Salicylic Acid Treatment Increasing Storability Of Harvested Longan Fruits And Its Action Mechanism

Longan(Dimocarpus longan Lour.)is characteristic tropical and subtropical fruit in southern China,which unique flavor,exceptionally tasty,higher nutritional value,commercial value and health efficacy.However,longan fruit matured at high temperature and humidity season,meanwhile,the longan fruit deteriorated rapidly due to physiological metabolic activity,accelerated respiration rate,pericarp browning,aril breakdown and pathogen infection after harvest.This study investigated the effects of the optimum concentration of salicylic acid(SA)treatment on postharvest physiology,quality and storability of harvested longan fruit in association with metabolisms of respiration,reactive oxygen species,membrane lipids,energy,cell wall materials and disease resistance with’Fuyan’ longan fruit as material,aimed to illuminate the mechanisms of improving the storability of harvested longan fruit by SA treatment.The results of this study were showed as follows:1.The effects of SA treatment on postharvest physiology,quality and storability of harvested longan fruit were studied.For the control fruits,SA treatment delayed the rise of fruit respiration rate and pericarp cell membrane permeability;kept higher L*value in epicarp,and the contents of chlorophyll,carotenoid and anthocyanin,flavonoids,total phenolic in pericarp of longan fruit,so as to hold the quality in pericarp of harvested longan fruit.The SA-treated fruit also maintained higher contents of total soluble solid(TSS),total sugar,sucrose and vitamin C in aril of longan fruit,but lower contents of titratable acid(TA)and reducing sugar;kept lower indices of pericarp browning,fruit disease,aril breakdown and lower fruit weight loss percentage,and held higher commercially acceptable fruit rate,so as to improve quality and storability of harvested longan fruit.2.The effects of SA treatment on respiration metablolism in pericarp of harvested longan fruit were studied.In contast to the control fruits,SA treatment declined the activities of respiratory terminal oxidase,like cytochrome C oxidase(CCO)and ascorbic acid oxidase(AAO)in the pericarp,and delayed fruit respiration rate of harvested longan fruit.Meanwhile,SA treatment retarded the activities of phosphohexose isomerase(PGI)and succinodehydrogenase(SDH)in pericarp,ultimately impaired embden-meyerhof-parnas(EMP)-tricarboxylic acid cycle(TCA);the nicotinamide adenine dinucleotide kinase(NADK)activity and the total activity of glucose-6-phosphate dehydrogenase(G-6-PDH)and 6phosphogluconate dehydrogenase(6-PGDH)in pericarp of SA-treated fruits were elevated.Also,SA treatment kept higher contents of nicotinamide adenine dinucleotide phosphate(NADP)and its reduced form(NADPH),but lower contents of nicotinamide adenine dinucleotide(NAD)and its reduced form(NADH)in pericarp,causing the higher NAD/NADH ratio in pericarp and pentose phosphate pathway(PPP).3.The effects of SA treatment on active oxygen species metabolism in pericarp of harvested longan fruit were studied.In contast to control longan fruits,SA treatment improved the activities of reactive oxygen species(ROS)scavenging enzymes including superoxide dismutase(SOD),catalase(CAT)and ascorbate peroxidase(APX),and the contents of reduced ascorbic acid(AsA)and reduced glutathione(GSH)in pericarp,thereby kept higher 1,1-diphenyl-2-picrylhydrazyl(DPPH)radical scavenging ability and reducing power in pericarp,which were beneficial for retarding the rise of superoxide free radical(O2-)production rate and malondialdehyde(MDA)content in pericarp of longan fruit,and delaying pericarp membrane lipid peroxidation and improving the integrity of pericarp cellular membrane structure of harvested longan fruit.4.The effects of SA treatment on membrane lipids metabolism in pericarp of harvested longan fruit were studied.Compared with the control fruits,SA-treated fruits could keep lower the activities of pericarp phospholipids and fatty acids degrading enzymes such as phospholipase D(PLD),lipase and lipoxygenase(LOX),thereby delayed the decline of contents of phosphatidylcholine(PC)and phosphatidylinositol(PI),kept higher unsaturated fatty acids relative contents like oleic acid(C18:1),linoleic acid(C18:2)and linolenic acid(C18:3),but lower PA content and relative contents of saturated fatty acids,like palmitic acid(C16:0),stearic acid(C18:0)and lignoceric acid(C24:0)in pericarp,thereby kept higher the ratio of unsaturated fatty acids to saturated fatty acid and index of unsaturated fatty acids.Therefore,SA treatment protected the cell membrane fluidity and cellular membrane structure in pericarp of harvested longan fruit.5.The effects of SA treatment on energy metablolism in pericarp of harvested longan fruit were studied.For the control fruits,SA treatment improved pericarp energy level of loangan fruit,that is increasing the contents of adenosine triphosphate(ATP),adenosine diphosphate(ADP)and energy charge,and decreasing adenosine monophosphate(AMP)content.In addition,SA treatment also kept higher activities of Ca2+ATPase,Mg2+-ATPase and H+-ATPase in pericarp membranes of protoplasma,vacuole and mitochordria,finally regulared ions equilibrium and stabilized pericarp membrance structure.6.The effects of SA treatment on cell wall materials metablolism in pericarp of harvested longan fruit were studied.As compared with the control fruits,SA treatment inhibited the activities of pectinesterase(PE),polygalacturonase(PG),cellulose(Cx)and β-galactosidase(β-Gal)in the pericarp,and controlled the degradation of cell wall materials(CWM),and then kept higher contents of chelate-soluble pectin(CSP),sodium carnonate-soluble pectin(NSP),cellulose and hemicellulose,but lower water-soluble pectin(WSP)content in pericarp.Thereby,SA treatment maintained the integrity of pericarp cell wall materials structure and improved storability of harvested longan fruit.7.The effects of SA treatment on disease resistance in pericarp of harvested longan fruit were studied.In contrast to control longan fruits,SA treatment enhanced the activities of phenylalnine ammonia-lyase(PAL),peroxidase(POD),polyphenol oxidase(PPO),chitinase(CHI)and β-1,3glucanase(GLU)in pericarp,and kept higher the contents of pericarp lignin and hydrogen peroxide(H2O2).Therefore,SA treatment could induce fruit disease resistance and control the disease development,improve the fruit quality harvested longan fruit.