Summary• Little is known about the spatial distribution of excess manganese (Mn) in the leaves of tolerant plants. Recently, the first such study of a Mn hyperaccumulator showed that the highest localized Mn concentrations occur in the photosynthetic tissue. This is in contrast to reports based on localization of foliar accumulation of other heavy metals.• Here, four tree species, Gossia bidwillii , Virotia neurophylla , Macadamia integrifolia and Macadamia tetraphylla , which hyperaccumulate or strongly accumulate Mn, were studied. Cross-sectional foliar Mn localization was carried out in situ using proton-induced X-ray emission/energy dispersive X-ray analysis (PIXE/EDAX).• All four species contained photosynthetic tissues with multiple palisade layers. These were shown to be the primary sequestration sites for Mn. Mn was not detected in the epidermal tissues.• The findings of this study demonstrate a concurrence of three traits in four tree species, that is, accumulation of excess Mn in the leaves, its primary sequestration in the photosynthetic tissues, and multiple-layer palisade mesophyll.

The occurrence of arbuscular mycorrhiza (AM) was surveyed in ten endemic plant species of the Koniambo Massif (New Caledonia) and associated metal-enriched ultramafic soils along a topographic sequence ranging from a plateau at 900 m altitude to a valley at 700 m. In the four different plant formations (Araucaria group on the plateau, ligno-herbaceous maquis, Tristaniopsis maquis and Nothofagus forest in the valley), all plants were consistently colonised by AM fungi, even the sedges Costularia arundinacea, C. nervosa and Lepidosperma perteres and the nickel-hyperaccumulating plant Phyllanthus favieri. Dual (AM and ectomycorrhiza EM) colonisation was observed in the two plant formations dominated by the ectomycorrhizal plants Nothofagus balansae for the forest (site 4) and Tristaniopsis guillainii and T. calobuxus for the Tristaniopsis maquis (site 3). In the soils, there are strong positive correlations between microbial activity, black AM spore abundance and concentrations of available metals indicating the role of the biotic component in the release of metals. These results suggest that these symbioses are important in the adaptation of the endemic plants to these soils, and may be relevant to ecological restoration of the ancient nickel mines.

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 Sebertia acuminata, 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.

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