Environmental context. Nickel (Ni) may be present in soil at phytotoxic levels as a result of weathering of ultramafic (serpentine) minerals or activities such as mining and metal ore processing. We assessed the tolerance of two grasses to excess Ni and used electron microscopy to examine the distribution of the Ni within the root tissue. This study provides information on the influence of excess Ni on the growth of these two grasses with consideration to their suitability for the revegetation of areas contaminated with Ni.

Abstract. Toxic effects of Nickel (Ni) in solution were evaluated in signal grass and Rhodes grass, two species commonly used for the revegetation of contaminated sites in the tropics and sub-tropics. Both grasses had a similar response to Ni, a Ni²⁺ activity ({Ni²⁺}) of 14 × 10^–6 M, which reduced the fresh mass by 50%. The sub-cellular distribution of Ni in the roots was similar for both species, with Ni accumulating primarily as particles <5 nm in the vacuoles of rhizodermal and outer cortical cells. The reduction in growth at elevated {Ni²⁺} caused a loss of apical dominance in the roots and a Ni-induced Fe deficiency in the shoots. Root hair growth was not reduced by Ni²⁺ toxicity and was prolific even at the highest {Ni²⁺} (35 × 10^–6 M). The translocation of Ni to the plant tops of both grasses resulted in concentrations that exceeded the guidelines for Ni toxicity to grazing animals (100 μg g^–1) when grown with ≥11 × 10^–6 M {Ni²⁺} in solution.