Reduction of Potassium (K) Content in Strawberry Fruits through KNO_3 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 KNO_3 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 KNO_3 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 KNO_3 at 1/16 of the normal level. Citric acid and ascorbic acid contents of ‘Toyonoka’ fruit were reduced with decreasing KNO_3 concentrations in the nutrient solution. Although the reduced <NO_3>^- of the nutrient solution was adjusted by using Ca(NO_3)_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 170mg/100g FW (Standard Tables of Food Composition in Japan, 2011), a 23.5% decrease (130mg/100g FW) in K was found in 1/32 level of KNO_3. The K contents of plant parts suggested that the low KNO_<3-> 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 KNO_3 in nutrient solution lowers the fruit K content considerably.