The hyperaccumulation of metals by plants

Hyperaccumulation is a fascinating phenomenon in which plants accumulateunusually large amounts of metals in their aerial parts. In this review the definition of the phenomenon is considered, and it is noted that there has probably been overreporting of it, certainly only the hyperaccumulation of nickel and zinc, and to a lesser extent cadmium and arsenic, have been established and studied experimentally. The character appears to have evolved independently on a number of occasions, but the member of evolutionary events is much less than the number of observed species, since some whole clades appear to show the phenomenon. Hyperaccumulation appears generally to be a species-level phenomenon, though there is within-species variation in degree and specificity of accumulation. The possible mechanisms of accumulation are reviewed. There is evidence that hyperaccumulators preferentially grow roots in areas of high metal concentration; have elevated levels of uptake into root cell symplasm; and have reduced root vacuolar transport. The potential reasons for the evolution of the character are explored. There is some evidence that the elevated level of metals may protect plants from herbivory and parasitism, but there are problems with this hypothesis, especially for the evolution of zinc hyperaccu mulation. The ecological effects of hyperaccumulation are less well studied, but should be a priority for research if the phenomenon is utilised to remediate metal contaminated land, or to mine metals on a widespread scale. © 2003 Elsevier Ltd.