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Heavy Metal Farming

The sap exuding from Pycnandra acuminata in New Caledonia

The sap exuding from Pycnandra acuminata in New Caledonia contains up to 25% nickel.

By Antony van der Ent

Special plants called hyperaccumulators can extract valuable metals from mineralised soils, yielding metallic crops that are more valuable than food grown in soils that are unsuitable for normal agriculture.

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Hyperaccumulators are rare plants that have the unusual ability to concentrate heavy metals, like nickel, into their living shoots. These plants can contain more than 3% nickel in their leaves and up to 25% nickel in their sap, making them truly metallic plants. Unlike the silvery shine of the metal, nickel compounds typically have a bright green colour, and hence the sap of the extreme hyperaccumulators is literally green from nickel. Some of these plants can grow quite big, so a 20-metre high hyperaccumulator tree can contain more than 5 kg of pure nickel.

To most plants, heavy metals such as zinc, copper and nickel are toxic when present in the soil in high concentrations. Therefore, the majority of plants have evolved highly efficient mechanisms to regulate their uptake and keep these metals out of their living shoots. But the behaviour of hyperaccumulator plants is the exact opposite.

So far about 450 hyperaccumulators have been discovered around the world. The greatest numbers have been found in Cuba and New Caledonia, but two species are also known from Australia, one from Queensland (Stackhousia tryonii) and the other from Western Australia (Hybanthus floribundus). My work in Malaysia and Indonesia focuses on discovering more of these plants, and trying to understand how they are able to take up and concentrate metals in their shoots.

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