'Jinju' Pomelo pollen enhances fruit set and development in Citrus maxima 'Tomentosa': physiological and proteomic...

Citrus maxima (Burm.) Merr. 'Tomentosa' ('Luchuan Juhong') is a valuable medicinal and edible germplasm, yet its commercial cultivation is severely constrained by strong self-incompatibility, which results in poor fruit set and excessive physiological fruit drop. While artificial cross-pollination can alleviate this limitation, the relative efficacy of different pollen donors and the underlying molecular mechanisms remain largely unknown.

We evaluated four pomelo cultivars as pollen donors and identified 'Jinju' Pomelo as the most effective. Following 'Jinju' pollination, fruit set rates reached 78.00% at 7 days and 41.56% at 8 weeks post-pollination, significantly outperforming self-pollination (42.67% and 8.00%, respectively) and the widely used 'Hongrou' Pomelo (62.00% and 24.22%). Moreover, cross-pollination with 'Jinju' resulted in significantly larger vertical and transverse fruit diameters during early development. Physiological analyses showed that 'Jinju' pollen had superior viability, with the shortest germination time (6.67 h) and the highest germination rate (89.68%). Fluorescence microscopy confirmed rapid, unimpeded pollen tube growth reaching the ovary base prior to stigma senescence. Cross-pollination also induced dynamic hormonal shifts, marked by a synergistic increase in indole-3-acetic acid (IAA) and zeatin (ZT) alongside a sharp decline in abscisic acid (ABA) at 5-7 days post-pollination. Quantitative proteomics of developing ovaries identified 386 differentially expressed proteins between cross and self-pollinated groups. Notably, pathways associated with pollen tube growth regulation and phenylpropanoid biosynthesis, including upregulation of the core enzymes Phenylalanine Ammonia-Lyase (PAL), 4-Coumarate: CoA Ligase (4CL), and Cinnamyl Alcohol Dehydrogenase (CAD) were significantly enriched.

Compatible pollination with 'Jinju' Pomelo enhances fruit set and early fruit development in C. maxima 'Tomentosa' through coordinated hormonal and proteomic responses linked to improved pollen tube progression, pedicel structure, and reduced fruit abscission. These findings provide a robust scientific basis for optimizing pollination strategies in the cultivation of this economically important germplasm.