Influence of Melatonin Treatment on Peel Browning of Cold-Stored "Nanguo" Pears

The effects of melatonin (MT) on chilling injury-induced peel browning of "Nanguo" pears were investigated during the fruit shelf-life at 20 degrees C, after storage at 0 degrees C for 120 days. Melatonin treatment alleviated the severity of peel browning, as evidenced by the lower incidence of peel browning and browning index of the MT-treated "Nanguo" pear, and delayed browning time compared to that observed in control fruit. In addition, MT treatment reduced lipoxygenase activity and malondialdehyde content and enhanced the activities of phenylalanine ammonia lyase, shikimate dehydrogenase, 4-coumarate:coenzyme A ligase, cinnamate-4-hydroxylase, and 6-phosphogluconate dehydrogenase but inhibited the activity of polyphenol oxidase, which catalyzes the oxidation of phenolic compounds to quinones. Furthermore, MT treatment enhanced the accumulation of total phenolic compounds, thereby improving antioxidant capacity and maintaining cell membrane integrity, and stimulated the accumulation of proline by promoting the activities of key enzymes involved in proline synthesis, namely Delta 1-pyrroline-5-carboxylate synthase and ornithine delta-aminotransferase, which collectively contribute to improved chilling tolerance and delayed browning. These findings indicate that MT treatment effectively relieves peel browning of cold-stored "Nanguo" pears by regulating the metabolism of phenolic compounds and proline.