The aim of the research was to find out how foliar selenium (Se) treatment in open field conditions affects plant nutrition, yield and bulb bioactive properties. 10, 50 or 100 µg·mL^-1 Se, referred to as Se10, Se50 and Se100 treatments, was applied to onion ‘Hercules’ in 2008 and 2009. Bulb weight, total yield, content of total Se, free selenomethionine and selenomethylselenocysteine, total S, N, P, K, Ca and Mg, content of total phenolics, pungency and total antioxidant capacity (TAC) were determined. All Se treatments significantly reduced bulb S content. The Se100 treatment had a tendency to decrease bulb size and yield. The Se50 treatment increased total Se content, total phenolics and TAC and had a tendency to increase the yield. A larger proportion of total Se was converted into organic compounds in the Se50 than in the Se100 treatment. In Se 10 treatment, bulb TAC was the highest among treatments in 2009. Considering both agronomic and human health benefits, Na₂SeO₄ solution at the rate of 50 μg·mL^-1 Se can be recommended for bulb onion.

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