Effects of Melatonin Treatment on Postharvest Ripening, Fruit Quality, and Sugar–Acid Metabolism in ‘Saimaiti’ Apricots

This study investigated whether exogenous melatonin (MT) treatment was associated with delayed ripening and changes in sugar and organic acid metabolism in Xinjiang ‘Saimaiti’ apricot fruit during cold storage. Fruit were immersed in 100 μmol·L−1 MT solution under vacuum at a pressure 0.05 MPa below atmospheric pressure for 2 min, followed by immersion for 8 min after pressure restoration, and stored at 0 ± 0.5 °C for 49 d. The control (CK) underwent identical procedures using distilled water. Compared with the CK, MT-treated fruit exhibited better maintenance of firmness, soluble solids content, and titratable acidity, together with lower weight loss during storage. The respiratory climacteric peak occurred at 21 d in CK fruit and at 28 d in MT-treated fruit, with a similar shift observed in the ethylene production peak. Sucrose content declined during storage but remained higher in MT-treated fruit from 7 d onward, whereas glucose and fructose contents increased, with larger treatment differences during 35–49 d. Differences in malic and citric acid contents were most pronounced around 28–35 d, accompanied by changes in related metabolic enzyme activities. At 49 d, MT-treated fruit received higher scores for sweetness acceptability, sourness acceptability, sugar–acid balance, and overall acceptability. Overall, these findings suggest that MT treatment may delay postharvest ripening and help maintain fruit quality in ‘Saimaiti’ apricots during cold storage, accompanied by changes in sugar and organic acid metabolism.