Developmental Stage-Specific Modulation of Sugar–Acid Metabolism in ‘Fengtang’ Plum as Revealed by UPLC-MS/MS-Based Metabolomics

‘Fengtang’ plum, a specialty fruit variety from Guizhou Province, is characterized by its high total soluble solids (TSS) and well-balanced sugar–acid ratio. In this study, widely targeted metabolomics (UPLC-MS/MS) combined with multivariate statistical analysis was employed to systematically monitor dynamic changes in sugar–acid metabolism across three critical developmental stages of the fruit: pre-veraison stage (PR), veraison stage (VR), and full veraison stage (FR). The aim was to elucidate the intrinsic mechanisms underlying high sugar accumulation. The results indicated that TSS content increased significantly during fruit maturation, reaching up to 17%, while titratable acid (TA) content continuously decreased. Consequently, the sugar–acid ratio rose substantially from 21.5 at the early stage to 43.8. A comprehensive metabolomic analysis led to the identification of 2003 metabolites, with notable emphasis on phenolic acids, amino acid derivatives, sugars, and other major classes. Key metabolic pathways from the KEGG database were also examined. Enrichment analysis of these pathways revealed that central metabolic routes—including fructose and mannose metabolism, galactose metabolism, and the tricarboxylic acid (TCA) cycle—played crucial roles in the dynamic accumulation of sugar–acid components. Notably, oligosaccharides such as cottonseed sugar and pine trisaccharide were observed to accumulate significantly during fruit ripening. These findings clarify the metabolic foundation and mechanisms contributing to the high sugar quality of ‘Fengtang’ plum, thereby providing a theoretical basis for precise fruit quality regulation and the optimization of post-harvest preservation strategies.