Analysis Of The Mechanism For The Difference In Citric And Malic Acid Accumulation Between Citrus Leaves And Pulp

Organic acid is an important factor in determining flavor and quality of citrus fruits.And different cultivars have their own citric acid accumulation and sugar content,which confer specific flavor in citrus fruits.Generally,the accumulation of organic acid in citrus fruit is a complex and precise metabolic process and co-regulated by synthesis,decomposition and utilization,and storage and transportation.Thus,researches on citrus fruit organic acid accumulation theory make great sense in regulating organic acid content,improving the quality and flavor and satisfying the consumers’ demand.We collected the mature fruit on 10.27 and 10.7,and also collected the new leaves and old leaves on the same tree.Five cultivars,Newhall navel orange(NHR,Citrus sinensis cv.Newhall),Guoqing No.1 mandarin(WM,C.unshiu cv.Guoqing No.1),Bendizao tangerine(BDZ,C.reticulata cv.Bendizao),Hongmeiren(HMR,C.reticulata cv.Aiyuan28),and HB pomelo(HB,C.grandis cv.HB pomelo)were used in this study.The contents of citric acid and malic acid in different organs were measured.We also measured the expression of key genes related to organic acid accumulation.The results are as follow:1.The contents of organic acid in leaf and fruit have significant differences.In leaves,malic acid content is more than citric acid content,while the accumulation in fruit is the opposite.Meanwhile,malic acid content in leaves is more than that in fruit,while citric acid in leaves is significant lower than that in fruit.2.The analysis of citric acid metabolism related genes indicated that: the expression of CS,ACO,ACL enzyme gene is basically higher while the expression of CsPH8 is significantly lower in citrus leaves than in fruits.In different cultivars,the activity of VHA gene varied.In BDZ,HB,and HMR,the activity of CsCit1 were significantly higher in leaves than in fruits.These result revealed that citric acid low accumulation is mainly because the low activity of CsPH8 and high expression levels of citric.Besides,the high activity of CsCit1 in vacuole,which lead to more citric acid transported out of the tonoplast in some cultivars,is also related.3.The analysis of malic acid metabolism related genes suggest: Malic acid synthesis gene CsMLS,CsFUM,CscNAD-MDH1 and transportational gene CsMLT,CsALMT1 basiclly have a higher activity in leaves than in fruit.Thus we infer that high activity of synthesis and transportational ability result in the higher accumulation in citrus leaves than in fruit.