Effects Of Environmental Factors On Citric Acid Accumulation And Its Related Genes In Citrus

The content and composition of organic acids greatly affect the flavor and quality of fruits.Citric acid is a predominant organic acid in citrus fruits.The accumulation of citric acid depends on the integrative process of citric acid synthesis,storage,and decomposition.The increase and decrease of citric acid enzyme’s activity affects the citric acid accumulation.The content of citric acid in fruits is not only affected by internal genetic factors,but also by different environmental and growth factors.Studying the effects of different environmental conditions on the changes of citric acid content and citric acid accumulation related genes in citrus,it is of great significance to improve the citrus accumulation mechanism of citrus fruits and improve the flavor quality.Therefore,a study was conducted to elucidate the mechanism of citric acid accumulation in citrus fruit under different environmental conditions.Fruits of satsuma mandarin cultivar were used as a plant material and subjected to temperature,light,drought,pH,and nutrients(nitrogen,potassium,phosphorus,and magnesium)treatments.Moreover,citrate content and expression of citrate accumulation related genes in juice sacs were determined.Followings are the main findings of our experiments:(1)Low-temperature(4℃)significantly increased the citrate content in citrus juice sacs,which may be related to the down-regulation of ACOs and ACLβ1.The increment in pH of growing media increased the citrate content in citrus juice sacs.The significant increase in citric acid content was found at pH 6.8 which may due to the down-regulation of ACLs and ACOs and the up-regulation of CSs genes.(2)Foliar application of nitrogen fertilizer increased the citrate content and possibly due to the down-regulation of citrate degradation genes(ACO1,AC02,and ACLs).On the other hand,foliar application of phosphorous fertilizer decreased the citrate content which resulted from the up-regulation of ACOs and ACLs.Similarly,significant reduction was observed under potassium application and was specifically related to the up-regulation of ACOs and ACLs,and the down-regulation of CsPH8.Foliar application of magnesium fertilizer reduced the citric acid content in fruits,which was related to the up-regulated expression of ACO1,AC02,and ACLs and the significant down-regulation of CSs and CsPH8.(3)The up-regulation of CSs and the significant down-regulation of ACO1,AC02,ACLal,and ACLa2 may be the possible reason for significant increase in citric acid content in fruit,under drought stress.(4)The citric acid content in fruits was significantly higher under shading treatment than that under non-shading treatments,which was correlated with down-regulated expressions of ACOs and ACLs.In summary,by analyzing the effects of different environmental factors on citrate-accumulated enzymes,it is found that the key genes affecting citric acid accumulation under different treatments were ACOs and ACLs.In addition,CSs play an important role in the regulation of citric acid accumulation by pH changes,drought stress and foliar nutrient application(magnesium fertilizer and combine application of phosphorous and potassium fertilizers).However,down-regulation of CsPH8,in Mg,K and combine application of K and P fertilized fruits resulted in reduced citrate content as compared to the control fruits.