| Summary: | Biofortification of <i>Brassica oleracea</i> with selenium (Se) is highly valuable both for human Se status optimization and functional food production with direct anti-carcinogenic activity. To assess the effects of organic and inorganic Se supply for biofortifying <i>Brassica</i> representatives, foliar applications of sodium selenate and selenocystine (SeCys<sub>2</sub>) were performed on Savoy cabbage treated with the growth stimulator microalgae <i>Chlorella</i>. Compared to sodium selenate, SeCys<sub>2</sub> exerted a stronger growth stimulation of heads (1.3 against 1.14 times) and an increase of leaf concentration of chlorophyll (1.56 against 1.2 times) and ascorbic acid (1.37 against 1.27 times). Head density was reduced by 1.22 times by foliar application of sodium selenate and by 1.58 times by SeCys<sub>2</sub>. Despite the greater growth stimulation effect of SeCys<sub>2</sub>, its application resulted in lower biofortification levels (2.9 times) compared to sodium selenate (11.6 times). Se concentration decreased according to the following sequence: leaves > roots > head. The antioxidant activity (AOA) was higher in water extracts compared to the ethanol ones in the heads, but the opposite trend was recorded in the leaves. <i>Chlorella</i> supply significantly increased the efficiency of biofortification with sodium selenate (by 1.57 times) but had no effect in the case of SeCys<sub>2</sub> application. Positive correlations were found between leaf and head weight (r = 0.621); head weight and Se content under selenate supply (r = 0.897–0.954); leaf ascorbic acid and total yield (r = 0.559), and chlorophyll (r = +0.83–0.89). Significant varietal differences were recorded for all the parameters examined. The broad comparison performed between the effects of selenate and SeCys<sub>2</sub> showed significant genetic differences as well as important peculiarities connected with the Se chemical form and its complex interaction with <i>Chlorella</i> treatment.
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