Potassium fertilization arrests malate accumulation and alters soluble sugar metabolism in apple fruit

Effects of different potassium (K) levels, which were K0 (no fertilizer), K1 (71.5 g KCl plant^-1 year^-1), K2 (286.7 g KCl plant^-1 year^-1), and K3 (434 g KCl plant^-1 year^-1), were evaluated based on sugar and organic acid metabolism levels from 70-126 days after bloom (DAB) in the developing fruit of potted five-year-old apple (Malus domestica, Borkh.) trees. The results indicate that K fertilization promoted greater fruit mass, higher Ca²⁺ and soluble solid levels, and lower titratable acid levels, as well as increased pH values at harvest. With the application of different levels of K fertilizer, fructose, sorbitol, glucose and sucrose accumulation rates significantly changed during fruit development. Fruit in the K2 group had higher fructose, sucrose and glucose levels than those in other treatment groups at 126 DAB. These changes in soluble sugar are related to the activity of metabolic enzymes. Sucrose synthase (SS) and sorbitol dehydrogenase (SDH) activity in the K2 treated fruit was significantly higher than those in other treatment groups from 70-126 DAB. Malate levels in K-supplemented fruit were notably lower than those in non K-supplemented fruit, and K3 treated fruit had the lowest malate levels during fruit development. Cytosolic malic enzyme (ME) and phosphoenolpyruvate carboxykinase (PEPCK) activity significantly increased in fruit under the K2 treatment during 112-126 DAB and 98-126 DAB, respectively. In addition, Ca²⁺ concentration increased with increasing K fertilization levels, which promoted a maximum of 11.72 mg g^-1 dry weight in apple fruit. These results show that K levels can alter soluble sugar and malate levels due to the interaction between sugars and acid-metabolic enzymes in fruit.