Effect Of Exogenous Melatonin On Postharvest Browning And Disease In Harvested Litchi Fruit

Litchi?Litchi chinensis Sonn.?is an important economic crop in the south of China.However,postharvest litchi fruit is extremely susceptible to browning and be infected by pathogenic microorganisms during storage and transportation,which leads to severe decay and quality deterioration.The litchi downy blight caused by Peronophythora litchii is one of the most common diseases in litchi.Using safe,efficient and environmentally friendly natural bioactive products is considered as a promising strategy to maintain the postharvest quality and reduce the disease occurrence in harvested crops.Melatonin is an important bioactive molecule that widely exists in nature and participates in a variety of physiological processes in plants.In this study,using‘A4 Wuhe'and‘Feizixiao'litchi fruit as experimental materials,we investigated the inhibitory effects of exogenous melatonin on postharvest browning and disease and explored the possible physiological mechanisms.The results are presented as follows:?1?Exogenous melatonin dipping treatment at 0.4 mmol L^-1effectively delayed the browning process while prevented the loss of pericarp color and maintained the integrity of cell membrane in postharvest‘A4 Wuhe'litchi fruit during storage.?2?Exogenous melatonin treatment at 0.4 mmol L^-1significantly inhibited the increases in activities of phosphatidylase D?PLD?,lipase and lipoxygenase?LOX?in litchi fruit while delayed the decrease of phosphatidylcholine?PC?content and the increase of phosphatidylcholine?PA?content.After 8 d of storage,the content of PC in melatonin-treated fruit was 11.9%higher than that control fruit while the content of PA in melatonin-treated fruit was 7.0%lower than that in control fruit.?3?Exogenous melatonin at 0.4 mmol L^-1significantly delayed the increases in relative content of saturated fatty acids?SFA?including palmitic acid and stearic acid,and delayed the decreases in relative content of three kinds of unsaturated fatty acids?USFA?including oleic acid,linoleic acid and linolenic acid.Moreover,the USFA/SFA ratio of melatonin-treated fruit were 22.16%,34.94%and 32.52%higher than those in control fruit at 4,6 and 8 d of storage,respectively.?4?Exogenous melatonin at 0.4 mmol L^-1significantly delayed the decreases in ATP and ADP content and energy charge?EC?.Furthermore,melatonin treatment inhibited the decreases in activities of the mitochondrial energy metabolism-related enzymes including H⁺-ATPase,Ca²⁺-ATPase,succinate dehydrogenase?SDH?,and cytochrome C oxidase?CCO?,contributing to maintainence of the higher energy status.?5?Exogenous melatonin can inhibit the growth of P.litchii in vitro,with a concentration-dependent pattern,among which 2 mmol L^-1melatonin showed the most significant effect.The results of the in vivo experiment showed that exogenous melatonin treatment at 0.25 mmol L^-1could inhibit the incidence of litchi downy blight and its lesion expansion in litchi fruit after inoculation with P.litchii.?6?Melatonin at 0.25 mmol L^-1reduced the decreases in ATP and ADP contents and energy charge?EC?and delayed the increase of AMP content,resulting in higher energy status than the control fruit in litchi fruit inoculated with P.litchii.Moreover,melatonin at0.25 mmol L^-1slowed the decrease in activities of energy metabolism-related enzymes including H⁺-ATPase,Ca²⁺-ATPase,SDH and CCO in litchi fruit inoculated with P.litchii.?7?Melatonin at 0.25 mmol L^-1effectively induced the increases in activities of phenylalnine ammonialyase?PAL?,cinnamate acid-4-hydroxylase enzyme?C4H?activity and 4-coumaryl:Co A ligase?4CL?in phenylpropanoid metabolic pathway,which might result in higher accumulation of metabolites?total phenolics and flavonoids?,and thus contribute to enhanced disease resistance in litchi fruit.?8?Exogenous melatonin treatment at 0.25 mmol L^-1significantly inhibited the decrease of NADPH content,while increased the activities in glucose-6-phosphate dehydrogenase?G6PDH?and 6-phosphate-gluconate dehydrogenase?6GPDH?,which could be of benefit to improve the requirement of the reducing power for defense reaction in litchi fruit.