Research On Synthesis And Degradation Of Citric Acid In Jing'an Ponkan Fruit

Citric acid,the major organic acid in citrus,is one of the important indicators which influence flavor quality of citrus fruit.In this research,Jing'an Ponkan(Citrus reticulata Blanco cv.Ponkan)fruits were used as materials to investigate the effects of development stage and postharvest treatments on organic acids content,citric acid metabolism-related genes expression of fruit by high performance liquid chromatography(HPLC)and real-time fluorescence quantification(Q-PCR).The main results are as follows:1.During the development of Jing'an Ponkan fruit,weight of singe fruit,total soluble solids(TSS),citrus color index(CCI)andtotal sugar content were gradually increased with fruit maturation.Citric acid was the main organic acid in fruit,which increased and then decreased during fruit development,was consistent with the variation of total organic acids content and titratable acidity(TA),.The results of citric acid metabolism-related genes expression and correlation analysis revealed that CitCS1 relative expression decreased slightly and CitCS2 relative expression increased first and then decreased in Jing'an Ponkan fruit,both of them had no significant correlation with citric acid content;CitAco1 relative expression showed a slow decline as a whole,which also had no significant correlation with citric acid content,CitAco2/3 relative expression rose slowly at early developmental stage then increases rapidly during fruit maturation,which showed a significant negative correlation with citric acid content;CitIDH1 relative expression trend showed decreased firstly then increased,but CitIDH2/3,CitGADs and CitGS2 relative expressionpattern was reverse,which increased firstlyfollowed by decreased,CitIDH1/2 relative expression and citric acid content showed a significant negative correlation,but CitGADs or CitCS2 gene expression wasn't relevant to the amount of citric acid.The results indicated that organic acid content in Jing'an Ponkan fruit was determined by citric acid and citric acid accumulation was mainly regulated by CitAco2/3 and CitIDH1/2.2.Jing'an Ponkan fruitlet(120d after full bloom)were used to investigated the effects of hot air(40?,48 h)treatment(HAT)on fruitlet quality and citric acid metabolism.The results showed that HAT significantly increased weight loss rate and total sugar content of fruit,had no significant effect on CCI and TSS content;and it significantly decreased organic acids especially citric acid content of fruitlet.Gene expression results showed that HAT could induced the gene expression of CitCS1,which had significantly different level in CitCS1 expression.The expression levels of CitAcos family member genes were also promoted.HAT could induced CitIDH2 temporary expression and promoted the gene expression of CitIDH1/3.In addition,HAT also significantly upregulated CitGAD4 and CitGAD5 genes expression,but had no significant effect on CitGS2 expression.These results suggested that HAT(40?,48 h)could promoted the degradation of citric acid in Jing'an ponkan fruitlet,then organic acids content was decreased,which was guided by the changes of citric acid metabolism-related gene expression,and the upregulation of CitAcos,CitIDHs and CitGAD4 expression levels maybe the major reason for organic acids decline in citrus fruit during maturation.3.The effected of postharvest HAT and chitosan coating on fruit quality and citric acid metabolism in mature Jing'an Ponkan fruits were investigated.The results showed that compared with the control,fruit weight loss rate,the decline of TSS and total sugars content were accelerated and delayed by HAT(40 ?,48 h)and 1.0% chitosan coating treatment respectively,but both treatments had no significant effect on CCI.During the whole storage period,the trend of Jing'an Ponkan fruit citric acid content was consistent with total organic acids and TA content,which decreased with the prolongation of storage time,hot air treatment significantly promoted but chitosan coating significantly delayed citric acid degradation in Jing'an Ponkan fruit,.Consistent with the fruitlet research results,the effects of hot air and chitosan coating treatment on fruits organic acids had no direct relation to CitCSs expression.HAT could induced CitAco2/3,CitIDH2/3 and CitGAD4 expression in Jing'an Ponkan during fruit storage period,but had no significant effect on CitGSs expression,The enhanced expression of degradation-related genes was closely related to the degradation of citric acid.Chitosan coating treatment could inhibited CitAco3,CitGAD4 expression during fruit storage,which leading to the degradation rate of citric acid was slowed by chitosan coating during storage.These results showed that the degradation of organic acids in Jing'an Ponkan mature fruit was regulated by GABA pathway.