Pre-harvested Spraying Diethyl Dithiocarbamate For Controlling Pericarp Browning Of Harvested Longan Fruit And Its Action Mechanism

Longan(Dimocarpus longan Lour.)is a famous subtropical fruit that has high nutritional and commercial value,and thus favored by the consumers.However,in China,longan ripens from August to September with high temperature,therefore fruit deterioration like pericarp browning occurs rapidly after harvest,which has a serious impact on the quality and shelf life of longan fruit,restricting the development of longan industry.'Fuyan' longan(Dimocarpus longan Lour.'Fuyan')fruits,the major longan cultivar in Fujian Province,were used as the experiment material.The objectives of this research were to investigate the effects of pre-harvest spraying sodium diethyldithiocarbamate(DDTC)on postharvest physiology,quality and storability of harvested 'Fuyan' longan fruits on the basis of reactive oxygen species(ROS)metabolism,membrane lipid metabolism,energy metabolism,phenolics metabolism.The results were showed as follows:1.The effects of pre-harvest spraying DDTC on physiology,quality and storability of harvested longan fruit were studied.The results showed that as compared to control fruit,pre-harvest spraying DDTC could decrease respiration rate of harvested longan fruits,keep higher contents of TSS,soluble sugar,sucrose,vitamin C in aril of longan fruits and higher chlorophyll and carotenoid content in pericarp of longan fruits,inhibit the increase of contents of TA in aril of longan fruits,delay the pericarp browning and aril breakdown,and lower weight loss and decay to keep higher healthy fruit rate.2.The effects of pre-harvest spraying DDTC on ROS metabolism in pericarp of harvested longan fruit were studied.The results showed that as compared to control fruit,pre-harvest spraying DDTC could keep higher activities of ROS scavenging enzymes including catalase(CAT)and ascorbate peroxidase(APX),and higher contents of endogenous antioxidant substances such as ascorbic acid(AsA)and glutathione(GSH);inhibit the production rate of O2-and the increase of malondialdehyde(MDA)content.These results suggested that pre-harvest spraying DDTC could increase ROS scavenging capacity to reduce the production and accumulation of ROS,alleviate membrane lipid peroxidation,maintain the integrity of cellular membrane structure in pericarp,and then delay pericarp browning of longan fruits.3.The effects of pre-harvest spraying DDTC on membrane lipid metabolism in pericarp of harvested longan fruit were studied.The results showed that as compared to control fruit,pre-harvest spraying DDTC could decrease the activities of phospholipase D(PLD),lipase and lipoxygenase(LOX),reduce the degradation of unsaturated fatty acids such as oleic acid,linoleic acid and linolenic acid,and inhibit the increase of the relative content of the saturated fatty acid like palmitic acid,stearic acid and Lignoceric acid,to maintain higher index of unsaturated fatty acids(IUFA)and the ratio of unsaturated fatty acids versus saturated fatty acids(U/S).From the above results,it can be concluded that pre-harvest spraying DDTC could maintain the integrity of cell membrane structure and delay pericarp browning of longan fruits via reducing the activity of membrane lipid degradation enzyme which might decrease unsaturated fatty acid degradation,.4.The effects of pre-harvest spraying DDTC on energy metabolism in pericarp of harvested longan fruit were studied.The results indicated that as compared to control fruit,pre-harvest spraying DDTC treatment could inhibit the decrease of ATP and ADP contents,maintain relatively higher energy charge level,keep higher activity of H+-ATPase,Ca2+-ATPase,Mg2+-ATPase,and NAD kinase,as well as higher content of NADPH and NADP,decrease NADH content and NAD content.These results suggested that pre-harvest spraying DDTC could maintain the energy level needed by normal physiological metabolism of harvested longan fruit,and keep the balance of ion concentration between inside and outside cells to maintain the integrity of cell membrane structure,delaying pericarp browning of longan fruits.5.The effects of pre-harvest spraying DDTC treatment on phenolics metabolism in pericarp of harvested longan fruit were studied.The results showed that as compared to control fruit,pre-harvest spraying DDTC treatment could effectively decrease the activities of polyphenol oxidase(PPO)and peroxidase(POD),and maintain higher content of total phenolic.These results suggest that pre-harvest spraying DDTC could reduce the activities of phenols oxidation enzymes of harvested longan fruit which delayed enzymatic oxidation of polyphenols to postpone pericarp browning of longan fruits.