Effects Of Exogenous Melatonin On Reducing Chilling Injury Of Postharvest Apricot Fruits

Apricot is a kind of drupe with very juicy pulp,smooth skin and excellent taste.Apricots are very sensitive fruits that rot and ripen quickly at room temperature.Cold storage is a common method to delay the deterioration of products,but this process may cause physiological obstacles such as chilling injury.This kind of damage is caused by physiological low temperature,which affects the quality of fruit,reduces the ground and shelf life of fruit,and damages the sensory characteristics of fruit.Therefore,the physiological mechanism of reducing chilling injury of apricot treated with melatonin was analyzed from the aspects of cold storage quality,active oxygen metabolism,cell wall metabolism,polyamine synthesis metabolism and proline synthesis metabolism,transcriptome sequencing of apricot fruit,GO and KEGG pathway analysis of DEGs,to explore the molecular mechanism of reducing chilling injury of apricot fruit treated with melatonin from the point of view of cold storage quality,active oxygen metabolism,cell wall metabolism,polyamine synthesis metabolism and proline synthesis metabolism.The main results are as follows:（1）Different concentrations（50,100 and 200μM）of melatonin could effectively delay the onset of chilling injury and reduce the incidence of chilling injury,delay the decrease in fruit firmness,SSC,TA,As A content and the pericarp color turning.The optimum concentration of melatonin was 100μM.The results showed that melatonin treatment could maintain the storage quality of apricot fruit and improve the cold resistance of apricot fruits.（2）Melatonin treatment significantly increased the activities of SOD and GR,delayed the decrease of POD,CAT and APX activities during storage,effectively suprressed the enhancement of H₂O₂content and the production rate of O₂^-·,and inhibited the increase of malondialdehyde content and cell membrane permeability.The results showed that the occurrence of chilling injury of apricot fruit was closely related to the metabolic balance of active oxygen.Melatonin treatment could maintain high activity of antioxidant enzymes,inhibit the production of reactive oxygen free radicals,maintain the integrity of membrane,and reduce the damage caused by oxidation.（3）Melatonin treatment maintaied the lower activity of pectinase and polygalacturonase in apricot fruit,and effectively inhibited the increase in activity of cellulase andβ-galactosidase.The degradation of CDTA-soluble pectin,sodium carbonate soluble pectin and cellulose content and the increase in water soluble pectin content were delayed.The results showed that the reduction of chilling injury of apricot fruits by melatonin treatment was closely related to the integrity of cell wall.（4）Melatonin treatment could enhance the activity ofΔ¹-pyrroline-5-carboxylic acid synthetase and ornithine-δ-aminotransferase and decrease proline dehydrogenase activity,thus promoting the accumulation of proline.In addition,MT treatment increased the activities of arginine decarboxylase and ornithine decarboxylase and the contents of putrescine,spermidine and spermine.These results showed that the effective reduction of chilling injury by exogenous MT treatment was related to the increase of endogenous osmotic substances proline and PAs.（5）The effects of melatonin treatment on transcriptome sequencing and WRKY gene expression in postharvest apricot fruits were studied.The transcriptome sequence of apricot fruit during storage was determined,and the differentially expressed genes in apricot fruit sequencing were analyzed by GO and KEGG,respectively.It was found that melatonin treatment could improve the defense of apricot fruit to cold stress by increasing the expression level of WRKY gene and regulating MPAK pathway.To sum up,exogenous melatonin was effective in maintaining the cold storage quality of postharvest apricot fruit,regulating the physiological metabolism of active oxygen metabolism,cell wall metabolism,polyamine metabolism and proline metabolism,and increasing the expression level of WRKY gene,thereby reducing the chilling injury of apricot fruits.