Effects Of Melatonin Combined With Ethanol Treatment On Cold Resistance Enhancing Of Bitter Melon Fruit

Bitter melon（Momordica charantia L.）,a kind of seasonal fruit,is widely distributed in tropical,subtropical and temperate regions.It is easy to appear such symptoms as the enlarged seed,the soft tissue,the yellow pericarp and the loss of bitterness during the storage at room temperature.Low temperature storage is an important measure for postharvest bitter melon preservation.However,as a cold sensitive fruit,bitter melon is prone to chilling injury during cold storage.The chilling injury is initially manifested as the appearance of water stains on the pericarp of fruit,and then softens with the increase of weight loss,moreover,the water stains are easy to be infected by pathogenic bacteria in the later period,which greatly reduces the marketability of bitter melon.Therefore,it’s vital to understand the mechanism of chilling injury and seek appropriate technology to improve the cold resistance of bitter melon fruit.In the present study,to reveal the mechanism of cold resistance induced by exogenous melatonin（MT）and ethanol of bitter melon,effects of exogenous melatonin and ethanol treatment on fruit quality,antioxidant metabolism,polyamine metabolism,γ-aminobutyric acid（GABA）metabolism and proline metabolism metabolism of postharvest bitter melon were studied,which can provide reference for postharvest treatment and low-temperature preservation of bitter melon fruit.The results are as follows:1.In the pre-experiment,100μmol L^-1of melatonin and 6%of ethanol were adopted respectively when they were applied to the treatment of bitter melon alone,and the soking time was 10 min.Based on that,models for the effects of different concentrations of melatonin(x₁,50-150μmol L^-1)and ethanol（x₂,2.5-9.5%）and different soaking time（x₃,5-15 min）on the response value of hardness（Y₁）,chilling injury index（Y₂）and weight loss（Y₃）were established by response surface methodology（RSM）.The firmness,chilling injury and weight loss models were all remarkably significant（P<0.01）with high fit（R²>0.9）and small experimental error.The interaction of melatonin concentration,ethanol concentration and soaking time on chilling injury of bitter melon fruit was analyzed.The optimal conditions for improving cold resistance of bitter melon fruit treated with melatonin and ethanol were optimized as:melatonin concentration of 120μmol L^-1,ethanol concentration of 5.5%and immersing time of 10 min.The subsequent validation test showed that melatonin-ethanol treated group could better alleviate chilling injury of bitter melon at the duration of storage at 4℃for twenty days and maintain better quality than the group treated with melatonin or ethanol alone.2.Melatonin(100μmol L^-1),ethanol（6%）and melatonin-ethanol treated group(melatonin concentration of 120μmol L^-1and ethanol concentration of 5.5%)can effectively reduce chilling injury and weight loss,maintain the high firmness,inhibit the accumulation of malondialdehyde and the increase of ion leakage of bitter melon fruit.In addition,treatment alone and in combination alleviated the decrease of chlorophyll content via inhibiting activities of chlorophyllase and Mg-dechelatase.Meanwhile,the content of soluble protein,total soluble solids and ascorbic acid in fruit was effectively maintained and the accumulation of total phenol and total flavonoids in fruit were promoted.Melatonin and ethanol alone and combined treatment maintained the activity of ascorbate peroxidase（APX）,peroxidase（POD）,catalase（CAT）,superoxide dismutase（SOD）and the DPPH（2,2-Diphenyl-1-picrylhydrazyl）scavenging capacity in postharvest bitter melon,which may be beneficial for the reducing of the superoxide anion and hydrogen peroxide accumulation,and the occurrence of chilling injury during low temperature storage of bitter melon fruit was slowed down.3.Melatonin and ethanol alone and combined treatment enhanced the activity of ornithine decarboxylase（ODC）,arginine decarboxylase（ADC）and glutamate decarboxylase（GAD）of bitter melon fruit during low temperature storage,thus promoting the increase of putrescine and spermidine content and GABA accumulation in varying degrees.Besides,the three treatments effectively increased the activity ofΔ¹-pyrroline-5-carboxylic acid synthase（P5CS）in bitter melon fruit,inhibited the activity of proline dehydrogenase（PDH）,and delayed the decrease of ornithineδ-aminotransferase（OAT）activity in the later period,thus enabling the accumulation of proline in fruit.Therefore,in the present study,the optimal conditions of a combined treatment of MT and ethanol were evaluated by central composite design（CCD）.Then,the combined treatment of 120μmol L^-1melatonin and 5.5%ethanol was used in the postharvest bitter melon fruit,which can not only maintained its quality at 4℃,but also increased the antioxidant capacity and promoted the accumulation of osmotic adjustment substances in fruit via affecting the activity of related enzymes.Finally,chilling injury of bitter melon fruit was delayed effectively.