Investigation of the cause of reduced sugar content in Kiyomi tangor fruit of Ziyang xiangcheng (Citrus junos Sieb. ex Tanaka) rootstock

Ziyang xiangcheng (Citrus junos Sieb. ex Tanaka) (Cj) rootstock is effective in Citrus production; however, when Cj rootstock was used, sugar content in Kiyomi tangor fruit was significantly lower than that in the fruit produced using Poncirus trifoliata (L.) Raf. rootstock (Pt). Therefore, using K. tangor, we explored the cause of this difference, determining sugar accumulation, sucrose-metabolism enzyme activities, and gene expression. Before ripening, sugar content in fruits with Cj rootstock was significantly lower than that in fruits with Pt rootstock, due to low fructose and sucrose content. Sucrose phosphate synthase (SPS) activity of Pt was higher than that of Cj in the early growth stage (at 90–210 days after anthesis), however it was opposite at 240–300 days after anthesis. Additionally, neutral invertase (NI) activity of Pt was higher than that of Cj. Gene expression in Pt was higher than that in Cj, but is was essentially the same at maturity. SPS and NI activities and CitSPS1 expression were positively correlated with sucrose, fructose, and glucose content, but CSCW1 expression was negatively correlated with the sugars. Overall, the weak flavour of K. tangor fruit with Cj rootstock was regulated by the sucrose metabolism-related enzymes and gene expression.