Ionomics and transcriptomics reveal nitrogen-driven modulation of fruit citric acid metabolism in pomelo

Fruit dimensions and caliber constitute pivotal determinants that significantly impact e-commerce sales performance. This study sought to investigate the correlations among fruit weight, nutrient composition, and acidity quality in pomelo ( Citrus grandis ), and to elucidate how nutrient management affects fruit quality, thereby offering a scientific foundation for improving pomelo cultivation practices and enhancing fruit quality. The results revealed that large fruits dominated pomelo production, with a high-quality fruit rate of 85.85%. Significant differences in titratable acidity (TA) content between fruit sizes were found, mainly due to variations in citric acid, the predominant organic acid. PCA revealed a distinct separation between the two fruit size groups, and correlation analysis showed multiple significant correlations among TA, citric acid, and nutrient levels. These findings imply that fruit size and nutrients, especially N, greatly influence fruit acidity, offering insights for pomelo cultivation and quality improvement. Differences in fruit structure also contributed to uneven nutrient distribution, with flavedo and vertical juice sacs identified as key sites for nutrient accumulation. It also indicated that optimizing nutrient management could improve fruit quality grades. Moreover, it has the capacity to modulate nutrient allocation, diminish citric acid levels, and elevate fruit quality. The TCA cycle, classified under energy metabolism and an in - depth analysis of 47 differentially expressed candidate genes in the TCA pathway revealed that CS family member genes' expression was notably affected by nutrient management, providing references for subsequent gene function validation. Notably, N has been identified as a pivotal factor influencing fruit acidity and citric acid content, thereby offering valuable insights for subsequent investigations into the molecular underpinnings of nutrient regulation and the sustainable advancement of the pomelo industry.