Characterizing the Metabolic Regulation of Organic Acids in the Fruits of ‘Minihyang’ Mandarin at Different Developmental Stages and on Different Rootstocks

This study was conducted to evaluate the effect of different rootstocks on the organic acid content of 'Minihyang' mandarin at different developmental stages. The titratable acidity of 'Minihyang' fruit was the highest in the early stage and decreased rapidly during maturity. The level of acidity in the Flying Dragon rootstock was significantly higher than that in the trifoliate orange rootstock at the early stage of growth, but the difference between them was insignificant during maturity. The content of organic acid showed a similar trend to that of titratable acidity, but the difference between rootstocks was very small. In the early stages of fruit development of 'Minihyang' mandarin, most of the organic acids were citric acid, but during maturation and ripening, malic acid was slightly more than citric acid. Unlike the decrease in citric acid, citrate synthase (CS) activity was maintained at a high level throughout the growing period. On the other hand, with the increase in malic acid, the activities of NAD-malic dehydrogenase (MDH) and NADP-malic enzyme(ME) gradually increased. The expression of CS genes related to citric acid synthesis increased gradually with fruit growth, while the expression of aconitase genes related to decomposition increased gradually. MDH and ME genes related to malic acid and oxalic acid synthesis in mitochondria increased with fruit ripening, while cytoplasmic MDH decreased. In addition, in the expression of phosphoenolpyruvate carboxylase (PEPC), PEPC1 was maintained high throughout the fruit growth period, while PEPC2 showed a gradual increase. The organic acid content and composition of 'Minihyang' fruit were regulated by the degree of citric acid synthesis and decomposition and malic acid synthesis. In terms of accumulation of organic acids in vacuole, ATPase genes were expressed at a constant level, while PPase genes were not expressed consistently. The results of this study showed that the organic acid content and composition of 'Minihyang' fruit were regulated by the degree of citric acid synthesis and decomposition and malic acid synthesis.