Isothiocyanate concentration in kohlrabi (Brassica oleracea L. var. gongylodes) plants as influenced by sulfur and nitrogen supply

Glucosinolates (GSss) represent bioactive compounds of Brassica vegetables whose health-promoting effects merely stem from their breakdown products, particularly the isothiocyanates (ITCs), released after hydrolysis of GSs by myrosinase. GSs are occasionally discussed as transient S reservoirs, but little is known concerning the interactive effect of S and N supply on ITC concentrations. Therefore, kohlrabi plants were grown in a pot experiment with varied S (0.00, 0.05, and 0.20 g pot(-1)) and N (1, 2, and 4 g pot(-1)) supplies. Plant growth exhibited a classical nutrient response curve with respect to both S and N. The ITC profile of kohlrabi tubers was dominated by methylthiobutyl ITC (11-1350 mu mol (g DM)(-1)), followed by sulforaphan (7-120 mu mol (g DM)(-1)), phenylethyl ITC (5-34 mu mol (g DM)(-1)), and allyl ITC (5-38 mu mol (g DM)(-1)), resulting from the hydrolysis of glucoerucin, glucoraphanin, gluconasturtiin, and sinigrin, respectively. The ITC profile was in agreement with reported data, and concentrations of all ITCs were substantially reduced in response to increasing N and decreasing S supply. A growth-induced dilution effect could be ruled out in most cases, and the results do not support the hypothesis that GS acts as transient reservoir with respect to S.