Abstract

The toxic effects of aluminum, cadmium, copper, manganese, nickel, and zinc stress on root growth of Triticum aestivum were modelled using a modified version of the Weibull frequency distribution. The modified Weibull function is a continuous, nonlinear mathematical function that provides direct estimates of several important biological parameters, including maximum growth, minimum growth, growth response, toxicity threshold, and maximum unit toxicity. The function was fit to experimental data using standard nonlinear regression techniques. Despite variation among metals in the characteristics of observed dose responses, the Weibull function provided a close fit for each of the metals tested (0.96 ≤ R2 ≤ 0.99). Estimates of maximum unit toxicities indicated that cadmium was most toxic to T. aestivum (152% growth reduction (GR)/(μmol∙L)), followed by copper (19.0% GR/(μmol∙L)), nickel (3.9% GR/(μmol∙L)), aluminum (1.3% GR/(μmol∙L)), zinc (0.5% GR/(μmol∙L)), and manganese (0.2% GR/((μmol∙L)). Estimated toxicity thresholds also indicated that cadmium was most toxic. Cadmium showed the lowest threshold for metal injury (0.02 μM), followed by copper (3.4 μM), nickel (11 μM), aluminum (18 μM), manganese (37 μM), and zinc (45 μM). The shift in the relative position of Mn and Zn suggests that unit toxicity and the toxicity threshold may be independent measures of the response of plants to metal stress. These experiments clearly demonstrate the utility of the Weibull function in modelling the response of plants to metal stress. Furthermore, the ability of the function to model primary growth data directly provides a suitable technique for investigation of potential interactions between phytotoxic metals. Key words: dose response, modelling, metal stress, toxicity threshold, unit toxicity.

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