Regression models to stratify the copper toxicity responses and tolerance mechanisms of Glycine max (L.) Merr. plants.

Root antioxidant defense, restricted root-to-shoot Cu translocation, altered nutrient partition, and leaf gas exchange adjustments occurred as tolerance mechanisms of soybean plants to increasing soil Cu levels. The intensive application of copper (Cu) fungicides has been related to the accumulation of this metal in agricultural soils. This study aimed to evaluate the effects of increasing soil Cu levels on soybean (Glycine max) plants. Soybean was cultivated under greenhouse conditions in soils containing different Cu concentrations (11.2, 52.3, 79.4, 133.5, 164.0, 205.1, or 243.8 mg kg), and biochemical and morphophysiological plant responses were analyzed through linear and nonlinear regression models. Although Cu concentrations around 50 mg kg promoted some positive effects on the initial development of soybean plants (e.g., increased root length and dry weight), these Cu concentrations also induced root oxidative stress and activated defense mechanisms (such as the induction of antioxidant response, N and S accumulation in the roots). At higher concentrations, Cu led to growth inhibition (mainly of the root), nutritional imbalance, and damage to the photosynthetic apparatus of soybean plants, resulting in decreased CO assimilation and stomatal conductance. In contrast, low translocation of Cu to the leaves, conservative water use, and increased carboxylation efficiency contributed to the partial mitigation of Cu-induced stress. These responses allowed soybean plants treated with Cu levels in the soil as high as 90 mg kg to maintain growth parameters higher than or similar to those of plants in the non-contaminated soil. These data provide a warning for the potentially deleterious consequences of the increasing use of Cu-based fungicides. However, it is necessary to verify how the responses to Cu contamination are affected by different types of soil and soybean cultivars.