TERRESTRIAL HIGHER PLANTS WHICH HYPERACCUMULATE METALLIC ELEMENTS. A REVIEW OF THEIR DISTRIBUTION, ECOLOGY AND PHYTOCHEMISTRY

This paper reviews the plant geography, ecology, metal tolerance and phytochcmistry of terrestrial higher plants which arc able to accumulate metallic elements in their dry matter to an exceptional degree. The review is limited to the elements Co, Cu, Cr, Pb. Mn. Ni and Zn. Hyperaccumulators of Co, Cu, Cr, Pb and Ni arc here defined as plants containing over 1000 u.g/g (ppm) of any of these elements in the dry matter; for Mn and Zn, the criterion is 10,000 u.g/g (1%). A unifying feature of hypcraccumula ting plants is their general restriction to mineralized soils and specific rock types. Lists of hypcraccumula ting species arc presented for the elements considered. These suggest that the phenomenon is widespread throughout the plant kingdom. For example, 145 hyper-accumulators of nickel are reported: these arc distributed among 6 supcrordcrs, 17 orders, and 22 families and include herbs, shrubs and trees from both the temperate and tropical zones. Although some phylogcnetic relationships emerge, the evolutionary significance of metal hyperaccumulation remains obscure. Phytochemical studies however suggest that hyperaccumulation is closely linked to the mechanism of metal tolerance involved in the successful colonization of metalliferous and otherwise phytotoxic soils. The potentialities of hyperaccumula ting plants in biorccovcry and soil detoxification arc indicated.