Exceptional Uptake and Accumulation of Chemical Elements in Plants: Extending the Hyperaccumulation Paradigm

Hyperaccumulation of elements in plants has been previously defined as an element-specific behaviour resulting in concentrations in foliar dry mass that are at least two orders of magnitude greater than the average concentrations of the same element in typical plants. The recognition of trace element hyperaccumulation of Ni, Co, Cu, Zn, Cd, Pb, Mn, Se, As and Tl has recently been clarified. In this review, instances of unusual accumulation of other elements in plants are assessed, including both common soil elements (such as Al and Si) and non-essential (ultra-)trace elements (such as Ba, Sr, Sb, W, Pd and rare earth elements). When sufficient data are available, threshold criteria for recognition of hyperaccumulation are proposed, but in other cases there is insufficient knowledge to make such a judgment. The physiological basis for hyperaccumulation is also discussed. Most of the elements described here are non-essential, with no known selective advantage for hyperaccumulation. It is therefore likely that these elements accumulate as an incidental side-effect of nutrient acquisition, either because they are chemical analogues of mineral nutrients, or because of shared uptake pathways.