Nickel hyperaccumulation in New Caledonian Hybanthus (Violaceae) and occurrence of nickel-rich phloem in Hybanthus austrocaledonicus

Hybanthus austrocaledonicus (Violaceae) is a Ni hyperaccumulator endemic to New Caledonia. One of the specimens stored at the local herbarium had a strip of bark with a remarkably green phloem tissue attached to the sheet containing over 4 Wt% Ni. This study aimed to collect field samples from the original H. austrocaledonicus locality to confirm the nature of the green "nickel-rich phloem" in this taxon and to systematically assess the occurrence of Ni hyperaccumulation in Hybanthus austrocaledonicus and H. caledonicus populations.X-ray fluorescence spectroscopy (XRF) scanning of all collections of the genus Hybanthus (236 specimens) was undertaken at the Herbarium of New Caledonia to reveal incidences of nickel accumulation in populations of Hybanthus austrocaledonicus and H. caledonicus. In parallel, micro-analytical investigations were performed via Synchrotron X-ray fluorescence microscopy (XFM) and Scanning electron microscopy with X-ray microanalysis (SEM-EDS).The extensive scanning demonstrated that Ni hyperaccumulation is not a characteristic common to all populations in the endemic Hybanthus species. Synchrotron X-ray fluorescence microscopy (XFM) revealed that nickel was exclusively concentrated in the epidermal cells of the leaf blade and petiole, conforming with the majority of (tropical) Ni hyperaccumulator plants studied to date. Scanning electron microscopy with X-ray microanalysis (SEM-EDS) of freeze-dried and frozen-hydrated samples revealed the presence of dense solid deposits in the phloem bundles that contained >8 Wt% nickel.The occurrence of extremely nickel-rich green-coloured phloem tissues appears to be a characteristic feature of tropical nickel hyperaccumulator plants