Effect of Different Interstocks on Fruit Quality, Amino Acids, and Antioxidant Capacity in ‘Yuanxiaochun’ Citrus

High grafting is a widely recognized technique for varietal renewal in aging citrus orchards. However, following high grafting, a specific 'rootstock-interstock-scion' combination is formed, yet the influence of interstock on scion fruit quality remains insufficiently explored. To address this gap, we conducted experiments by grafting 'Yuanxiaochun' ((<i>Citrus unshiu</i> Marcov × <i>Citrus sinensis</i> Osbeck) × (<i>Citrus reticulata</i> × <i>Citrus paradisi</i>)) onto three distinct interstocks ('Yuanxiaochun'/'Harumi'/'<i>Trifoliate orange</i>' (CJ), 'Yuanxiaochun'/'Ponkan'/'<i>Trifoliate orange</i>' (PG), 'Yuanxiaochun'/'Marumi Kumquat'/'<i>Trifoliate orange</i>' (JJ)), with 'Yuanxiaochun'/'<i>Trifoliate orange</i>' used as a control (CK), and comprehensively evaluated their impact on fruit quality of 'Yuanxiaochun'. Our research results show that interstock significantly increased the total soluble solids (TSSs) content of fruit. Additionally, interstocks also significantly increased the organic acid content in the fruit, particularly citric acid, which was on average 2.90 mg·g^-1 FW higher than CK. In terms of fruit flavor, interstocks significantly reduced the sugar/acid ratio and the sweetness/total acid ratio. However, CJ and PG showed markedly higher sweetness levels. Furthermore, interstocks led to a marked increase in both total amino acid content and flavor-active amino acid content in the fruit. Taste active values of γ-aminobutyric acid, asparagine, aspartic acid, glutamic acid, and arginine were all greater than 1, indicating a significant contribution to the fruit flavor. Moreover, interstocks increased the total flavonoid and phenol content in the fruit, thereby affecting its overall antioxidant capacity. These findings provide valuable and systematic insights for high grafting and variety renewal in citrus production.