Changes in organic acids content during‘Fengtang'plum (Prunus salicina) fruit development in relation to malic acid metabolism related enzymes

Abstract: 【Objective】In order to understand the mechanisms regulating acidity in plum fruit, the composition and contents of organic acids in‘Fengtang'plum (Prunus salicina‘Fengtang') fruit were determined, and the changes in organic acids content during the fruit development and their relationships with the activities of enzymes associated with malic acid metabolism were analyzed. The critical period and key enzymes of organic acid accumulation were elucidated.【Methods】‘Fengtang'and‘Siyue'fruits were used as experimental material. The composition and contents of organic acids in plum fruitsduring the development were analyzed by high performance liquid chromatography (HPLC) and the activities of enzymes related with malic acid metabolism were determined by spectrophotometer. The correlations between the changes in malic acid content and the activities of malic acid metabolism enzymes were analyzed in this study.【Results】The total acid content was 5.94 mg· g-1 at maturity in‘Fengtang', including 6 organic acids, malic acid, tartaric acid, citric acid, oxalic acid, shikimic acid and succinic acid. Among them, malic acid was the dominant acid accounting for 88% of the total acid content.‘Siyue'fruit had the same composition of organic acids. Both cultivars belong to the malic acid type. During the growth and development of plum, the content of malic acid increased first and decreased at the late phase. The malic acid content in‘Fengtang'reduced to a minimum when the fruit was ripe, and was always significantly lower than‘Siyue'. The content of oxalic acid and tartaric acid gradually decreased with the growth and development of fruit, and the amplitude was small at the ripening period, but the content of citric acid gradually increased. The changes in total acid content and malic acid content in the fruit of‘Siyue'were similar to those of‘Fengtang'. The difference in total acid content in the two cultivars was mainly due to the difference in malic acid content. The changes in malic acid content were closely related to the changes in NAD-malate dehydrogenase (NAD-MDH) , NADP-malosterase (NADP-ME) and phosphoenolpyruvate carboxylase (PEPC) activities. The activity of NADMDH in‘Fengtang'and‘Siyue'had a significant difference. The activity of NAD-MDH in the‘Fengtang'fruit fluctuated within the range of 82.24-169.12 U· mg-1. NAD-MDH activity in‘Siyue'showed an M pattern and was higher than that in‘Fengtang'. There was a significant positive correlation between malic acid content and NAD-MDH activity in the two cultivars with a correlation coefficient of0.97** and 0.95**, respectively. The change pattern of the activity of NADP-ME in‘Fengtang'was similar to that‘Siyue', increasing during young fruit enlargement period and decreasing during maturation. However, the activity of NADP-ME in the fruit of‘Fengtang'was always significantly lower than that of‘Siyue'. The activity of NADP-ME was significantly negatively correlated with malic acid content with a correlation coefficient of-0.86--0.87, respectively. The change in PEPC activity in the fruit‘Fengtang'exhibited an M pattern. Except for the fruit ripening period, the enzyme activity was always significantly higher in‘Fengtang'. The activity of PEPC in‘Siyue'fruit changed smoothly during the whole fruit development period. There was a significant positive correlation between the activity of PEPC and the content of malic acid in‘Fengtang'and‘Siyue'with a coefficient of 0.75* and 0.73*, respectively. The early stage of fruit development was the critical period of malic acid accumulation in‘Fengtang', when enhanced activity of PEPC promoted the synthesis of malic acid and the decreased activity of NADP-ME reduced the decomposition of malic acid, while NAD-MDH had little effect.However the key enzyme for accumulation of malic acid in‘Siyue'was NAD-MDH. At the later stage of fruit development, the activity of NADP-ME increased rapidly in both cultivars, while the activity of PEPC in‘Fengtang'was weakened and the NAD-MDH in‘Siyue'activity decreased. As a result, the degradation of malic acid was greater than synthesis and the content of malic acid became lower in the both cultivars.【Conclusion】‘Fengtang'is a low-acid plum variety, in which malic acid is the main organic acid. The critical period of malic acid accumulation is the early stages of fruit development. The change in malic acid content is coordinated by PEPC and NADP-ME. However, in‘Siyue'malic acid content is mainly controlled by NAD-MDH and NADP-ME.