Hyperaccumulators of metal and metalloid trace elements: Facts and fiction

Plants that accumulate metal and metalloid trace elements to extraordinarily high concentrations in their living biomass have inspired much research worldwide during the last decades. Hyperaccumulators have been recorded and experimentally confirmed for elements such as nickel, zinc, cadmium, manganese, arsenic and selenium. However, to date, hyperaccumulation of lead, copper, cobalt, chromium and thallium remain largely unconfirmed. Recent uses of the term in relation to rare-earth elements require critical evaluation. Since the mid-1970s the term 'hyperaccumulator' has been used millions of times by thousands of people, with varying degrees of precision, aptness and understanding that have not always corresponded with the views of the originators of the terminology and of the present authors. There is therefore a need to clarify the circumstances in which the term 'hyperaccumulator' is appropriate and to set out the conditions that should be met when the terms are used. We outline here the main considerations for establishing metal or metalloid hyperaccumulation status of plants, (re)define some of the terminology and note potential pitfalls. Unambiguous communication will require the international scientific community to adopt standard terminology and methods for confirming the reliability of analytical data in relation to metal and metalloid hyperaccumulators.