RESPONSE OF ROOTS OF TREES TO HEAVY-METALS

The heavy metal burden with which tree roots have to cope originates either from natural metalliferous soils or from continuous man-made inputs to forest ecosystems. Considering the interconnections with external factors in the rhizosphere, the responses of roots of various tree species to root-applied Pb, Cd, Zn, Cu, Ni, Mn, and Hg are reviewed. Data on stimulation, but mainly on the depression, of growth parameters such as root elongation, biomass production, root initiation and root hair formation in relation to the architecture of the whole root system, are derived from dose-response experiments with moderate or excess metal supply to young trees using water, sand or soil culture. Assessment of heavy metal toxicity also includes results on the disturbance to the mineral nutrition of tree roots, Effective metal levels from experiments are compared to field concentrations in forest soils and trees. Counteracting strategies such as the phytochelatin response, tolerance mechanisms, or raising of the rhizosphere pH are discussed with respect to tree species and include the crucial role of mycorrhizae.