Pumpkin rootstock-driven variation in mineral nutrient allocation determines differential melon fruit quality

Rootstocks are commonly used in melon cultivation to mitigate biotic and abiotic stresses and improve fruit quality. However, the extent to which rootstocks influence nutrient allocation, growth dynamics, and fruit quality formation in the scion remains insufficiently understood. This study compared the effects of two pumpkin rootstocks (Tianzhen No. 1 and Sizhuang No. 12) on melon fruit quality by examining nutrient partitioning, sugar accumulation, hormonal balance, and expression of sugar metabolism-related genes. Compared with non-grafted plants, rootstock grafting enhanced plant growth and increased total N, P, and K uptake (mg plant^-1) as well as tissue nutrient concentrations (mg g^-1 DW) during vine growth and fruit maturation stages. Tianzhen No. 1 showed higher K distribution to the scion (97.6%) and greater fruit K accumulation (45.88 mg/g) than Sizhuang No. 12. Fruits grafted onto Tianzhen No. 1 also exhibited higher total soluble solids (15.96 °Brix) and soluble sugar contents, including glucose (18.28 mg g^-1), fructose (20.20 mg g^-1), and sucrose (144.25 mg g^-1), whereas fruits grafted onto Sizhuang No. 12 showed higher firmness (4.18 kg cm^-2) compared with Tianzhen No. 1-grafted fruits (3.07 kg cm^-2). At fruit maturation, Tianzhen No. 1-grafted melons had higher ABA content (14.49 ng/g) and upregulated expression of sugar transporter genes (HT2, HT7, SWEET4, SWEET7) and genes encoding key metabolic enzymes (SPS1, NIN3) compared to Sizhuang No. 12. These observed associations suggest that rootstock-mediated differences in nutrient uptake may contribute to altered scion fruit quality metabolism. Overall, the findings provide insights into the physiological and molecular basis of rootstock-dependent variation in melon fruit quality and may contribute to the development of targeted rootstock selection strategies in melon breeding programs.