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

Abstract

Background and Aims 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. Methods 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) Key Results 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. Conclusions The occurrence of extremely nickel-rich green-coloured phloem tissues appears to be a characteristic feature of tropical nickel hyperaccumulator plants.

Intensive cycling of nickel in a New Caledonian forest dominated by hyperaccumulator trees

“…Hybanthus austrocaledonicus , one of the tree species in the plot (albeit none had a dbh of >10 cm), can accumulate concentrations up to 1000 μg g −1 Co and 58 000 μg g −1 Ni, making it a Co and Ni hyperaccumulator (Paul et al. , 2020). Conversely, concentrations of Mn were lower in the Ni hyperaccumulator species, suggesting that they do not possess an efficient Mn transport system.…”

Convergent patterns of tissue-level distribution of elements in different tropical woody nickel hyperaccumulator species from Borneo Island

“…This aligns with the results of this study on F. kinabaluensis , P. sarmentosa and A. alanbakeri with Ni enrichment in the phloem. In comparison with the New Caledonian Ni hyperaccumulator plants including Homalium francii (Phyllanthaceae), Hybanthus austrocaledonicus (Rubiaceae) and P. gabriellae (Salicaceae), Ni is also strongly localized in the epidermal cells and phloem bundles ( Gei et al 2020 ; Paul et al 2020 ), and likewise in Geissois pruinosa (Cunoniaceae). However, P. acuminata (Sapotaceae) has Ni-rich laticifers, which constitute an independent network of cells parallel to the vascular bundles ( Gei et al 2020 ).…”

Contrasting nickel and manganese accumulation and localization in New Caledonian Cunoniaceae

“…A handheld XRF device (Thermo Fisher Scientific Niton XL3t 950 GOLDD+) was used to measure the elemental concentrations of Ni and Mn in the leaves of the herbarium specimens. Following completion of the earlier extensive herbarium XRF screening study at the Herbarium of New Caledonia (NOU) (Gei et al 2020b), the calibration model for XRF measurements was further improved (Paul et al 2020a). This revised calibration (which has an improved regression fit of R 2 0.97 for Mn) was used in this study to determine the concentrations of Mn in the Cunoniaceae family with greater accuracy.…”

“…The samples were weighed and ground to a fine powder and subsequently acid digested with 4 mL HNO 3 (70%) in a digestion microwave (Milestone, model Start D) using a 45-min programme. The samples were brought to volume (40 mL) with ultrapure water before analysis using Inductively coupled plasma atomic emission spectroscopy (ICP-AES) with a Thermo Scientific iCAP 7400 instrument as described previously (Paul et al 2020a, b).…”

“…Soil pH was measured in a 1:2.5 soil:water slurry after 2 h of shaking using a glass electrode (Ionode using a 'soil probe'). Exchangeable transition elements were analysed by extraction using 0.1 M Sr(NO 3 ) 2 solution in a soil:solution ratio of 1:4 (10 g soil: 40 mL solution) and 2 h shaking time, while phytoavailable trace elements were extracted in diethylenetriamine penta-acetic acid (DTPA-extractant) solution adapted from (Lindsay and Norvell 1978) as described previously (Paul et al 2020a, b) and then analysed using ICP-AES as for the plant material samples. Collection and processing of P. reticulata and P. xaragurensis specimens for XFM Due to the remote access to the field locations in New Caledonia and logistical challenges with travel to Germany, we were unable to analyse fresh-hydrated specimens for P. reticulata.…”