Sugar and organic acid metabolism and accumulation in different cultivars during fruit development in hawthorn (<i>Crataegus pinnatifida</i>)

Abstract Objectives: Hawthorn fruit is beneficial for human health; however, the consumption of fresh hawthorn fruit is severely limited because of its extreme acidity. Elucidating the metabolic patterns of sugar and organic acid in fruits is crucial for improving fruit quality. Methods and Results: In this study, 16 hawthorn cultivars with different tastes were analyzed for sugar and acid metabolism in developing fruits, and the results revealed that the dominant sugar differed among varieties, while citric acid was predominant in all cultivars. Furthermore, enzyme activities and related gene expression levels associated with fruit sugar and organic acid metabolism were analyzed in four representative varieties. The results indicated that the abundant glucose accumulation observed in ‘Dawang’ and ‘Qiujinxing’ could be attributed to increased sorbitol oxidase (SOX), sucrose synthase (SS)-cleavage (SS-c), neutral invertase, and vacuole invertase (VI) activities, whereas in ‘Baiquan7901’ and ‘Xinglongzirou’, increased activity of the SS-synthesis enzyme caused increased sucrose storage, and elevated citrate synthase (CS) and decreased aconitase (ACO) activity caused the accumulation of citric acid. Conclusions: The findings of this study can provide a basis for further studies on improving hawthorn fruit quality.