Nickel, Zn and Cd localisation in seeds of metal hyperaccumulators using μ-PIXE spectroscopy

Metal hyperaccumulators are a rare group of plant species that accumulate exceptionally high concentrations of metals in above ground tissues without showing symptoms of phytotoxicity. Quantitative localisation of the accumulated metals in seed tissues is of considerable interest to help understand the eco-physiology of these unique plant species. We investigated the spatial localisation of metals within seeds of Ni hyperaccumulating Hybanthus floribundus subsp. adpressus, H. floribundus subsp. floribundus and Pimelea leptospermoides and dual-metal (Cd and Zn) hyperaccumulating Thlaspi caerulescens using quantitative micro-proton induced X-ray emission (μ-PIXE) spectroscopy. Intact seeds were hand-sectioned, sandwiched between Formvar films and irradiated using the 3 MeV high energy heavy ion microprobe at ANSTO. Elemental maps of whole H. floribundus subsp. adpressus seeds showed an average Ni concentration of 5.1 × 103 mg kg−1 dry weight (DW) with highest Ni concentration in cotyledonary tissues (7.6 × 103 mg kg−1 DW), followed by the embryonic axis (4.4 × 103 mg kg−1 DW). Nickel concentration in whole H. floribundus subsp. floribundus seeds was 3.5 × 102 mg kg−1 DW without a clear pattern of Ni localisation. The average Ni concentration in whole P. leptospermoides seeds was 2.6 × 102 mg kg−1 DW, and Ni was preferentially localised in the embryonic axis (4.3 × 102 mg kg−1 DW). In T. caerulescens, Cd concentrations were similar in cotyledon (4.5 × 103 mg kg−1 DW) and embryonic axis (3.3 × 103 mg kg−1 DW) tissues, whereas Zn was highest in cotyledonary tissues (1.5 × 103 mg kg−1 DW). In all species, the presence of the accumulated metal within the cotyledonary and embryonic axis tissues indicates that the accumulated metal was able to move apoplastically within the seed.