Melatonin Treatment Enhances Flavonol Content and Antioxidant Capacity in Bell Peppers ( Capsicum annuum L.) During Postharvest Storage

Abstract Postharvest oxidative stress accelerates senescence and flavonol loss in bell peppers, reducing nutritional quality and shelf life, while the role of exogenous melatonin in preserving specific flavonols and regulating the AsA–GSH cycle during ambient storage remains unclear. This study evaluated the effects of melatonin (100 and 200 μmol/L) on flavonol composition, antioxidant capacity, and antioxidant defense systems in bell peppers stored for nine days at 23±2 °C. Melatonin significantly preserved major flavonols, particularly quercetin, luteolin, and apigenin derivatives, with 200 μmol/L providing the greatest retention. Antioxidant capacity measured by DPPH and ABTS assays and activities of catalase, peroxidase, and ascorbate peroxidase were markedly enhanced by melatonin treatment. The AsA–GSH cycle was positively regulated, especially at 100 μmol/L, maintaining higher levels of reduced antioxidants. After nine days, ascorbic acid contents were 5.24 and 6.60 mg g-1 in peppers treated with 100 and 200 μmol/L melatonin, respectively, compared with 4.40 mg g-1 in controls. Glutathione levels were higher in treated samples (10.59–10.64 mg g-1) than in controls (9.22 mg g-1), while oxidized glutathione accumulation was significantly suppressed. Overall, exogenous melatonin mitigates oxidative stress and preserves flavonols, supporting its postharvest application in bell peppers during storage.