Effects Of Citrate Content On The Storage Performance Of ‘Bingtang’ Sweet Orange And Expression Analysis Of Genes In Citrate Metabolism

Organic acid is one of the important indexes that affect the flavor and quality of citrus fruit,its content is closely related to the storage performance and shelf life of the fruit.Citrate is the major organic acid in pulp juice cells of most citrus fruits.During postharvest storage,citrate is consumed gradually with respiration and energy metabolism,which can accelerate the process of fruit aging,lead to worse fruit quality and commercial value.Therefore,it is very important to maintain the fruit quality in the postharvest production of citrus industry.‘Bingtang’ sweet orange(BTO)(Citrus sinensis Osbeck),is an excellent variety selected in Hunan province,and belongs to the low-acid type.The fruits of BTO are very prone to off-flavor formation after harvest in production practice due to its low level of organic acids,which seriously affected the taste and flavor of the fruit.‘Qianyang Bingtang’ navel orange(QY-BTNO)and ‘Jinxiu Bingtang’ sweet orange(JX-BTO)are bud mutants of the common ‘Bingtang’ sweet orange(CK-BTO).Both mutants are characterized by higher and lower accumulation of organic acids in fruit juice sacs respectively,which were selected to compare the difference in titratable acid(TA),total soluble solid(TSS)and vitamin C(VC)content and disease incidence during storage.Further analysis of the contents of citrate and its metabolism related genes,ATP content and antioxidant enzyme activities in high acid QY-BTNO and low acid JX-BTO fruit during storage,aiming to explain the mechanism of high acid accumulation in QY-BTNO.The main results are as follows:1.The high expression of PH8 may be the main reason for the high accumulation of citrate in QY-BTNO fruits:these results indicated that more citrate in the mitochondrion transports into the cytoplasm by lower expression of ACO3 in QY-BTNO than in JX-BTO fruits at mature period and previous stage of storage.Moreover,the lower expression of cyt-ACO(ACO2),NADP-IDH1 and ACLα1decrease the ability to utilize citrate.While in mature QY-BTNO fruits,more citrate in cytoplasm does not utilize but accumulate in vacuole because the strikingly higher expression level of PH8 increase the ability of the proton pump to uptake citrate into the vacuole.2.Transcription factor Csb HLH063(TT8)and protein degradation related gene APD2 may be involved in the regulation of citrate metabolis:during the whole storage period,the expression of Csb HLH063(TT8)and APD2 is similar to that of PH8,which is significantly higher in QY-BTNO,and the difference of expression has maintained the whole storage period.3.The consumption speed of high-acid QY-BTNO was slower than that of low-acid JX-BTO:in the postharvest storage time:the ATP content and POD activity of QY-BTNO were significantly lower than that of JX-BTO,indicating that the respiration and metabolism of QY-BTNO fruits were slower and consumed less substrate during the whole storage period.4.The high-acid QY-BTNO fruits had better storage performance attributed to the lower disease incidence during postharvest storage.