Potassium fertilization affects nutritional quality and aroma in Haruka apple fruits: insights from multi-omics analysis

Apples are important cultivated fruit crops grown worldwide and represent one of the most nutritious foods in a healthy diet. They are rich in nutrients, such as sugars, organic acids, vitamins, and amino acids, and possess a pleasant aroma. Spraying different types of potash fertilizers on apples can affect the fruit quality at maturity. However, the underlying molecular mechanisms of this effect remain unclear. In this study, two types of potassium (K) fertilizers (K2SO4 and KH2PO4) were applied to the leaves of Haruka apples to elucidate the regulatory mechanisms of different K fertilizer types of on fruit nutritional quality and aroma, construct ceRNA network, and identify the most effective K fertilizer type. Seventy-seven and 45 differentially altered compounds (DCCs) were identified in the nutritional and volatile compounds of fruit flesh of apples under K2SO4 treatment, whereas 25 and 8 DCCs were identified under the KH2PO4 treatment, respectively. Both K2SO4 and KH2PO4 increased the content of tyrosine, vitamin B2, and lipids in mature apples. Additionally, K2SO4 enhanced the nicotinamide content. KH2PO4 improved the aroma of apples during ripening only through the lipoxygenase (LOX) pathway, whereas K2SO4 improved the aroma during ripening through the LOX, MEP, and shikimate pathways. K fertilizer treatment increased the expression of LOX and alcohol dehydrogenase (ADH) genes by regulating the LOX pathway, whereas the expression of LOX and ADH genes was downregulated in the KH2PO4 treatment compared with that in the K2SO4 treatment. ceRNA networks associated with apples sprayed with potash fertilizer were also established, showing that mdm-miR159a and PC-3p-45634_116 are involved in apple aroma and lipid synthesis. The application of K2SO4 was more effective than that of KH2PO4 at improving fruit nutritional quality and aroma, offering valuable guidance for production in orchards.