Abstract: 【Objective】 ‘Beni-Madonna’, also known as ‘Hongmeiren’, is favored by people because of its excellent quality. Fruit thinning is a technique commonly used by ‘Beni-Madonna’ to improve fruit quality in the process of agricultural production. It can not only significantly increase the volume of fruit, but also increase the content of soluble solids in the fruit. However, the specific mechanism of fruit thinning to increase the content of soluble solids in the fruit of ‘Beni-Madonna’ is still unclear. Therefore, in this study, Four-year-old ‘Beni-Madonna’ tangor (Citrus nanko ′ C. amakusa) plants, with trifoliate orange (Poncirus trifoliata) as a base rootstock and Ponkan as an intermediate stock, were used as experimental materials to explore the specific mechanism of fruit thinning affecting the change of sugar and acid content in fruits. 【Method】Fruit thinning was carried out at the young fruit stage of ‘Beni-Madonna’ fruit. The horizontal and vertical stems of fruit thinning and non fruit thinning were measured every half a month or so. Samples were taken back and the single fruit weight and sugar and acid content of each treatment were measured. After grinding into powder with liquid nitrogen, the activities of sugar and acid metabolism related enzymes and the relative expression of their corresponding coding genes were measured at the time when the sugar and acid content were significantly different. 【Result】Fruit thinning significantly increased the horizontal and vertical stems and single fruit weight of ‘Beni-Madonna’ fruit in the middle and late stages of fruit growth, significantly accelerated the degradation rate of citrate content, but did not affect the final citrate content at fruit maturity, significantly increased the fruit glucose and sucrose content, but had no significant effect on fructose content. Fruit thinning significantly increased the activities of sucrose synthase (SSⅡ, synthesis direction), sucrose synthase (SSⅠ, decomposition direction), sucrose phosphate synthase (SPS) in sucrose metabolism and cytosolic aconitase (Cyt-ACO), cytoplasmic isocitrate dehydrogenase (ICDH) and glutamine synthetase (GS) implicated in citric acid catabolism. In addition, fruit thinning significantly promoted the early relative expression of CitSS3 and the relative expression of CitSPSs at all stages in the process of sucrose metabolism, and the relative expression of CitACO3, CitNADP-IDH1, CitACLα1/β, CitGAD1 and CitGSs in the process ofcitric acid metabolismin the early and middle stages. 【Conclusion】Artificial fruit thinning mainly promotes the accumulation of soluble sugar in fruit by enhancing the enzyme activity of sucrose phosphate synthase in the process of sucrose anabolism, improves the activity and gene expression level of related enzymes in the process of citric acid catabolism, and accelerates citric acid degradation, thus playing an important role in improving fruit quality.

Key words: fruit thinning, ‘Beni-Madonna’, fruit quality, sucrose metabolism, citric acid metabolism