The Effect And Mechanism Of Postharvest Melatonin Treatment On Pakchoi Quality Deterioration

Pakchoi（Brassica rapa chinensis）is rich in glucosinolates,vitamin C,polyphenols and other nutrients.Pakchoi is popular among consumers because of its crisp taste and rich nutrition.The postharvest metabolism of pakchoi is vigorous.It is prone to wilting,yellowing and other quality deterioration.The postharvest loss rate is as high as 30%.Because of the rapid degradation of chlorophyll,postharvest aging and yellowing are serious,which greatly reduces the nutritional value and commercial value of pakchoi.According to statistics,due to improper storage and transportation of pakchoi,the loss rate is as high as 30%,causing large economic losses to farmers.Therefore,it is urgently needed in production practice to explore safe and effective methods of keeping fresh Chinese cabbage.In this experiment,the ’Shanghai Green’,variety of pakchoi was used as the experimental object,which was sprayed with a 500μmol/L exogenous melatonin solution.To investigate the effect of spraying with exogenous melatonin on the quality,physiological and biochemical indexes,and chlorophyll degradation metabolism of whole pakchoi under the storage condition of 20℃±1℃,to clarify the mechanism of melatonin delaying the degradation of chlorophyll in pakchoi.The results of the study are as follows:1.The effect of spraying with 500μmol/L exogenous melatonin on the color,weight loss rate,chlorophyll content of pakchoi was studied.During the early storage period（0-3d）,the yellowing of pakchoi is not obvious.As the senescence intensifies,the leaves turn from green to yellow.On the 5th day,the senescence rate of pakchoi reached its peak,and the weight loss rate of exogenous melatonin treatment group and control group reached 4.20%and 5.69%respectively;The yellowing index is 0.507,0.355,respectively;the chlorophyll content is 0.592mg^-1FW,0.746mg^-1FW,respectively.In the control group,the pakchoi was dehydrated,wilted and yellowed severely,and almost lost its commercial value on the 5th day.In contrast,exogenous melatonin treatment can significantly inhibit the quality loss and yellowing senescence of pakchoi,and can extend the shelf life of pakchoi by 2 days.2.The effects of spraying with 500μmol/L exogenous melatonin treatment on the relative conductivity,MDA,respiratory intensity and other physiological indicators of pakchoi were studied.With the extension of storage time,the relative conductivity,MDA content and LOX activity of pakchoi increased.The relative conductivity and MDA content of pakchoi in the treatment group and the control group reached the maximum on the 7th day,the value were 23.91%,19.26%,respectively;2.604μmol·g^-1 FW,3.182μmol·g^-1 FW respectively.Melatonin treatment can significantly inhibit the rise of these indicators and effectively alleviate the damage to the body caused by membrane lipid peroxidation.It suppresses the increase in respiratory intensity,delays the consumption of organic nutrients in the body by individuals after harvest,and prolongs the shelf life of pakchoi.3.The effect of spraying with 500μmol/L exogenous melatonin treatment on the content of active oxygen free radicals in pakchoi was studied.During storage,as the aging progresses,the content of superoxide anion free radicals and H₂O₂ in pakchoi gradually accumulates.It reached the peak on the 5th day,the superoxide anion content of the treatment group and the control group were 68.54nmol·min-1·g^-1 FW and 93.87nmol min-1·g^-1FW,respectively;the H₂O₂ content were 239.7μmol·g^-1 FW and 302.3μmol·g^-1 FW respectively.Exogenous melatonin treatment significant-ly inhibited the accumulation of active oxygen free radicals.4.The effects of spraying with 500μmol/L exogenous melatonin treatment on the activities of SOD,CAT,POD,APX and other antioxidant enzymes in pakchoi were studied.The experimental results showed that during storage,the antioxidant enzyme activity of the treated group of cabbage was significantly higher than that of the control group.Compared with the control group,on the third day,the activities of SOD and POD increased by 15.49%and 10.04%,respectively;the activities of CAT and APX increased by 22.53%and 34.38%,respectively.Melatonin treatment can inhibit the decline of CAT and APX activities,enhance SOD and POD activities,and maintain good free radical scavenging ability.5.The effect of spraying with 500μmol/L exogenous melatonin treatment on the non-enzymatic antioxidant system such as the content of dehydroascorbic acid and reduced glutathione in pakchoi was studied.Exogenous melatonin treatment can significantly delay the decrease of AsA and GSH content,and enhance the ability of AsA-GSH cycle to scavenging active oxygen free radicals.At the same time,it reduces the loss of vitamin C and other nutrients in pakchoi.On the 5th day,the loss rate of vitamin C content in the control group and the treatment group were 33.9%and 16.7%,respectively;the loss rate of GSH content was 47.4%and 33.6%,respectively.6.The effect of spraying with 500μmol/L exogenous melatonin treatment on the activities of CLH,MDCase,and PPH in the pathway of PAO degradation of pakchoi chlorophyll was studied.Compared with the control group,on the 5th day,the activities of CLH,MDCase and PPH decreased by 34.09%,23.37%,22.58%,respectively.Exogenous melatonin treatment can significantly inhibit the increase of CLH,MDCase,and PPH activities,reduce the decomposition of chlorophyll molecules by these three chlorophyll degrading enzymes,delay the decline of chlorophyll content in pakchoi,and extend the storage period of pakchoi.