It has always been asssumed, if not demonstrated unequivocally, that metal tolerance is an entirely genetically-based phenomenon, unaffected by environmental influences (Baker and Walker, 1989). Carefully controlled pretreatment and cultivation experiments with clonal plant materials have shown that a certain degree of metal tolerance can be induced by metal pretreatment and subsequently 'lost' when metal influences are removed. Thus Baker et al. (1986) were able to demonstrate the tolerances of both cadmium-tolerant and non-tolerant populations of the grasses Holcus lanatus, Agrostis capillaris, Festuca rubra and Deschampsia caespitosa were reduced by an overall 13% when clonal materials were cultivated in an uncon-taminated potting compost instead of native soils. Tolerance could also be induced in non-tolerant H. lanatus by transplantation of tillers into metalliferous soil. Subsequent experiments by Walker and Baker (1991 ) with seedlings of this grass have shown that zinc tolerance is similarly inducible, but like cadmium tolerance, much influenced by the nutrient status of the plant and growth medium. Outridge and Hutchinson (1991) have shown similar effects with the induction of cadmium tolerance in the clonal fern Salvinia minima where changes in tolerance of daughter ramets could be shown from prior acclimation to cadmium of parent plants. The evolutionary significance of inducible tolerances requires urgent investigation as the ubiquity of the phenomenon is not known. It could represent an important mechanism for survival of a pollution episode or establishment in marginally-contaminated habitats, such as those resulting from the deposition of airborne metal particulates (Baker et al., 1990).
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