Multiple metal stress in Triticum aestivum. Differentiation between additive, multiplicative, antagonistic, and synergistic effects

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Abstract
Aluminum toxicity is an important factor limiting plant growth in soils with pH below 5.0. Because metalliferous soils are commonly acidic, understanding plant growth dynamics on such soils may require insight into the interaction between aluminum and other phytotoxic metals. To investigate potential interactions in the phytotoxicity of nickel and aluminum, seedlings of Triticum aestivum L. cv. Neepawa were grown for 14 days in solution culture with varying concentrations of nickel (0–60 μM), with or without 40 μM aluminum. Increasing concentrations of nickel were observed in roots (232–2318 μg ∙ g−1) and leaves (25–152 μg ∙ g−1) with increasing nickel in solution, but concentrations were reduced with addition of aluminum as a second phytotoxic metal (roots: 125–1314 μg ∙ g−1; leaves: 20–108 μg ∙ g−1). Aluminum was only detectable in roots of seedlings grown with aluminum and declined with increasing nickel in the growth solution (1446–530 μg ∙ g−1). Addition of nickel to growth solutions reduced both root (0.981–0.193 g) and leaf (2.608–1.030 g) weights. Aluminum as a second phytotoxic metal further reduced root (0.698–0.194 g) and leaf (2.245–1.111 g) weights at low concentrations of nickel in solution. Analysis of variance of the root weight data indicated a significant nickel × aluminum interaction, but this may not have been indicative of a biological interaction. Root weight, both in the presence and absence of aluminum, decreased to a common minimum at high concentrations of nickel in the growth solution, possibly reflecting biomass present at the onset of experimental treatments and the lag time required for the expression of the phytotoxic growth response. When root growth relative to control was expressed as weight above the empirical minimum (the root weight index), results of the analysis of variance were different. Significant main effects of nickel were detected, while main effects due to aluminum and the nickel × aluminum interaction were not significant. Despite antagonistic effects observed with metal uptake and accumulation, such results suggest that the combined toxic effect of nickel and aluminum can be explained by a simple multiplicative model.