Analysis of organic acid components and malic acid metabolizing enzyme activity in Hongbaoshi apple fruits

Abstract:【Objective】The studies on the contents and components of sugars and acids in fruit have been widely concerned. Among them, the contents and components of organic acids are the important indicators to determine the quality and flavor of fruit. Thus, to determine and explore the physiological basis of low organic acid characteristics in Hongbaoshi apple fruit, the components and contents of organic acids in Hongbaoshi apple fruit and the relationship between the changes of malic acid contents and metabolism- related enzyme activities were analyzed.【Methods】In the current study, the Hongbaoshi and Fuji 2001 apple fruits at different growth stages were taken as experimental materials, the total sugar contents were measured by anthrone-colorimetric method, and the components and contents of organic acids were determined by high performance liquid chromatography. Finally, the activities of malate-related metabolic enzymes and their correlation with the malate content were determined and analyzed.【Results】The organic acids in Hongbaoshi fruit were mainly malic acid, oxalic acid, citric acid, tartaric acid and succinic acid. Among them, the content of malic acid was the highest (accounting for more than 89% of the total acid). The organic acids in Hongbaoshi fruits were consistent with those in Fuji 2001 fruits, and the malic acid was the main component. In addition, the malic acid content in Hongbaoshi fruits at different developmental stages accounted for 89.03%-91.80% of the total acid contents, and the latter was 97.65%-98.56%, indicating that the different components of organic acids in fruits showed that the difference in total acid content in fruits was mainly caused by the difference in malic acid content. During the whole fruit developmental period, the changes of total sugar and total acid contents in Hongbaoshi fruits were basically consistent with those of Fuji 2001, the content of malic acid in the former fruit was significantly lower than that in Fuji 2001, and it was found that the key period of malic acid accumulation was at the early stage of fruit development with an up-down-up trend, while the citric acid, oxalic acid and succinic acid showed a decreasing trend overall, and the content of tartaric acid increased sharply at the later stage of fruit development and then decreased. Compared with Fuji 2001, the changing trends of malic acid, citric acid, tartaric acid, succinic acid and total sugar were more consistent, the opposite was true for oxalic acid at the later stage of fruit development, and the contents of malic acid, total sugar and total acid were higher than those of Hongbaoshi during the whole fruit developmental process. Furthermore, at the early stage of fruit development, the activity of NADP-ME enzyme in the fruit of Hongbaoshi first decreased, and then continued to increase, the activity of PEPC enzyme decreased sharply as well as the NAD-MDH enzyme that caused the content of malic acid to accumulate slowly, while the NAD-MDH activity in the fruit of Fuji 2001 increased rapidly at the early stage of fruit development, which was exactly the opposite of Hongbaoshi. Then through analyzing the correlation between malate content and malate- related metabolic enzyme activities, it showed that the content of malic acid in the fruit of Hongbaoshi accumulated more at the early stage of fruit development because of the increased activity of NAD- malate dehydrogenase (NAD- MDH), which promoted the synthesis of malic acid in this period, and reduced NADP-malicase (NADP-ME) activity, which reduced the breakdown of malic acid. It had little relationship with phosphoenolpyruvate carboxylase (PEPC) activity and it was similar to Fuji 2001. At the later stage of fruit development, the NAD-MDH activity in both cultivars increased briefly and then decreased rapidly, while the NADP-ME activity of Hongbaoshi kept increasing, and the NADP-ME activity of Fuji 2001 first increased and then decreased, resulting in the degradation of malic acid in the two fruits being greater than the synthesis. In addition, the NAD-MDH was the key enzyme in the accumulation of malic acid in Hongbaoshi and Fuji 2001 fruits.【Conclusion】Hongbaoshi apple is a low-acid-type cultivar, which was dominated by malic acid. The early stage of fruit development is a critical period for the accumulation of malic acid. NADPME and NAD-MDH enzymes play a major synergistic role in the accumulation of malic acid in the fruit.【Objective】The studies on the contents and components of sugars and acids in fruit have been widely concerned. Among them, the contents and components of organic acids are the important indicators to determine the quality and flavor of fruit. Thus, to determine and explore the physiological basis of low organic acid characteristics in Hongbaoshi apple fruit, the components and contents of organic acids in Hongbaoshi apple fruit and the relationship between the changes of malic acid contents and metabolism- related enzyme activities were analyzed.【Methods】In the current study, the Hongbaoshi and Fuji 2001 apple fruits at different growth stages were taken as experimental materials, the total sugar contents were measured by anthrone-colorimetric method, and the components and contents of organic acids were determined by high performance liquid chromatography. Finally, the activities of malate-related metabolic enzymes and their correlation with the malate content were determined and analyzed.【Results】The organic acids in Hongbaoshi fruit were mainly malic acid, oxalic acid, citric acid, tartaric acid and succinic acid. Among them, the content of malic acid was the highest (accounting for more than 89% of the total acid). The organic acids in Hongbaoshi fruits were consistent with those in