The effect of nitrate and ammonium nutrition on the toxicity of zinc to zinc-tolerant and non-tolerant clones of Deschampsia cespitosa (L.) Beauv, was investigated. Zinc and nitrogen source interacted in their effect on root growth and zinc tended to be less toxic if ammonium was the nitrogen source. Root growth of the non-tolerant clone was inhibited by ammonium. Zinc caused chlorosis in nitrate-grown tolerant plants but not in ammonium-grown plants. Zinc caused a decrease in the concentrations of reduced nitrogen compounds in leaves and decreased nitrate translocation to them. The zinc concentration in roots was slightly decreased in ammonium-grown plants. In vivo nitrate reductase activity was stimulated by high zinc concentrations in the tolerant clone. Asparagine and proline accumulated in response to zinc treatment in the non-tolerant clone. Ammonium nutrition also induced asparagine accumulation and such accumulation was much greater in the non-tolerant clone.
The small decrease in zinc toxicity when ammonium is the nitrogen source could be only partly accounted for by lower zinc uptake. It is suggested that asparagine which accumulates in ammonium-grown plants, forms an intracellular complex with zinc and thereby decreases its toxicity.