Pre-harvested Spraying Salicylic Acid For Controlling The Development Of Aril Breakdown In Harvested Longan Fruit And Its Action Mechanism

Longan(Dimocarpus longan Lour.)is famous subtropical fruit specie in the South of China and rich in nutrition.Because longan fruits mature in the hot seasons during August to September,it is difficult to storage and have very short postharvest life for 3?4 days at ambient temperature.longan fruits deteriorate rapidly after harvest due to pericarp browing,aril breakdown and decay,which limited its long-distance transportation and marketing.In this paper,'Fuyan' longan(Dimocarpus longan Lour.Cv.'Fuyan'),the major longan cultivar in Fujian Province of China was used as meterial.The objectives of this research were to clarify the mechanism of pre-harvest spraying salicylic acid(SA)of delaying aril breakdown of harvested longan fruit,in the perspectives of active oxygen metabolism,membrane lipid metabolism,energy metabolism,cell wall metabolism and disease-resistant.The results were showed as follows:1.Pre-harvest spraying SA reduced aril breakdown index of harvested longan fruit,and delayed aril breakdown of harvested longan fruit.2.Effects of pre-harvest spraying SA on ROS metabolism in pulp of harvested longan fruit were investigated.The results showed that compared with control,pre-harvest spraying SA increased the acitives of ROS scavenging enzymes such as dismutase(SOD),catalase(CAT)and ascorbate peroxidase(APX),and elevated contents of endogenous antioxidant such as ascorbic acid(AsA)and glutathione(GSH),which resulted in decreased superoxide anion radical(O2-·)production rate and malondialdehyde(MDA)content.From these results,it could be concluded that pre-harvest spraying SA could enhance the capacity of active-oxygen-scavenging in longan pulp,inhibit membrane lipid peroxidation,and thus delaying aril breakdown of harvested longan fruit.3.Effects of pre-harvest spraying SA on membrane lipid metabolism in pulp of harvested longan fruit were studied.The results showed that compared with control,pre-harvest spraying salicylic acid inhibited the acitives of phospolipase D(PLD),lipase and lipoxygenase(LOX),which reduced the degradation of unsaturated fatty acids such as linoleic acid(C18:2)and linolenic acid(C18:3),inhibited the increase of the relative content of the saturated fatty acid like palmitic acid(C16:0)and stearic acid(C18:0)during storage,and maintained higher level of the index of unsaturated fatty acids(IUFA)and the ratio of unsaturated fatty acids to saturated fatty acids(U/S).From these results,it could be concluded that pre-harvest spraying SA could decrease the activities of membrane lipid degrading enzymes,inhibit membrane lipid peroxidation to maintain the integrity of cellular membrane structure,thus delaying aril breakdown of harvested longan fruit.4.Effects of pre-harvest spraying SA on energy metabolism in pulp of harvested longan fruit were analyzed.The results showed that compared with control,pre-harvest spraying SA inhibited the decrease of adenosine triphosphate(ATP)cotent and energy charge level,increased the activities of cytochrome c oxidase(CCO),Ca2+-ATPase?Mg2+-ATPase,H+-ATPase and nicotinamide adenine dinucleotide kinase(NADK),kept higher content of oxidized form of nicotinamide adenine dinucleotide phosphate(NADP)and lower content of nicotinamide adenine dinucleotide(NADH).From these results,it could be concluded that pre-harvest spraying SA could maintain regular energy metabolism and the ions concentration balance between extra and intracellular,so as to delay aril breakdown of harvested longan fruit.5.Effects of pre-harvest spraying SA on cell wall metabolism in pulp of harvested longan fruit were studied.The results showed that compared with control,pre-harvest spraying SA decreased activities of cell wall-degrading enzymes obviously,such as petinesterase(PE),polygalacturonase(PG),cellulose(Cx)and?-galactosidase,retarded the decrease of the contents of cell wall material,such as protopectin,cellulose and semicellulose,and delayed the increase in the content of soluble pectin.From these results,it could be concluded that pre-harvested spraying SA could obviously reduce activities of cell wall-degrading enzymes which result in decreasing cell wall material to maintain the integrity of cell wall,delaying the aril breakdown of harvested longan fruits.6.Effects of pre-harvest spraying SA on disease-resistance in pulp of harvested longan fruit were evaluated.The results showed that compared with control,pre-harvest spraying SA kept higher activities of disease defense-related enzymes,such as chitinase,?-1,3-gulcanase,phenylalanine ammonialyase(PAL)and peroxidase(POD),and increased the content of resistance substances,such as ligninignin.From these results,it could be concluded that pre-harvested spraying SA could obviously enhance the activities of disease defense-related enzymes and the content of resistance substances,resulting in retarding the decrease of disease resistance,and further delaying the aril breakdown of harvested longan fruits.