Reduction of Potassium (K) Content in Strawberry Fruits through KNO_3 Management of Hydroponics

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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.