Study on the differences in sugar-acid metabolism of Sweet Orange 'Newhall' with different rootstocks.

Background: Citrus rootstock selection plays a critical role in modulating sugar-acid metabolism, thereby influencing fruit flavor quality. This study systematically investigated the dynamic changes in sugar and acid components, key enzyme activities, and related gene expression in 'Newhall' navel orange (Citrus sinensis) grafted onto Citrus reticulata (Rt), Citrus junos (Cj), and Poncirus trifoliata (Pt) rootstocks during fruit development (150-240 days after flowering, DAF).

Results: Pt significantly enhanced late-stage sucrose accumulation (48.86 ± 2.63 mg·g⁻¹), surpassing the means of Rt and Cj by 26.78% and 60.72% at 240 DAF, respectively. This was associated with elevated sucrose synthase (SS) activity and upregulated CsSS2 expression. In contrast, Rt rootstock promoted malic and quinic acid retention and exhibited the highest citrate synthase (CS), isocitrate dehydrogenase (IDH), and isocitrate lyase (ICL) activities, supported by significantly increased upregulation of acid metabolism genes (CsCS1, CsNADP-IDH1/2/3). Comprehensive evaluation using the TOPSIS model, weighted by entropy analysis (with tartaric acid, fructose, and glucose identified as primary determinants), ranked fruit quality as Pt > Rt > Cj at maturity.

Conclusions: These findings establish a mechanistic framework linking rootstock-specific regulation of enzymatic activities and gene expression with dynamic sugar-acid profiles, providing valuable insights for optimizing rootstock-scion combinations to enhance citrus flavor quality.