Heavy metal pollution improves allelopathic effects of Canada goldenrod on lettuce germination

Large amounts of heavy metals have been released into the environment. Thus, the allelopathic effects of invasive alien species on the germination performance of co-occurring indigenous species may be altered or even heightened with the rapid growth in heavy metal pollution. This study evaluated the impacts of Canada goldenrod (Solidago canadensis L.) leaf extracts at concentrations of 0, 10 or 20 gl 1 on the germination of lettuce under different forms of heavy metal pollution (Cu²⁺ , Pb²⁺ or a combination of Cu²⁺ and Pb²⁺ ; 35 mgl 1) during incubation in Petri dishes for 10 days. Goldenrod leaf extracts (high concentration) reduced growth of aboveground and belowground parts of lettuce as well as competition for light and soil nutrients. However, low concentrations of goldenrod leaf extracts dramatically improved growth of lettuce roots, competition for light, soil nutrient availability, leaf photosynthetic area and growth competitiveness. The combination of goldenrod leaf extracts and heavy metal pollution was synergistic on most lettuce germination parameters, probably because high concentrations of goldenrod leaf extracts together with heavy metal pollution had a synergistic negative impact on lettuce germination. Consequently, increased levels of heavy metal pollution may favour invasion of invasive alien species while largely suppressing germination of indigenous species.