Evaluation of an electrostatic toxicity model for predicting Ni2+ toxicity to barley root elongation in hydroponic cultures and in soils

Assessing environmental risks of metal contamination in soils is a complex task because the biologically effective concentrations of metals in soils vary widely with soil properties. The factors influencing the toxic effect of nickel (Ni) on root growth of barley (Hordeum vulgare) were re-evaluated using published data from both soil and hydroponic cultures. The electrical potential (psi(o)(0)) and ion activities ({I-z}(0)(o)) at the outer surfaces of root-cell plasma membranes (PMs) were computed as the basis of the re-evaluation. The reanalyses demonstrated that root growth was related to: the Ni2+ activity at the PM surface, ({Ni2+}(0)(o)); calcium (Ca) deficiency (related to {Ca2+}(0)(o)); osmotic effects; and modification of intrinsic Ni2+ toxicity by magnesium (Mg2+; this appeared to exert an intrinsic (specific) ameliorating effect on intrinsic Ni2+ toxicity). Electrostatic toxicity models (ETM) were developed to relate root growth to these factors (R-2 > 0.751). Based on the ETM developed in soil culture and a Ni2+ solid-solution partitioning model, critical metal concentrations in soils linked to a biological effect were well predicted for 16 European soils with a wide range of properties, indicating the potential utility of ETM in risk assessment of metals in terrestrial ecosystems.