Variability in enzymatic and non-enzymatic antioxidants in red and green-leafy kale in relation to soil type and N-level

•There was no evidence of negative impact of increasing N-level on antioxidants.•N-fertility considerably affected glutathione reductase activity in both cultivars.•Antioxidants were more frequently affected by soil type than by applied N-level.•Green and red-leafy kale distinctly differed in the antioxidant potential.•Genotypes differed slightly in the pattern of response to N-level.

The main objectives of this study were: (i) to determine the differences between guide values concerning soil N-level for yielding and the content of health-promoting compounds; (ii) to evaluate the positive/negative relationships between the enzymatic and non-enzymatic antioxidants and the increasing N-fertility; (iii) finally, to assess the degree to which genotype and soil type may modify the N-dependent response. Two kale genotypes, Redbor F1 and Winterbor F1 characterized by red and green colour of leaves, and two types of soil, mud and lessive soil, were selected to test these interrelationships.The best yield and internal quality of kale plants was obtained at the N range between 100 and 200 mg N dm−3. Of these two, the lower N-level should rather be recommended for the red-leafy kale and in the case of the mud soil. Compared to control, there was no clear negative impact of increasing N-level (in the range 100–300 mg N dm−3) on non-enzymatic antioxidants determined in both kale genotypes in this study. N-fertility considerably affected glutathione reductase in both cultivars, whereas ascorbate peroxidase and catalase activity remained relatively stable. In general, kale plants grown on the lessive soil expressed significantly higher antioxidative enzyme activity as well as ascorbate, thiols and dry matter content as compared to mud soil. Green-leafy kale was characterized by a considerably higher yield and total ascorbate and glutathione contents, whereas red-leafy Redbor F1 expressed significantly higher concentration of global phenolics and total antioxidant ability (FRAP-assay). There were no substantial differences between cultivars in N-dependent response of the examined bioactives.

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