Heavy-metal uptake by metal-tolerant Elsholtzia haichowensis and Commelina communis from China

Elsholtzia haichowensis Sun and Commelina communis L. are the most widely distributed heavy-metal-tolerant plants in the areas along the middle and lower streams of the Yangtze River, China. Both plant species were grown in pot trials involving three different treatments containing 3-12 mg kg(-1) cadmium (Cd), 37-140 mg kg(-1) copper (Cu), 30-101 mg kg(-1) chromium (Cr), <3-9 mg kg(-1) nickel (Ni), 66-224 mg kg(-1) lead (Pb), and 1,000-2,850 mg kg(-1) zinc (Zn). Hydroponic trials were also conducted using Cd, Cu, and Zn only. Both species accumulated much higher concentrations (by a factor of about ten) of copper and other heavy metals in the roots than in the shoots, following a typical pattern of metal-excluder plants. The metal concentrations in the plant tissues, especially the roots, of E. haichowensis and C. communis were positively correlated to the metal concentrations in the growth media. With increasing heavy metal concentrations in the root media, the shoot biomass yields of E. haichowensis increased, but the root yields showed no significant variation. Shoot or root dry biomass of C. communis increased in the case of the pot trials but decreased in the hydroponic culture fi om the low metal to high metal treatments. The results suggest that E. haichowensis may be a better biogeochemical indicator than C. communis for ore exploration and both may also be used fur phytostabilization of toxic mine spoils because of their tolerance to heavy metal. It is pro posed that although either roots or shoots could be used for biogeochemical prospecting, shoot sampling would be easier and more cost-effective.