ISSRNS 2012: Abstracts / Synchrotron Radiation in Natural Science Vol. 11, No 1 – 2 (2012) P 37
DISTRIBUTION AND SPECIATION OF NICKEL
IN HYPERACCUMULATING PLANTS FROM SOUTH AFRICA
J. Mesjasz-Przybylowicz1, E. Montarg`es-Pelletier2, A. Barnabas1, G. Echevarria3, V. Briois4, P. Sechogela1, S. Groeber3, and W. Przybylowicz1,5∗
1Materials Research Department, iThemba LABS, Somerset West 7129, South Africa
2Laboratoire Environnement et Min´eralurgie, CNRS Universit´e de Lorraine, F–54500 Vandoeuvre les Nancy, France
3Laboratoire Sols et Environnement, INRA Universit´e de Lorraine, F–54505 Vandoeuvre-les-Nancy cedex, France
4SOLEIL, SAMBA beamline, l’Orme des Merisiers, Saint Aubin, 91192 Gif sur Yvette cedex, France
5on leave from the Faculty of Physics and Applied Computer Science, University of Science and Technology, Krakow, Poland
Keywords: synchrotron radiation, speciation, nickel, hyperaccumulation, Senecio coronatus, Berkheya coddii, Asteraceae, ultramafic soil, microspectroscopy, XANES, EXAFS, X-ray fluorescence,
x-ray microanalysis, micro-PIXE
∗e-mail : przybylowicz@tlabs.ac.za
Hyperaccumulation is an unusual plant response to soils enriched with heavy metals such as Ni, Co, Zn, and Cd. Most plants growing on these metal- rich soils exclude metals from their shoots as exces- sive accumulation of heavy metals is toxic to the majority of them. However, about 2% of plants in- habiting these soils take up and accumulate large quantities of heavy metals in their shoots: a phe- nomenon known as hyperaccumulation. It has been reported for more than 450 species, mainly Ni accu- mulating species (about 400).
The uptake mechanism is still not well understood, despite increasing number of inves- tigations concentrating on different aspects of hyperaccumulation. The mechanisms underlying the process of hyperaccumulation have been stud- ied in relatively few species, all of which be- long to the Brassicaceae family, known to be pri- marily of European/Mediterranean origin. Most of the results so far are from model plants like Thlaspi sp. and Alyssum sp., but the diversity within hyperaccumulating plant species suggests that several mechanisms are responsible for this phenomenon.
Two South African Ni-hyperaccumulating plants from the Asteraceae: Berkheya coddii Roessler and Senecio coronatus (Thunb.) Harv. were collected from their native habitat on ultramafic soils in the Barberton area (Mpumalanga Province, South Africa). Bulk elemental analyses of different plant parts and related soil were done using AA and ICP.
Plant samples were cryo-fixed in liquid propane and freeze-dried. Microanalyses of elemental concentra- tion and distribution were performed using parti- cle induced X-ray emission (PIXE), complemented by simultaneously used proton backscattering for matrix corrections (nuclear microprobe at iThemba LABS, South Africa). Elemental localization was also examined with SEM-EDXS and micro-PIXE on frozen hydrated material. Plant anatomy and
cytology were studied using light and electron mi- croscopy.
Spatially resolved X-ray absorption experiments were performed on SAMBA beamline to investi- gate nickel distribution and speciation within cross- sections of leaves, stems and roots. Incident X-ray beam was reduced in size using a pinhole for a first series of experiments, and using a monocapillary for a second series of experiments. Regions of interest were selected within the different plant tissues by direct visualisation, with the support of elemental profiles and maps acquired by X-ray fluorescence.
XAS spectra were then collected at Ni K-edge, at room temperature.
Furthermore, in order to check the preserva- tion of nickel status within freeze-dried plant sam- ples, bulk X-ray absorption spectra obtained on frozen-hydrated and freeze-dried plant specimens were compared. XAS data were then collected at liquid N2 temperature.
The results show that Ni is predominantly com- plexed by weak or middle-strong organic ligands, through the presence of carboxylate groups in the first sphere of coordination. Chelation could be def- initely excluded from major accumulation mecha- nism. Spectroscopic signals strongly support the predominance of citrate and malate, low molecular weight ligands, for nickel transport and storage in the hyperaccumulators S. coronatus and B. coddii, growing in their natural environment.
Acknowledgments: This work is based upon re- search supported by the South African National Re- search Foundation and the French Ministries of Research and Foreign Affairs. Any opinion, finding, conclusion or recommendation expressed in this material are those of the authors and therefore the NRF does not accept any liability in regards thereto. The authors greatly acknowledge Mpumalanga Parks Boards, SAFCOL and SAPPI Forestry for permission to access sites and all assistance.
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