Reduction of Potassium (K) Content in Strawberry Fruits through KNO3 Management of Hydroponics

The consumption of vegetables and fruits rich in potassium (K), such as melons and strawberries, is restricted in chronic kidney disease (CKD) patients. Therefore, we attempted to produce low-K strawberry fruits through management of a KNO3 fertilizer in nutrient solution from anthesis to the harvest period. A general trend of decreasing K content in fruit was observed with the decrease of KNO3 concentration in the nutrient solution. Among four strawberry cultivars, the fruit of the ‘Toyonoka’ exhibited a K reduction of about 64% when plants were grown in nutrient solution with KNO3 at 1/16 of the normal level. Citric acid and ascorbic acid contents of ‘Toyonoka’ fruit were reduced with decreasing KNO3 concentrations in the nutrient solution. Although the reduced NO3− of the nutrient solution was adjusted by using Ca(NO3)2 to obtain low-K strawberries, growth, yield, and quality did not vary with this adjustment. Compared with the typical level of K in strawberry fruit of 170 mg/100 g FW (Standard Tables of Food Composition in Japan, 2011), a 23.5% decrease (130 mg/100 g FW) in K was found in 1/32 level of KNO3. The K contents of plant parts suggested that the low KNO3 level was responsible for the low K absorption, which may have affected the translocation and accumulation of K into fruit. Therefore, 1/32 level of KNO3 in nutrient solution lowers the fruit K content considerably.