Comparative effects of different potassium sources on soluble sugars and organic acids in tomato

•Tomato sugars and acids were differentially regulated by different K fertilizers.•K enhanced sugar accumulations by improving sucrose metabolism and transport in tomato.•K increased citrate accumulations by improving citrate synthesis and TCA cycle.•The accumulation of soluble sugar and organic acid exhibited specific K fertilizer-effect. Soluble sugars and organic acids are mainly decisive factor of flavor quality in tomato. Potassium (K), an essential macroelement, plays key roles in regulation of plant growth, stress responses and fruit quality. However, the comparative effects of different K fertilizer on soluble sugars and organic acids accumulation and the differential mechanisms are still unknown. The aim of this study was to investigate how K fertizliers (K_Cl, K_S) affect the accumulations of soluble sugars and organic acids in tomato fruits. The results indicated that K applications improved fruit flavor quality of tomato, as supported by the increased concentrations of soluble solid, titratable acid, fructose, glucose, sucrose (Suc), citric acid (CA) and malic acid (MA). Further study showed that K application increased the accumulations of soluble sugars by improving Suc metabolism, sink strength and Suc transport due to the increased expressions of Suc-metabolizing enzyme genes and SlSWEETs and SlSUTs in tomato. K applications increased the accumulations of MA and CA by improving CA synthesis and TCA cycle due to the increased expressions CA-metabolizing enzyme genes. Moreover, more Suc of leaves were transferred into fruits in K4_Cl treatment, which was supported by the lower concentrations of soluble sugars and higher expressions of SlSUT1 and SlSWEET11b in leaves. More carbohydrates were allocated to form yield and metabolic substrate in K4_S treatment, which was supported by the higher expressions of SlSUT4 and more differential metabolic pathway in fruits. Finally, the carbohydrates transports and allocations were involved in the differential mechanisms of soluble sugar accumulations in response to K_S and K_Cl in tomato.