Effects Of γ-aminobutyric Acid(GABA) Treatment On Chilling Tolerance Of Cold-Stored Chinese Olive Fruit And Its Mechanism

Chinese olive(Canarium album(Lour.)Raeusch)is one of the unique subtropical fruit with high nutritional value in southern China.With the increasing consumption demand,low-temperature storage has become one of the most effective means for the export and sale of Chinese olive fruit.However,Chinese olive fruit is a cold-sensitive fruit,which is easy to produce chilling injury in the low-temperature environment,resulting in inferior quality.In this study,’Tanxiang’ Chinese olive(Canarium album(Lour.)Raeusch cv.Tanxiang)fruit,the main cultivated Chinese olive variety in Fujian Province of China,was used as the experimental material and stored at(2±1)℃.The objectives of this study were to select the optimum concentration of y-aminobutyric acid(GABA)for cold-stored Chinese olive fruit and according to storage effects of different concentrations of GABA,and investigate the effects of the optimum concentration of GABA treatment on physiological change and chilling tolerance of Chinese olive fruit.Therefore,the effects of optimum concentration of GABA treatment on quality change,storage behavior and chilling tolerance of cold-stored Chinese olive fruit in perspectives of its respiratory metabolism,reactive oxygen species(ROS)metabolism,membrane lipid metabolism,energy metabolism,phenolic substances metabolism were studied in this research.The results are showed as follows:1.The preservation effects of GABA treatments at different concentrations on cold-stored Chinese olive fruit were studied.Compared with control fruit stored at(2±1)℃,GABA treatments at different concentrations(0.5 mmo1·L-1,1.0 mmol·L-1,1.5 mmol·L-1 and 2.0 mmol·L-1 GABA)delayed the fruit respiratory climacteric and the increase in cell membrane permeability,maintained higher contents of TSS,total sugar,titratable acid,vitamin C and tannin in pulp of cold-stored Chinese olive fruit,and retarded the decline in contents of pulp pigment.GABA treatments also inhibited chilling injury index and browning index,and kept higher rate of commercially acceptable fruit and lower water loss percentage.Especially,the effects of 1.0 mmol·L-1 GABA treatment was optimum among thetreatments with different GABA concentrations.According to these results,the 1.0 mmol·L-1 GABA treatment was used as the optimum concentration for maintaining storage quality and enhancing chilling tolerance of ’Tanxiang’ cold-stored Chinese olive fruit.2.The effects of GABA treatment on physiology,quality and storability of cold-stored Chinese olive fruit were investigated.Compared with control fruit,GABA treatment decreased the respiration intensity,delayed the increase in cell membrane permeability of cold-stored Chinese olive fruit,kept higher pulp nutrient contents including TSS,sugar,acid,vitamin C and tannin,and delayed the decline of chlorophyll content in the pericarp.GABA treatment also decreased the chilling injury index,browning index of cold-stored Chinese olive fruit,and maintained higher commercially acceptable fruit rate and lower water loss percentage.According to these results,GABA treatment enhanced cold tolerance and storage quality of cold-stored Chinese olive fruit.3.The effects of GABA treatment on respiratory metabolism in pulp of cold-stored Chinese olive fruit were studied.The results showed that,compared with control fruit,GABA treatment kept lower activities of ascorbic acid oxidase(AAO),cytochrome C oxidase(CCO),succinodehy drogenase(SDH),and phosphohexose isomerase(PGI)of cold-stored Chinese olive fruit,maintained higher activities of nicotinamide adenine dinucleotide kinase(NADK)and total activity of glucose-6-phosphate dehydrogenase(G-6-PDH)and 6-phosphate gluconate dehydrogenase(6PGDH),as well as the content of oxidized form of nicotinamide adenine dinucleotide phosphate(NADP)and its reduced form(NADPH),but kept lower level of oxidized form of nicotinamide adenine dinucleotide(NAD)and its reduced form(NADH),hence maintaining higher ratio of NAD/NADH.4.The effects of GABA treatment on ROS metabolism in pulp of coldstored Chinese olive fruit was studied.The results showed that,compared with control fruit,GABA treatment reduced the increase of superoxide anion radical(O2-·)production rate and malondialdehyde(MDA)content,improved the activities of ROS scavenging enzymes including superoxide dismutase(SOD),catalase(CAT)and ascorbate peroxidase(APX),kept higher contents of glutathione(GSH)and ascorbic acid(AsA),and kept higher DPPH scavenging activity and reducing power.5.The effects of GABA treatment on membrane lipid metabolism in pulp of cold-stored Chinese olive fruit were investigated.The results showed that,compared with control fruit,GABA treatment reduced the activities of lipase,phospolipase D(PLD)and lipoxygenase(LOX),delayed the increase of phosphatidic acid(PA)content,maintained higher contents of phosphatidylinositol(PI)and phosphatidylcholine(PC),kept higher relative contents of unsaturated fatty acids and index of unsaturated fatty acids(IUFA),and maintained lower relative content of saturated fatty acids in pulp of cold-stored Chinese olive fruit.6.The effects of GABA treatment on energy metabolism in pulp of cold-stored Chinese olive fruit were studied.The results showed that,compared with control fruit,GABA treatment kept higher ATP,ADP content and energy charge,maintained the activities of H+-ATPase,Ca2+ATPase,Mg2+-ATPase.According to these results,GABA treatment balanced osmotic pressure of Ca2+,Mg2+,H+of cold-stored Chinese olive fruit through higher energy charge level and the activities of ATPase.7.The effects of GABA treatment on phenolic substance metabolism in pulp of cold-stored Chinese olive fruit were studied.The results showed that,compared with control fruit,GABA treatment kept thecontents of total phenolics and flavonoid,maintained phenylalanine ammonia-lyase(PAL)activity,and delayed the rise of polyphenol oxidase(PPO)and peroxidase(POD)activities,inhibiting browning of cold-stored Chinese olive fruit.