Nickel toxicity in <em>Brassica rapa</em> seedlings: Impact on sulfur metabolism and mineral nutrient content

Abstract

Throughout the world anthropogenic activity has resulted in enhanced soil nickel (Ni²⁺) levels, which may negatively affect plant productivity. The physiological background of Ni²⁺ phytotoxicity is still largely unclear. Ten-day exposures 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 Ni²⁺. At 5 μM Ni²⁺ 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. Ni²⁺ exposure at ≤ 2 μM only slightly affected the mineral nutrient content of both shoot and root. Hence, Ni²⁺ exposure hardly affected the sulfur metabolite 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 Ni²⁺ tolerance mechanisms of B. rapa.