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

Abstract

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 × 10 3 mg kg −1 dry weight (DW) with highest Ni concentration in cotyledonary tissues (7.6 × 10 3 mg kg −1 DW), followed by the embryonic axis (4.4 × 10 3 mg kg −1 DW). Nickel concentration in whole H. floribundus subsp. floribundus seeds was 3.5 × 10 2 mg kg −1 DW without a clear pattern of Ni localisation. The average Ni concentration in whole P. leptospermoides seeds was 2.6 × 10 2 mg kg −1 DW, and Ni was preferentially localised in the embryonic axis (4.3 × 10 2 mg kg −1 DW). In T. caerulescens, Cd concentrations were similar in cotyledon (4.5 × 10 3 mg kg −1 DW) and embryonic axis (3.3 × 10 3 mg kg −1 DW) tissues, whereas Zn was highest in cotyledonary tissues (1.5 × 10 3 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.