Thresholds of heavy-metal toxicity in cuttings of European black poplar (Populus nigra L.) determined according to antioxidant status of fine roots and morphometrical disorders

The effects of increasing concentrations of polluted soils collected from two different stands in the direct neighbourhood of a copper smelter were studied by analyzing the activity of lipid peroxidation and antioxidant enzymes in the fine roots of cuttings of black poplar (Populus nigra L.). Morphological parameters of affected and unaffected plants were compared and concentrations of heavy metals in their fine roots were estimated. Copper and lead were the major pollutants, and their threshold concentrations were determined on the basis of positive or negative impact on shoots and root growth, morphological disorders of the roots (thickening, decreasing of fine roots volume), stimulation or inhibition of the most important antioxidant enzymes: catalase (CAT), superoxide dismutase (SOD), guaiacol and ascorbate peroxidases (GPOX, APOX) and glutathione reductase (GR) as well as increasing MDA concentration. Finally, three thresholds of Cu and Pb concentrations resulting in increasing toxicity against plants were proposed, splitting the ranges of the non toxic presence of these metals into soil (Cu < 100 ppm, Pb < 30 ppm), full tolerance (Cu 100–220 ppm, Pb 30–70 ppm), limited tolerance (Cu 220–650 ppm, Pb 70–200 ppm) and breakdown of tolerance (Cu > 650 ppm, Pb > 200 ppm).