Characteristics of heavy metal uptake by plant species with potential for phytoremediation and phytomining

Genetically transformed hairy root cultures were established for a range of plant species and applied in studies of growth and accumulation of heavy metals. Experiments were conducted using liquid nutrient medium containing elevated concentrations of Ni, Cd or Cu. Hairy roots of three hyperaccumulator species were tested for Ni uptake; of these, Alyssum bertolonii accumulated the highest Ni contents in the biomass after exposure to 20 ppm Ni for up to 9 h. Ni uptake was relatively slow with 5-7 h required to achieve equilibrium conditions, suggesting the involvement of intracellular processes in Ni accumulation and/or detoxification. In contrast, uptake of Cd and Cu by hairy roots of several hyperaccumulator and non-hyperaccumulator species was fast with equilibrium conditions achieved after only 30-60 min. Cd uptake during the first 9 h of exposure was increased by treatment with H+-ATPase inhibitor and was similar in live and autoclaved roots, suggesting that Cd uptake was due, at least initially, to sorptive rather than intracellular mechanisms. Up to 10,600 mu g(-1) dry weight Cd was accumulated by growing Thlaspi caerulescens hairy roots from a liquid-phase concentration of 100 ppm. In contrast, growth of Nicotiana tabacum hairy roots was severely retarded at 20 ppm Cd and negligible at 100 ppm. Similar Cu levels were accumulated by Hyptis capitata, Polycarpaea longiflora and N. tabacum hairy roots after short-term exposure to 1000 ppm Cu; under the same conditions, the Cu content in Euphorbia hirta hairy roots was 28% lower. Growth of H. capitata roots was slightly reduced in the presence of EDTA, but was unaffected by addition of both EDTA and 20 ppm Cu to the medium. This work demonstrates the utility of hairy roots for screening a range of plant species for their biosorption and long-term metal uptake capabilities. (C) 2000 Elsevier Science Ltd. All rights reserved.