Melatonin Improved the Postharvest Quality of Cold-stored Phyllanthus emblica Fruit and Its Relationship with the Metabolisms of Reactive Oxygen Species and Membrane Lipid

In this study, the harvested Phyllanthus emblica (P. emblica) fruit cv. 'Lanfeng' was used as material to investigate the effects of melatonin treatment on the postharvest quality of cold-stored P. emblica fruit and its relationship with reactive oxygen species (ROS) and membrane lipid metabolisms. The fruit was treated by soaking in 0.4 mmol/L melatonin and distilled water (as control), respectively, then stored at (4±1) ℃ and relative humidity of 85%~90% for 49 days. During storage, the indicators related to storage quality, cell membrane permeability (CMP), contents of ROS and malondialdehyde (MDA), levels of ROS-scavenging enzyme activities and substance contents, activities of membrane lipid metabolism-related enzymes and contents of membrane lipid components were measured every 7 days. Results showed that, compared with control P. emblica fruit, melatonin-treated P. emblica fruit maintained the notably lower chilling injury index, weight loss rate and values of a^* and ΔE^*, but kept the clearly higher commercially acceptable fruit rate, values of L^* and b^*, and amounts of total sugar, TSS and TA, thus retaining the postharvest quality during cold storage. Moreover, melatonin lowered CMP, superoxide anion production rate and MDA content, raised activities of superoxide dismutase, catalase and ascorbate peroxidase, increased contents of ascorbic acid and glutathione, and rose scavenging ability of 1,1-diphenyl-2-trinitrophenylhydrazine radical of P. emblica fruit. In addition, in melatonin-treated fruit, the activities of phospholipase D, lipoxygenase and lipase were decreased. Meanwhile, the levels of phosphatidic acid, palmitic acid and stearic acid were reduced, whereas the levels of phosphatidylcholine, phosphatidylinositol, oleic acid, linoleic acid and linolenic acid were raised. Therefore, melatonin could retain the postharvest quality of cold-stored P. emblica fruit by regulating their ROS and membrane lipid metabolisms. Specifically, it enhanced the ROS-scavenging capacity of the fruit to reduce ROS level, and decreased the activities of membrane lipid metabolism-related enzymes to stabilize the contents of membrane lipid components. These regulatory effects helped maintain the stability of cell membrane structure, thereby improving the postharvest quality during cold storage, and extending the fruit's storage time.