Effect of Potassium-Restricted Cultivation on Fruit Quality and Cation Dynamics in Potted Satsuma Mandarin

Patients with chronic kidney disease are often advised to restrict potassium (K) intake, making low-K crop production highly desirable. Although low-K blueberries have been reported, studies on other fruit tree species are required. We applied root-zone K restriction to potted satsuma mandarin (Citrus unshiu Marc.), a major fruit crop that is naturally rich in K, over 18 months across two seasons (2023–2024). First-year fruit data were treated as preliminary due to limited yield. In 2024, fruit K content was successfully halved under K restriction. Although single fruit weight and yield showed a non-significant downward trend, the soluble solid content to titratable acidity ratio significantly increased, primarily driven by a marked drop in acidity. Sodium, calcium, and/or magnesium contents also shifted in the fruits and leaves, with the strongest elevation observed in sodium. However, the resulting increase in fruit sodium was clinically minor relative to the substantial decrease in K. Furthermore, because sodium hydroxide was used for pH adjustment and because leaf cation levels varied by Year and Sampling Period, these shifts cannot be attributed solely to K restriction. Overall, these findings demonstrate the technical potential to reduce fruit K content in satsuma mandarin, though further studies are required to evaluate commercial production feasibility.