Effect of Melatonin on Browning, Membrane Lipid and Energy Metabolisms in Postharvest Rambutan Fruit

In this study, to investigate the effects of melatonin on pericarp browning in rambutan fruit, postharvest ‘Baoyan7’ rambutan fruit was immersed in melatonin solutions at varying concentrations for 15 min, air-dried, and then stored at 25 ℃ for 6 days. The results demonstrated that melatonin treatment at each concentration delayed browning and the decline in the chromaticity a* value of rambutan pericarp, with the most pronounced effect being observed at 0.125 mmol/L melatonin. Physiological and biochemical analyses showed that melatonin treatment at 0.125 mmol/L repressed the increase in relative conductivity (RC) and malondialdehyde (MDA) contents, while retarding the increase in lipoxygenase (LOX), lipase (LPS) and phospholipase D (PLD) activities in rambutan fruit during storage, thereby preserving the integrity of the cell membrane. In addition, treatment with 0.125 mmol/L melatonin increased the contents of nicotinamide adenine dinucleotide (NADH) and flavin dinucleotide (FADH2), enhanced the activities of energy metabolism-related enzymes including succinate dehydrogenase (SDH), cytochromic c oxidase (CCO), H+-ATPase and Ca2+-ATPase, and maintained high levels of adenosine triphosphate (ATP) and energy charge, thus improving cellular energy status. Moreover, this treatment enhanced γ-aminobutyric acid (GABA) content, and maintained high activities of glutamate decarboxylase (GAD) and GABA aminotransferase (GABA-T), thus contributing to enhanced cellular energy supply. These findings suggest that melatonin could retard postharvest browning and senescence in rambutan fruit by modulating membrane lipid and energy metabolisms.