Studies On The Mechanisms Of Chitosan Controlling The Disease Development Of Harvested Litchi Fruit Infected By Peronophythora Litchii

Litchi(Litchi chinensis Sonn.)is a famous fruit in the south of China and a featured fruit in subtropical areas.Nowadays,litchi has been planted globally in subtropical and tropical areas and litchi fruit has been favored by the consumers globally since its good flavor and rich nutrient.However,the high temperature and humidity of harvest season makes litchi fruit vulnerable to pathogen infection,which might lead to serious fruit decay.Peronophythora litchii is the principal pathogen that can cause disastrous postharvest disease of litchi fruit,which leads to the damage of fruit quality and the limitation of economic benefit.In order to reduce the use of artificial chemical fungicides,novel method that can improve and maintain the postharvest quality of litchi fruit should be offered.Previous researches indicated that chitosan treatment was able to control the postharvest disease of litchi fruit,but the mechanism of it was unclear.Therefore,this study investigated the effects of chitosan on the disease development of harvested litchis caused by P.litchii,regarding the metabolisms of respiration,energy,reactive oxygen species,membrane lipid and disease resistance substance,with the ’Wuye’ litchi fruit as material.Also,the effects of chitosan treatment on postharvest quality and storage behavior of litchis would be investigated.The objectives of this work were to find out the mechanism underlying the control effects of chitosan on litchi fruit disease and offer the scientific theoretical foundations,guidance and feasible methods for disease control of litchi fruit.The main results this research found are listed below:1.Chitosan treatment could effectively decrease the disease index and browning index of the P.litchii-inoculated litchis.It was indicated that chitosan treatment could control the development of P.litchii induced disease.2.For the P.litchii-inoculated litchis,the chitosan treatment could effectively decrease the respiration rate of litchi fruit,lower the activities of phosphohexose isomerase(PGI)and succinate dehydrogenase(SDH),and increase the total activities of glucose-6-phosphate dehydrogenase(G-6-PDH)and 6-phosphogluconate dehydrogenas(6-PGDH).Meanwhile,the treatment maintained higher activity of nicotinamide adenine dinucleotide kinase(NADK),lowered the contents of oxidized form of nicotinamide adenine dinucleotide(NAD)and reduced form of nicotinamide adenine dinucleotid(NADH),while increased the contents of oxidized form of nicotinamide adenine dinucleotide phosphate(NADP)and nicotinamide adenine dinucleotide phosphate(NADPH),enhanced pentose phosphate pathway(PPP)of respiration.The activities of respiratory terminal oxidases such as cytochrome C oxidase(CCO),ascorbic acid oxidase(AAO)and polyphenol oxidase(PPO)in the pericarp were also inhibited by chitosan treatment.It was indicated that the chitosan treatment could control the disease development by changing the activities of respiration related enzymes and the pathway of respiration.3.For the P.litchii-inoculated litchis,the chitosan treatment could maintain higher adenosine triphosphate(ATP),adenosine diphosphate(ADP)level and energy charge in the pericarp,meanwhile the chitosan treatment could remain higher activities of H+-ATPase,Ca2+-ATPase and Mg2+-ATPase in protoplasm,vacuole and mitochondria,retaining the balance of H+ and Ca2+,which might be beneficial to the normal osmotic pressure in protoplasm and the maintenance of membrane structure integrity.It was indicated that the chitosan treatment could control the disease development by offering more efficient ATP synthesis to satisfy the energy demand of resistance.4.For the P.litchii-inoculated litchis,chitosan treatment could significantly reduce the O2-·production rate and malondialdehyde(MDA)content,keep higher contents of ascorbic acid and reduce glutathione during storage.Chitosan could also keep higher activities of reactive oxygen species(ROS)scavenging enzymes(superoxide dismutase,catalase and ascorbate peroxidase)and 1,1-diphenyl-2-picrylhydrazyl(DPPH)scavenging ability and reducing power in pericarp of litchis.It was indicated that the chitosan treatment could reduce the accumulation of reactive oxygen species by retaining the activity of active oxygen scavenging in litchi pericarp,which could decrease the membrane lipid peroxidation and maintain the cell membrane structure of litchis,therefore,the chitosan treatment could control the P.litchii induced disease.5.For the P.litchii-inoculated litchis,chitosan treatment could effectively retard the rapid increase of membrane permeability,reduce the activities of fatty acid degradation enzymes(phospholipase D,lipase and lipoxygenase)in the pericarp,therefore retained higher contents of phosphatidylcholine(PC)and phosphatidylinositol(PI)and lower content of phosphatidic acid(PA),retarded the degradation of oleic acid(C18:1),linoleic acid(C18:2)and linolenic acid(C18:3),kept lower levels of myristic acid(C14:0),palmitic aicd(C16:0)and stearic acid(C18:0),consequently maintained higher ratio of unfatty acid to fatty acid(U/S)and index of unsaturated fatty acid(IUFA).It was demonstrated that the chitosan treatment could control the disease development by reducing the degradation of membrane lipid and alleviating the damage of membrane structure integrity.6.For the P.litchii-inoculated litchis,chitosan treatment could maintain higher phenylalanine ammonia-lyase(PAL)activity in the pericarp,which was beneficial to the synthesis of lignin and the resistance of pathogen invasion,keep lower polyphenol oxidase(PPO)activity,which could inhibit the phenolics oxidation followed by lower browing index,retain higher chitinase(CHI)and β-1,3-glucanase(GLU)activities,consequently enhance the disease resistance.It was indicated that chitosan treatment could control the development of P.litchii-induced disease through maintaining the contents of endogenous resistant substances and activities of disease resistance related enzymes.7.Compared with the control litchi fruit,the chitosan treatment could significantly reduce fruit respiration rate,delay an increase of cell membrane permeability in pericarp,maintain higher contents of chlorophyll,carotenoid,anthocyanin and flavonoid in pericarp and subsequently retard the change of apparent color of litchis.The application of chitosan could also effectively inhibit the decreases of contents of total soluble solids(TSS),titratable acid(TA),total soluble sugars and vitamin C in litchi pulp,and subsequently maintain better quality and flavor in litchi pulp.Furthermore,chitosan treatment could maintain higher rate of commercially acceptable fruit,and effectively reduce weight loss from litchis,with the developments of pericarp browning and fruit disease in litchi fruit retarded.It could be concluded that chitosan treatment could effectively delay fruit senescence,maintain higher fruit quality,and exhibit better storage behavior of litchi fruit.Thus,it is suggested that application of chitosan may be a promising method for maintaining quality and extending shelf life of harvested’Wuye’ litchi fruit.