Research On Synthesis And Degradation Of Citric Acid In 'Newhall' Navel Orange Fruit

To study the relationship between the changes of citric acid and the expression of related genes during fruit development and postharvest storage of ‘Newhall' navel orange fruit(Citrus sinensis Osbeck ‘Newhall'),the changes of organic acid components in fruit were determined by high performance liquid chromatography(HPLC)and the expression of citric acid metabolism related genes was analyzed by real-time quantitative PCR(Q-PCR).The main results are as follows:1.During the development of ‘Newhall' navel orange fruit,fruit weight,diameter and longitudinal diameter,citrus color index(CCI)and total soluble solids(TSS)showed a gradually increased trend.The fluctuation of vitamin C(Vc)content showed a M-type mode,and the content of total sugars increased first then decreased.Citric acid was the major organic acid in ‘Newhall' navel orange,followed by quinic acid,and malic acid was the least.With the ripening of fruits,the content of citric,quinic and tartaric acid showed a downward trend,and the change of total organic acids and TA content was the same as that of citric acid.The expression of CitCSs was generally stable and not significantly correlated with citric acid accumulation.The expression of CitPEPCKs was significantly changed,but negatively correlated with the content of citric acid.However,the expression levels of Cit Acos,CitIDH1/3,Cit GAD4,CitACL?1/? and CitPEPCKs increased with the decrease of citric acid in the ‘Newhall' navel orange fruit,and were negatively correlated with the change of citric acid content which might be a major cause that lead to the decrease of organic acids content in navel orange during fruit ripening.2.The young(90 d after full blossom)and mature fruit of ‘Newhall' navel orange were used to investigated the effects of hot air treatment(HAT,40? and 72 h)on citric acid metabolism.The results showed that HAT could help the young fruit of ‘Newhall'navel orange to become more chlorotic,but it was not benefit to the ripening of navel orange fruit from yellow to orange.In addition,the weight loss rate of ‘Newhall' navel orange increased linearly throughout the whole storage period.HAT significantly increased the weight loss rate of fruit,and the weight loss rate of young fruit was higher than mature fruit during the whole storage stage.Moreover,citric acid is the major organic acid in‘Newhall' navel orange fruit,HAT promotes the degradation of citric acid during storage time which showed a gradual downward trend as the same as organic acids,TA changes.Throughout storage,young fruit has a higher citric acid,organic acid,and titratable acidity(TA)level than mature fruit.HAT inhibited the expression of citrate synthesis related genes include CitCSs and CitPEPCs in fruit,but the expression of CitAcos,Cit IDH3,CitGAD4,CitGS2,CitACLs,CitPEPCKs and CitFBPases which involved in citrate degradation were up-regulated.In addition,the citric acid metabolism related genes(CitCS1,CitGAD4,CitACLs,CitPEPCKs and CitFBPases)in high acid fruit(young fruit)showed high abundance expression and HAT transiently induced CitIDH2 expression.In addition,only CitPEPC2,CitGAD4 and CitFBPase1 showed higher expression in low acid fruit(mature fruit),and HAT increased the expression of CitIDH2 at later storage period.The results of this experiment showed that citric acid was degraded by HAT through three routes(the routes of GABA,GS and ACL)especially the ACL pathway in ‘Newhall' navel orange young fruit treated,and mature fruit mainly through the GABA and ACL pathway.3.The response of citric acid metabolism and gene expression to exogenous citric acid in postharvest ‘Newhall' navel orange fruit was studied.Exogenous citric acid treatment increased the content of citric acid in navel orange fruit and accelerated the degradation rate of citric acid.This could be achieved by the induction of the fruit GABA pathway(CitAco1,CitGAD4),but not the glycolysis pathway and ACL pathway.In summary,the content of organic acid in fruit is mainly affected by the change of citric acid content which is the main organic acid of ‘Newhall' navel orange fruit.The accumulation of citric acid in the fruit is a complex process which correlates with citrate synthesis,transport mechanisms and degradation metabolism and other aspects.The GABA,GS and ACL pathway,which are three routes in citric acid degradation,may play different role in the degradation of citric acid during development and postharvest storage periods in ‘Newhall' navel orange fruit.