Tolerance and accumulation of heavy metals by wild plant species grown in contaminated soils in Apulia region, Southern Italy

Many sites inside a protected area in Apulia region (Italy) have been contaminated with heavy metals (Cd, Cr, Cu, Ni, Pb, Zn) because an inadequate disposal of a variety of wastes with different sources of origin. As first measure in-situ phytoremediation techniques were evaluated using only the natural plants that grew wildly on the contaminated soils, in order to minimize the environmental impact on this fragile ecosystem. Total contents of metals in soils generally exceeded the maximum levels indicated in Italian and European regulations, specially Cr and Zn. Although the extractable fraction was element-dependent, the metal immobilization was enhanced by the components of soils. Maximum mobilizable fractions (%DTPA of total content) were 30% Cd, 0.01% Cr, 11.5% Cu, 4.1% Ni, 13.3% Pb and 13.8% Zn. The general trend of metal accumulation in plants was Zn > Cu > Cr > Pb > Ni > Cd and statically accumulation differences were found to largely depend on plant species. Thus different metal uptake and translocation strategies were suggested in the studied species: exclusion for Stipa austroitalica and Dasypyrum villosum, whereas tolerance mechanisms for Carduus pycnocephalus, Silybum marianum and Sinapis arvensis. The metal contents in above ground parts of these species were within the values of normal in plants and below phytotoxic levels, thus faraway from phytoextraction applicability. These species can be considered as metal excluder or tolerant plants with ability of growing in soils with a wide range of heavy metal concentrations, mainly immobilized by soil conditions. Thus they accomplished the criteria to be considered for phytostabilization technique in these contaminated sites.