Chitosan Treatment Increasing Storability Of Harvested Longan Fruits And Its Action Mechanism

Longan(Dimocarpus longan Lour.)is a major cultivated subtropical fruit in the south of China which has high nutritional and commercial value.However,longan fruit mature in warm season,and can be stored only for 3 to 4 days at ambient temperature.Longan fruit deteriorate easily due to percarp browing,aril breakdown and disease,which seriously influenced the postharvest quality,transportation and consumption.'Fuyan'longan(Dimocarpus longan Lour.Cv.'Fuyan')fruit,the major cultivar in Fujian Province of China,was used as the experimental material.The objectives of this research were to select the optimum concentration of chitosan for postharvest treatment,and to discuss the effects of chitosan treatment on postharvest physiology,quality and storability,reactive oxygen species(ROS)metabolism,membrane lipid metabolism,energy metabolism,antifungal substances metabolism,and therefore to explain the mechanism of improved storability of longan fruit by chitosan treatment.The results were showed as follows:1.The optimum concentration of chitosan treatment was selected for'Fuyan' longan fruit.Compared with control fruit,chitosan treatment of different concentrations(Kadozan were diluted 250,500,750and 1000 times,respectively)could decrease the respiration rate and pericarp cell membrane permeability of longan fruit,keep higher contents of TSS,total sugar,sucrose,reducing sugar and vitamin C in aril of longan fruit,delay the decline in contents of pericarp pigment,and maintain higher flavonoids and total phenolic contents in pericarp of longan fruit.Chitosan treatment could also decrease pericarp browning index,aril breakdown index and fruit decay index,and maintain higher rate of commercially acceptable fruit and lower water loss percentage.According to these results,the Kadozan diluted by 500 times was the appropriate concentration for maintaining storage quality and storability of 'Fuyan' longan fruit during the postharvest storage at(25±1)?.2.The effects of appropriate concentration of chitosan treatment on physiology,quality and storability of harvested longan fruit were studied.Compared with control,the results showed that chitosan treatment could decrease the respiration rate and pericarp cell membrane permeability of longan fruit,keep higher nutrient contents including TSS,total sugar,sucrose,reducing sugar and vitamin C in aril of longan fruit,maintain higher pericarp flavonoids,total phenolic content,and delay the decline of chlorophyll,carotenoid and anthocyanin contents in the pericarp.Chitosan treatment could also decrase the pericarp browning index,aril breakdown index,fruit decay index,and maintain higher the rate of commercially acceptable fruit and lower water loss percentage.3.The effects of the appropriate concentration of chitosan treatment on ROS metabolism in pericarp of harvested longan fruit were studied.The results showed that,compared with control,the appropriate concentration of chitosan treatment could decrease superoxide anion radical(O2-.)production rate and malondialdehyde(MDA)content,increase the activities of ROS scavenging enzymes including superoxide dismutase(SOD),catalase(CAT)and ascorbate peroxidase(APX),delay the decline in antioxident substances contents such as ascorbic acid(AsA)and glutathione(GSH),and keep higher DPPH scavenging activity and reducing power.Therefore,it could be concluded that the appropriate concentration of chitosan treatment could improve the scavenging capacity of ROS,reduce the reactive oxygen species accumulation and membrane lipid peroxidation,maintain the integrity of cellular membrane structure in pericarp of longan fruit,and thus improve the storability of harvested longan fruit.4.The effects of the appropriate concentration of chitosan treatment on membrane lipid metabolism in pericarp of harvested longan fruit were studied.Compared with control,the results showed that chitosan treatment could decrease the pericarp cell membrane permeability,inhibit the activities of membrane lipid-degrading enzymes such as lipoxygenase(LOX),lipase and phospolipase D(PLD),keep higher relative contents of unsaturated fatty acids and lower relative content of saturated fatty acids,maintain higher index of unsaturated fatty acids(IUFA)and the ratio of unsaturated fatty acids to saturated fatty acids(U/S)in pericarp of longan fruit.According to the result,the improved storability of harvested longan fruit by chitosan treatment may due to the reduced fatty acids degrading enzymes activities of cell membrane,leading to delayed degradation of cell membrane unsaturated fatty acids,and thus the cell membrane structure integrity was kept.5.The effects of the appropriate concentration of chitosan treatment on energy metabolism in pericarp of harvested longan fruit were studied.The results showed that,compared with control fruit,the appropriate concentration of chitosan treatment could keep higher ATP,ADP content and energy charge level,increase the activities of H+-ATPase,Ca2+-ATPase,Mg2+-ATPase and NAD kinase,and decrease the activity of cytochrome c oxidase(CCO)and the contents of NAD and NADH,and maintain higher contents of NADP and NADPH.According to these results,the improved storability of harvested longan fruit is related with the higher ATP content and energy charge maintained by chitosan treatment.6.The effects of the appropriate concentration of chitosan treatment on antifungal substances metabolism in pericarp of harvested longan fruit were studied.The results showed that chitosan treatment could enhance the activities of disease defense-related enzymes such as phenylalanine ammonialyase(PAL),?-1,3-gulcanase(GLU)and chitinase(CHI),induce the increase of activities of PPO and POD at early storage period,and increase the contents of antifungal substances such as lignin,flavonoid and total phenols.According to these results,the elevated storability of harvested longan fruit is associated with the increased activities of disease defense-related enzymes and contents of antifungal substances induced by chitosan treatment.