Various toxicity models such as the Biotic Ligand Model (BLM), Tissue Residue Approach (TRA), and Predicted No Effect Concentration (PNEC) calculator were evaluated for their utility in predicting nickel toxicity to aquatic (Lemna minor) or terrestrial (Avena sativa) plants grown in complex media. For the study on L. minor, exposure chemistry was altered by addition of major cations (Ca2+, Mg2+, Na+, K+) or environmental ligands (dissolved organic carbon (DOC), flotation ligands), or altered by pH. Major cations did not inhibit Ni accumulation via competitive inhibition as expected by the BLM framework. In addition, the counter anion of the added Ca greatly affected L. minor growth response to Ni. The effect of DOC was explained solely by complexation with Ni in solution (conforms to BLM), while the effect of flotation ligands was more complex and better described by a TRA model. Nickel toxicity increased linearly with increasing pH; however, Ni accumulation kinetics suggested that the mechanism of interaction between Ni and protons for plant uptake was not competitive as expected by BLM. In general, although the physiological response of L. minor was affected by exposure chemistry and tissue Ni related well with observed toxicity, total Ni and free Ni activity better related to the majority of the plant’s response. For terrestrial plants, Ni PNEC values were calculated and compared to previously published data measuring A. sativa toxicity in agricultural soils near Port Colborne, Ontario. In addition, metal speciation estimated by Windermere Humic Aqueous Model (WHAM) 6 was utilized in Multiple Linear Regression (MLR) analyses to determine factors important for a Terrestrial BLM (TBLM). Calculated PNEC values were conservative (two orders of magnitude lower than no observed effect concentrations). This suggests that the additional assessment factor incorporated into the PNEC calculator is not necessary. In addition, soil type affected Ni’s relationship to toxicity (i.e. clay soil was not well predicted). Overall, the results of this thesis suggests that the simplified approach of relating total dissolved Ni concentration to observed toxicity is sufficient for the purposes of plant Ni risk assessment.