Nickel toxicity in Brassica rapa seedlings: Impact on sulfur metabolism and mineral nutrient content

Throughout the world anthropogenic activity has resulted in enhanced soil nickel (Ni 2+) levels, which may negatively affect plant productivity. The physiological background of Ni 2+ phytotoxicity is still largely unclear. Ten-day expo-sures of Brassica rapa seedlings to 1, 2 and 5 μM NiCl2 resulted in strongly enhanced tissue Ni levels, a decreased biomass production and leaf chlorosis at ≥ 2 μM Ni 2+. At 5 μM Ni 2+ plant growth was completely halted. Ni toxicity occurred when the content of the shoot exceeded 1.0 μmol g –1 dry weight and that of the root, 23 μmol g –1 dry weight. Ni 2+ exposure at ≤ 2 μM only slightly affected the mineral nutrient content of both shoot and root. Hence, Ni 2+ exposure hardly affected the sulfur metabo-lite content of the plant. At ≥ 1 μM Ni2+ the total sulfur content of the root was only slightly lowered, which could fully be ascribed to a decreased sulfate content. Moreover, the water-soluble non-protein thiol content of both shoot and root was only enhanced at 5 μM Ni2+ . From these results it was clear that sulfur metabolism was unlikely to be directly involved in the Ni 2+ tolerance mechanisms of B. rapa.