Potassium stimulates fruit sugar accumulation by increasing carbon flow in Citrus

Soluble sugar is a key factor of flavor quality in citrus. Potassium (K) is known as a quality element, which plays key roles in improving sugar accumulation and fruit quality, but the mechanisms are largely unknown. This study aims to elucidate how K improves sugar accumulation by regulating carbon flow between source and sink in Newhall navel orange (Citrus sinensis). The results demonstrated that appropriate K concentration improved fruit quality and sugar accumulations in citrus, and 1.5% of K concentration in pulp was the optimal concentration for fruit quality. K increased strength of sink and source, as supported by the increased fruit growth rate, gene expressions related to sugar metabolism and sugar transport in fruit, and photosynthesis, gene expressions of sugar metabolism in leaf, respectively, which contributed to increasing sugars accumulation. Further study indicated that K improved carbon flow from source to sink by boosting symplastic and apoplastic loading of phloem, as supported by the increased CF signal intensities, plasmodesmata densities, and the expressions of CsSUT1 and CsSUT2 in leaf at early and mid stages of fruit development, finally increasing sugar accumulation in fruit. Conclusively, K stimulates fruit sugar accumulation by increasing carbon flow in Newhall navel orange. HighlightK application stimulated carbon flow between source and sink through symplastic and apoplastic loading, which were supported by the structural characteristics of phloem and the expression of CsSUTs and CsSWEETs, ultimately promoting sugar accumulation in Citrus sinensis.