Physiological Mechanism Of Salicylic Acid Induces Disease Resistance Of Harvested Longan Fruit Against Phomopsis Longanae Chi

Longan(Dimocarpus longan Lour.)is a famous fruit in southern China,and produced mainly in tropical and subtropical provinces including Fujian,Hainan,Guangdong,Guangxi.Longan fruit has high nutritional value,but matures at high temperature and high humidity season from July to September,and thus the postharvest physiological metabolism of longan fruit is vigorous.After 3 or 4 days of storage at room temperature,the pericarp was being browned,and aril was breakdown,finally caused deterioration.It was previously found that the Phomopsis longanae Chi was the main pathogenic fungus inducing fruit decay of harvested longan.Our previous studies found that salicylic acid(SA)treatment can effectively inhibit the occurrence of fruit decay of harvested longan.However,the mechanism of inhibition is still unknown.In present research,the disease resistance mechanism of harvested longan fruit induced by salicylic acid in vivo was researched through the aspects of respiratory metabolic pathway,antioxidant reaction,disease resistance defense response,tissue energy formation characteristics,membrane system integrity,membrane lipid compositions and membrane lipid degradation related enzyme activities with ’Fuyan’ longan fruit as material.The results are expected to obtain a new theoretical basis for elucidating the possible mechanism of SA inhibiting longan fruit decay,and further to provide a scientific basis and practical guidance for controlling the postharvest disease and prolonging the fruit storage period of harvested longan fruit.The main results of this study are listed as below:1.Compared with P.longanae-inoculated longans,SA treatment decreased fruit respiration rate,pericarp cell membrane permeability,the indices of fruit disease,pericarp browning,aril breakdown,and fruit weight loss percentage.Meanwhile,SA treatment delayed the decrease in L*value,and contents of chlorophyll,carotenoid,anthocyanin,flavonoids,and total phenolic in the pericarp,maintained higher contents of total sugar,sucrose,total soluble solid(TSS)and vitamin C in pulp of longan fruit,and delayed the increase of titratable acid(TA)and reducing sugar content in P.longanae-infected longan fruit.Therefore,it was indicated that SA treatment could maintain better post-harvest storage quality,and improve its storability.2.Compared with P.longanae-inoculated longans,SA treatment reduced the activities of respiratory terminal oxidase including cytochrome C oxidase(CCO)and ascorbic acid oxidase(AAO),as well as the activities of phosphohexose isomerase(PGI)and succinodehydrogenase(SDH),maintained higher activities of nicotinamide adenine dinucleotide kinase(NADK),total activity of glucose-6-phosphate dehydrogenase(G-6-PDH)and 6-phosphogluconate dehydrogenase(6-PGDH),enhanced the contents of nicotinamide adenine dinucleotide phosphate(NADP)and its reduced form(NADPH),but decreased the contents of nicotinamide adenine dinucleotide(NAD)and its reduced form(NADH),keep higher NAD/NADH ratio.The results demonstrated that SA treatment delayed the development of postharvest disease in P.longanae-infected longans through increasing pentose phosphate pathway(PPP)and decreasing activities of respiratory terminal oxidase.3.Compared with P.longanae-inoculated longans,SA treatment decreased the superoxide free radical(O2-·)production rate,inhibited the production of malondialdehyde(MDA),and increased the activities of reactive oxygen scavenging(ROS)enzymes including superoxide dismutase(SOD),catalase(CAT),and ascorbate peroxidase(APX).Additionally,SA treatment maintained the contents of ascorbic acid(AsA)and glutathione(GSH),enhanced the DPPH scavenging ability and reduction ability.The results demonstrate that SA treatment delayed the development of postharvest disease in P.longanae-infected longans through enhancing ROS-scavenging ability,inhibiting the accumulation of ROS effectively,and decreasing membrane lipid peroxidation in pericarp of harvested longan fruit.4.Compared with P.longanae-inoculated longans,SA treatment decreased the activities of fatty acid degrading enzymes including hospholipase D(PLD),lipase and lipoxygenase(LOX),maintained higher contents of phosphatidylcholine(PC)and phosphatidylinositol(PI),as well as the unsaturated fatty acid including oleic acid(C18:1),linoleic acid(C18:2)and linolenic acid(C18:3),inhibited the increase of phosphatidic acid(PA)content and saturated fatty acids including palmitic acid(C16:0),stearic acid(C18:0)and lignoceric acid(C24:0),and thereby maintained higher ratio of unsaturated fatty acids to saturated fatty acid(U/S)and the index of unsaturated fatty acids(IUFA).The results demonstrate that SA treatment delayed the development of postharvest disease in P.longanae-infected longans through decreasing the activities of fatty acid degrading enzymes,inhibiting the degradation of membrane lipid,and preserving maintaining the integrity of cell membrane structure of longan pericarp5.Compared with P.longanae-inoculated longans,SA treatment maintained higher contents of adenosine triphosphate(ATP)and adenosine diphosphate(ADP),as well as energy charge,maintained lower adenosine monophosphate(AMP)content,increased the activities of Ca2+-ATPase,Mg2+-ATPase and H+-ATPase on plasma,vacuole and mitochondria.The results demonstrate that SA treatment delayed the development of postharvest disease in P.longanae-infected longans through maintaining higher energy charge and increasing the activities of ATPase.6.Compared with P.longanae-inoculated longans,SA treatment delayed the decrease of cell wall materials(CWM),maintained higher contents of chelate-soluble pectin(CSP),sodium carbonate-soluble pectin(NSP),cellulose and hemicellulose,alleviated the increase of water-soluble pectin(WSP)content,inhibited the activities of cell wall degrading enzymes like pectinesterase(PE),polygalacturonase(PG),cellulose(Cx)and β-galactosidase(β-Gal).The results demonstrate that SA treatment delayed the development of postharvest disease in P.longonae-infected longans through decreasing the activities of cell wall degrading enzymes and inhibiting degradation of CMW to maintain cell wall structure integrity,keep higher mechanical strength in pericarp of harvested longan fruit,and then improving the ability to resist the infection of pathogens.7.Compared with P.longanae-inoculated longans,SA treatment promoted the generation of H2O2 and lignin during the middle and late storage period,maintained higher activities of chitinase(CHI),β-1,3-glucanase(GLU),peroxidase(POD),polyphenol oxidase(PPO)and phenylalnine ammonia-lyase(PAL).The results demonstrate that SA treatment delayed the development of postharvest disease in P.longanae-infected longans through maintaining higher activities of desease resistance enzymes and contents of disease resistance substances in pericarp of harvested longan fruit.