Effects of Training Systems on Sugar Accumulation and Metabolism in ‘Kyoho’ Grape Berries

To investigate the effects of straight-line-shape (SL) and inverted-umbrella-shape (IU) training systems on sugar accumulation and metabolism in ‘Kyoho’ grape berries in Fujian, this study used 16-year-old ‘Kyoho’ grapevines trained in the two systems. Fruit samples were collected from 45 to 95 days after flowering (DAF) to measure soluble sugar content. Transcriptome sequencing was performed to analyze the differential expression of sugar metabolism-related genes, combined with KEGG enrichment analysis and RT-qPCR validation of key genes. The results showed that, at the same stage, the soluble sugar content in berries under the SL training system was significantly higher than that under the IU training system, especially from 45 to 65 DAF, where sugar accumulation was faster. Transcriptome analysis revealed that the SL training system showed 6274, 5597, and 2064 differentially expressed genes at 45, 65, and 95 DAF, respectively. Key sugar metabolism-related genes, such as fructokinase (FK), phosphofructokinase (PFK), and sucrose phosphate synthase (SPS), exhibited significantly higher expression levels in the SL training system than in the IU training system. KEGG enrichment analysis indicated that the SL training system significantly enriched sugar metabolism and transport pathways during the early fruit ripening stage. RT-qPCR validation confirmed that genes related to sugar metabolism and transport (such as FK7, SUS3, SPP1) were expressed at significantly higher levels in the SL training system than in the IU training system. In conclusion, the SL training system significantly promoted soluble sugar accumulation and accelerated fruit ripening in ‘Kyoho’ grapes by regulating the expression of sugar metabolism and transport-related genes, providing a theoretical basis for promoting the SL training system in production.