ABSTRACT
This chapter presents knowledge of the mechanisms and genetics of heavy metal tolerance in hyperaccumulating and nonhyperaccumulatitig metallophytes. Strongly heavy metal-enriched substrates are hostile to plant growth. The colonization of metalliferous substrates involves evolutionary adaptation. Plants of facultative metallophyte populations from metalliferous soil almost invariably exhibit higher levels of metal tolerance than those of normal soil populations. Metalliferous soils are often enriched in combinations of different heavy metals and, therefore, local metallophyte populations often exhibit combined tolerance to different metals. The physiological and biochemical mechanisms of metal tolerance are poorly understood. In general, tolerance in unadapted and metal-adapted plants might depend on qualitatively different mechanisms. Hyperaccumulators, just like nonhyperaccumulating metallophytes, may show relatively high levels of tolerance to metals that are not present at toxic concentrations in soil at the sites of population origin. Hyperaccumulators seem to exhibit a high degree of metal specificity with regard to foliar accumulation.
