Nickel distribution in Stackhousia tryonii shown by synchrotron X-ray fluorescence micro-computed tomography

Context. Hyperaccumulator plants are of considerable interest for their extreme physiology. Stackhousia tryonii is a nickel (Ni) hyperaccumulator plant endemic to ultramafic outcrops in Queensland (Australia) capable of attaining up to 41 300 mu g g(-1) foliar Ni. Aims. This study sought to elucidate the distribution of Ni in S. tryonii by using synchrotron X-ray fluorescence micro-computed tomography (XFM-CT), complemented with elemental maps acquired from physically sectioned plant organs. Its Ni-enriched cylindrical photosynthetic stems make them particularly well suited samples for synchrotron XFM-CT. Method. XFM-CT enables 'virtual sectioning' of a sample, avoiding artefacts arising from physical sample preparation. The method can be used on fresh samples that are frozen during the analysis, which preserves life-like' conditions by limiting radiation damage. It also prevents/minimises other artefacts. Key results. The results showed that Ni is mainly concentrated in the apoplastic space surrounding epidermal cells, and in some epidermal cell vacuoles. This finding is significant because this 'free' solute Ni is likely to be lost during physical sectioning. Conclusions and implications. This case study has highlighted the utility of the XFM-CT approach for visualising metals within intact plant organs, which may be used across the plant sciences.