Effect of arsenic, lead and zinc on seed germination and plant growth in black nightshade (Solanum nigrum L.) vs. clover (Trifolium incarnatum L.)

S. nigrum was found to proliferate in sediments with high levels of metal pollution. Toxicity tests were performed using seed germination, root elongation, shoot length and biomass as parameters for establishing growth inhibition of S. nigrum and T. incarnatum by arsenic, lead and zinc. All the elements had a higher inhibitory action on the germination and growth of T. incarnatum - inhibition occurring at 3 mg element per kg of dry sand. S. nigrum presented germination and growth at higher concentrations of those elements - 7 mg Pb kg^-1, 10 mg Zn kg^-1 and 10 mg As kg^-1. Although inhibitory concentrations for each plant were different, increasing the elements concentration reduced seed germination, root elongation and shoot length in T. incarnatum, but not always in S. nigrum. For the latter, low concentrations (3 mg kg^-1 dry sand) of Zn and As seemed to have stimulatory effects on germination. The addition of nutrients did not cause a significant (P < 0.05) difference in the germination of both plants, except for high metal/ metalloid concentration in S. nigrum. Concerning growth characteristics, nutrient addition was only a differentiation factor for shoot length at low metal/metalloid concentrations. S. nigrum could be a promising plant to be used in further experiments concerning the potential of phytoremediation of metal contaminated sites, as long as an adequate planting strategy is followed.