Abstract

The increase of copper concentration in soils due to human activities may result in plant toxicity, particularly in sensitive species. However, it has been postulated that some edaphic factors may determine the bioavailable copper fraction in soils and thus the intensity of fitotoxicity, such as the physicochemistry of the substrate, besides the intrinsic solubility of the chemical form in which copper is incorporated to soil. To determine the importance of the metal buffering capacity of the plant-growth substrate and the copper form on the intensity of copper toxicity, seedlings of a sensitive plant species were exposed to increasing copper concentrations, added as a high water soluble form (CuSO4) or as a low water soluble form (CuO) to substrates with high (commercial compost, high content of organic matter and neutral pH) and low (solution of 2 mmol L-1 calcium nitrate with no organic matter and slightly acidic pH) copper buffering capacity. Significant effects of copper toxicity were found in roots of seedlings grown in solutions spiked with either CuSO4 or CuO, at both experimental concentrations, as growth of primary roots was inhibited and the number of secondary roots was reduced. Shoots showed less effects of toxicity as expected, but a significant reduction in the number of leaves was detected on seedlings grown in solutions spiked with CuSO4, at both concentrations, and with high concentration of CuO. No toxicity effects were detected on seedlings grown in commercial compost spiked either with copper sulphate or copper oxide, at any experimental concentration, because the high buffering capacity of the organic matter and the neutral pH of the substrate resulted in low availability of copper ions for plant uptake. Therefore, plant response to elevated copper concentrations is multifactorial and cannot be only assessed from the total copper levels added to soils.