‘Jinju’ Pomelo Pollen Enhances Fruit Set and Development in Citrus maxima ‘Tomentosa’: Physiological and Proteomic Insights

Citrus maxima ‘ Tomentosa ’ , (commonly known as ‘Luchuan Juhong’) is a valuable medicinal and edible germplasm resource in China, renowned for its unique pharmacological properties. However, its commercial cultivation is hindered by strong self-incompatibility, leading to poor fruit set and significant physiological fruit drop. While artificial cross-pollination can mitigate this issue, the effectiveness of different pollen donors and the underlying molecular mechanisms remain poorly understood. In this study, we assessed four pomelo cultivars as pollen donors and identified ‘Jinju’ Pomelo to be the most effective, achieving fruit set rates of 78.00% (7 days post-pollination) and 44.00% (8 weeks post-pollination)—significantly higher than self-pollination (42.67% and 8.0%) or pollination with the commonly used ‘Hongrou’ pomelo (66.67% and 26.67%). Physiological analysis revealed that ‘Jinju’ Pomelo pollen had superior viability, with the shortest germination time (6.67 h) and the highest germination rate (89.68%), enabling rapid pollen tube growth before stigma senescence. Cross-pollination also triggered dynamic changes in endogenous hormone, characterized by a synergistic increase in auxin (IAA) and zeatin (ZT) alongside a sharp decline in abscisic acid (ABA) during the critical fruit-set window (5–7 days post-pollination). Quantitative proteomics further identified 367 differentially expressed proteins (DEPs) in cross-pollinated ovaries, enriched in pathways related to pollen recognition, pollen tube guidance, and phenylpropanoid biosynthesis. These findings suggest that compatible pollination triggers a coordinated hormonal and proteomic response, reducing abscission and enhancing fruit development. This study provides scientific insights for optimizing pollination strategies and supports the sustainable cultivation of C. maxima ‘Tomentosa’.