Studies on spatial distribution of nickel in leaves and stems of the metal hyperaccumulator Stackhousia tryonii Bailey using nuclear microprobe (micro-PIXE) and EDXS techniques.

Stackhousia tryonii Bailey is one of the three nickel hyperaccumulators reported from Australia. It is a rare, herbaceous plant that accumulates (Ni) both in leaf and stem tissues. Localisation of Ni in leaf and stem tissues of S. tryonii was studied using two micro-analytical techniques, energy dispersive X-ray spectrometry (EDXS) and micro-proton-induced X-ray emission spectrometry (micro-PIXE). Dimethylglyoxime complexation of Ni was also visualised by bright- and dark-field microscopy, but this technique was considered to create artefacts in the distribution of Ni. Energy dispersive X-ray spectrometric analysis indicated that guard cells possessed a lower Ni concentration than epidermal cells, and that epidermal cells and vascular tissue contained higher levels of Ni than mesophyll, as reported for other Ni hyperaccumulators. The highest Ni concentration was recorded (PIXE quantitative point analysis) in the epidermal cells and vascular tissue (5400 µg g-1 DW), approximately double that recorded in palisade cells (2500 µg g-1 DW). However, concentrations were variable within these tissues, explaining, in part, the similarity between average Ni concentrations of these tissues (as estimated by region selection mode). Stem tissues showed a similar distribution pattern as leaves, with relatively low Ni concentration in the pith (central) region. The majority of Ni (73-85% for leaves; 80-92% for stem) was extracted from freeze-dried sections by water extraction, suggesting that this metal is present in a highly soluble and mobile form in the leaf and stem tissues of S. tryonii.

Elemental mapping using PIXE shows the main pathway of nickel movement is principally symplastic within the fruit of the hyperaccumulator Stackhousia tryonii.

• Metal concentrations within reproductive tissues of metallophytes are rarely reported. Here, the spatial distribution of nickel (Ni) within the fruits (seeds) of the Ni hyperaccumulator Stackhousia tryonii was investigated. • Two microanalytical techniques, energy dispersive x-ray spectrometry (EDXS) and nuclear microprobe (micro-proton-induced x-ray emission spectrometry; micro-PIXE) were employed for qualitative and quantitative assessment, respectively, of localized Ni, within the fruits of S. tryonii. The results were compared with quantitative analysis made using inductively coupled plasma-optical emission spectrometry (ICP-OES). • Nickel analysis made using micro-PIXE was consistent with bulk (ICP-OES) analysis (at 1800 µg g-1 d. wt), however, a beam resolution of approx. 2 × 2 µm2 allowed tissue localization. Nickel was partitioned to the fruit wall (pericarp) (4433 µg g-1 ), while endospermic and cotyledonary tissues possessed little Ni (309 and 182 µg g-1 d. wt, respectively). • This distribution is consistent with the interpretation that principal pathway of Ni movement within the fruit is symplastic rather than apoplastic (as the filial generation lacks symplastic connection with the parent).