Foliar spraying of potassium sulfate during fruit development comprehensively improves the quality of citrus fruits

Sugar accumulation is influenced by various fertilizer treatments, of which potassium spraying is the most effective. However, the effect of different potassium sources as a foliar application at different fruit development stages on citrus fruits is still unclear. In this study, three different potassium fertilizers and one water (F1: 0.65% KNO₃, F2: 0.88% KH₂PO₄, F3: 0.56% K₂SO₄, and F4: water) were sprayed on Citrus reticulata cv. Nanfeng at cell division, cell expansion, fruit ripening, and throughout fruit developmental stages, respectively. Results showed that the six-time K₂SO₄ application had the best function in enhancing fruit physiological attributes such as fruit weight and the total carotenoids; also, this treatment significantly decreased TA (titratable acid), increased fruit TSS (total soluble solids), soluble sugar content, and TSS:TA ratio. Furthermore, K₂SO₄ spraying obviously increased the expression of CsCWINV-2/6 in the segment membrane, and CsSUT-1/2 and CsVPP-1/2 in fruit juices. Taken together, six-time application of K₂SO₄ throughout fruit developmental stages produced better fruit quality, at least through enhancing sink strength and promoting sugar transportation in citrus fruits. This study might effectively contribute to maximizing fruit quality and its marketability.