had the best effect on black spot diseases of apricot fruits. In the early stage of storage, 100 μmol/L exogenous melatonin treatment increased the apricot fruit superoxide dismutase activity, decreased the rate of superoxide anion, inhibited catalase activity, and induced rapid accumulation of H₂O₂ in apricot fruit. At the end of storage, catalase and peroxidase activities in the 100 μmol/L melatonin-treated group were 39.13% and 78.91% higher than those in control, respectively (P<0.05). The H₂O₂ content of apricot fruit in the 100 μmol/L melatonin-treated group was significantly lower than that in the control group by 19.74% (P<0.05), which reduced the accumulation of H₂O₂ in apricot fruit, and delayed the rise of cell membrane permeability and malondialdehyde content. These findings suggested that melatonin could regulate reactive oxygen species metabolism to induce resistance to black spot in postharvest apricot fruit.