Understanding tissue-specific mineral allocation during fruit development is essential for improving melon quality, yet quantitative, dynamic evidence remains limited. Here, we conducted tissue-resolved ionomic profiling of ten essential elements in the rind, flesh, and seed cavity at the stages of cell division, fruit expansion, and maturation. Rubidium-to-strontium molar ratios were used to trace the relative contributions of phloem and xylem transport to each tissue. Mineral allocation followed tissue-specific patterns governed by vascular connectivity and elemental mobility: the xylem-dependent rind preferentially accumulated Ca and B, whereas the phloem-dependent seed cavity accumulated Zn, Cu, and Mn. In the flesh, a significant shift from xylem to phloem dependence occurred during late fruit expansion, as indicated by the increased Rb:Sr ratios. This vascular transition restricted Ca entry into the flesh while facilitating K and carbohydrate import, establishing flesh sweetness. However, progressive flesh growth led to dilution and declining mineral concentrations in the edible parts, with Zn and Fe reaching only ∼0.5 and 1.7 µg g −1 fresh weight at maturity. These results identify early development as a critical window for Ca supply, a later window for K fertilization, and highlight the need for breeding strategies that enhance transporter-mediated micronutrient loading into the flesh.
Yan et al. (Fri,) studied this question.