Relationships between Ni-hyperaccumulation and mycorrhizal status of different endemic plant species from New Caledonian ultramafic soils

For a long time, Ni-hyperaccumulating plants have been considered to be non-mycorrhizal species. However, two recent publications have reported arbuscular mycorrhizal fungi ( AMF) colonisation in Ni-hyperaccumulators. In this work, 9 endemic Ni-accumulators of unknown mycorrhizal status, from New Caledonia, were studied. All were mycorrhizal, but some were poorly colonised by the symbiots. Only AMF were observed. We analysed the relationships between Ni-hyperaccumulation ability and AMF colonisation of the plants. The roots of the three strongest hyperaccumulators, namely Sebertiaacuminata, Psychotria douarrei and Phyllanthus favieri, were characterised by a lower mycorrhizal colonisation than the others. Mycorrhizal density varied with the level of Ni concentration in soil and plant. Root-colonisation by AMF was negatively correlated with leaf Ni content and with extractable-Ni concentration in soil. The roots of Ni-hyperaccumulators and the soils collected under these plants clearly inhibited germination of AMF spores. Hence, it appears that mycorrhizal colonisation is inhibited above a certain threshold of Ni concentration in soil and plant and becomes either absent or very low. However AMF isolated from the roots of strong Ni-hyperaccumulators have developed a very high level of Ni-tolerance and are then able to colonize at least parts of their roots.