Effect of Exogenous Melatonin Application on Maintaining Physicochemical Properties, Phytochemicals, and Enzymatic Activities of Mango Fruits During Cold Storage

Mango fruits are susceptible to cold stress under prolonged storage. Melatonin (MT) is a phytohormone well known for enhancing the tolerance and overall quality of various tropical and subtropical fruits during cold storage. This study investigated the effects of MT treatment on the postharvest quality of mango fruits during prolonged cold storage. Mangoes were treated with different concentrations of MT (1.0 mM (T1), 1.5 mM (T2), 2.0 mM (T3), and 2.5 mM (T4)) and stored for 45 days under cold conditions (15 °C and 90% relative humidity). Control fruits had no MT treatments. Various physicochemical, phytochemical, antioxidant, and enzymatic activities were monitored every 5 days throughout the storage period. MT treatment significantly reduced the weight loss and decay rates compared to control samples, with T3 and T4 treatments showing superior effectiveness. Due to severe decay in the control samples, the storage period was terminated on day 25, whereas the MT treatment protected the mango fruits and allowed for the completion of all 45 days of storage. The MT treatments effectively maintained color characteristics, reduced respiration rates, and suppressed ethylene production in mango fruits compared to the control samples. Higher MT concentrations preserved firmness and controlled malondialdehyde accumulation (<i>p</i> < 0.05). Chemical properties, including the starch content, total soluble solids, and titratable acidity, were better maintained in MT-treated fruits. The treatments also enhanced the retention of phytochemicals (ascorbic acid, total phenolic, and total flavonoid contents) and improved antioxidant activities against DPPH and ABTS radicals. Furthermore, MT treatment effectively regulated the activities of browning-related enzymes (polyphenol oxidase (PPO) and peroxidase (POD)), cell wall-degrading enzymes (polygalacturonase (PG), pectin methylesterase (PME), and lipoxygenase (LOX)), and antioxidant enzymes (superoxide dismutase (SOD) and ascorbate peroxidase (APX)). The results demonstrate that MT treatment, particularly at higher concentrations (T3 and T4), effectively extends the storage life and maintains the quality of mango fruits during prolonged cold storage.