Foliar application of potassium-based fertilizer improves strawberry fruit quality F. Valentinuzzi 1 , M. Maver 1 , S. Fontanari 2 , D. Mott 2 , G. Savini 2 , R. Tiziani 1 , Y. Pii 1,a , T. Mimmo 1 and S. Cesco 1 1 Faculty of Science and Technology, Free University of Bolzano, 39100 Bolzano, Italy; 2 Società Cooperativa Sant'Orsola, Pergine Valsugana (TN), Italy. Abstract Among the different mineral nutrients, potassium (K) is recognized as the macronutrient that most strongly affects the quality parameters of crops. An adequate K nutrition is in fact associated with increased fruit size, enhanced nutraceutical properties (e.g., content of sugar, organic acids, antioxidants), improved color and increased shelf life for many agricultural products. Previous studies have shown that, in controlled conditions, the foliar application of K-based fertilizers during the fruit development period could improve fruit quality; however, it is not clear whether such positive results can also be obtained in field-cultivated plants. For these reasons, the aim of the present study was to assess if the repeated applications of K-based fertilizers during the production period (mid-June to late August) might affect the quality parameters of strawberry (Fragaria × ananassa ‘Murano’) fruits. Frigoplants, transplanted in April, were cultivated in grow bags on cocopeat (8 plants per linear meter) and treated foliarly spraying K2SO4 (6 g L -1 ) either once or twice a week. Control plants were sprayed with distilled water. As expected, foliar K-fertilization increased the K concentration by 8-11% in fruits and by 7-5% in leaves according to the frequency of application (once and twice a week, respectively). In addition, strawberry fruits exhibited an enhanced total soluble solids content (approximately 12%) and sweetness index (approximately 13%). Conversely, other nutraceutical compounds (e.g., phenolics, flavonoids and flavonols) as well as the fruit yield were not affected by K treatments, regardless the fertilization levels. In conclusion, results here presented show that at the field scale the foliar K-fertilization during the development of strawberries can enhance the quality and the nutraceutical value of fruits. Keywords: foliar fertilization, potassium, strawberry, fruit quality INTRODUCTION Potassium (K) is an essential mineral nutrient for plants (Marschner, 2011). Despite not being involved in the formation of either organic molecules or cell structure, it is required for a plethora of biochemical and physiological processes that are fundamental for plant growth, resistance to stresses, yield, fruit quality as well as its shelf life (Cakmak, 2005; Lester et al., 2005, 2006; Marschner, 2011). In plants, K is taken up by roots as monovalent cation (K + ) from soil solution (Marschner, 2011) and its absorption is determined by several factors including environmental conditions and genetics (Rengel and Damon, 2008). In many different plant species, the uptake of K occurs mainly during the vegetative stage, when photosynthetates are largely available for root growth. On the contrary, during the reproductive stage, there is a strong competition between developing fruits and vegetative organs for these photoassimilates. This phenomenon may have an impact on the K acquisition at root level (Ho, 1988) and, in turn, on the yield and quality (Lester et al., 2005, 2006). Indeed, under such conditions, the soil supplementation with K-based fertilizers can a E-mail: youry.pii@unibz.it Acta Hortic. 1217. ISHS 2018. DOI 10.17660/ActaHortic.2018.1217.48 Proc. VIII International Symposium on Mineral Nutrition of Fruit Crops Eds.: T. Mimmo, Y. Pii and F. Scandellari 379