School of Botany - Theses
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ItemHyperaccumulation of metals in Australian native plants( 2000)The central aim of this thesis was to study the accumulation of metals in Australian native plants. Firstly, this involved searching for new hyperaccumulators on serpentine outcrops in the Wellington region of Victoria and serpentine areas between Rockhampton and Marlborough in central Queensland. A new hyperaccumulator of Mn was found in central Queensland, Austromyrtus bidwillii. This is the first Australian native plant to be reported to hyperaccumulate Mn. Manganese accumulated throughout the above-ground parts of this dry rainforest tree, reaching concentrations up to 19,200 µg g-1 and 26,500 µg g-1 in leaves and young bark, respectively. Concentrations of Ni, Cr, Co, Mn, Mg and Fe in the other species sampled were generally either lower than or similar to the concentrations reported to be typical for serpentine vegetation throughout the world (96 species were sampled from Victoria and 138 species from central Queensland). The aim of the second part of this thesis was to examine possible mechanisms involved in detoxification of metal ions and storage of metal complexes in two known hyperaccumulators of Ni from Australia, Hybanthus floribundus and Stackhousia tryonii, and the hyperaccumulator of Mn identified in the surveys described above. This involved identifying and quantifying organic acids present in leaf extracts and determining whether there were sufficient concentrations of these compounds to sequester the metal. In leaves of Austromyrtus bidwillii, a number of organic acids (oxalic, malic, malonic and succinic acids) were present in relatively high concentrations compared with normal plants and the total concentration of organic acids exceeded the concentration of cations (Mn, Mg and Ca). Purification of the Mn rich extracts of leaves by size exclusion chromatography showed that these organic acids could be associated with the Mn. In both Stackhousia tryonii and Hybanthus floribundus, citric acid was the dominant organic acid present in leaf extracts, but it was present in concentrations that are considered as normal for plant leaves. The effect of treating plants with Ni on citric acid content in leaves and roots of Hybanthus floribundus was investigated. Ni treatment had no significant effect on citric acid content in leaves, but a strong and significant correlation was found between Ni and citric acid content in roots. Citric acid was present in sufficient concentrations in both leaves and roots to complex the Ni in most treated plants, except for those treated with the highest dose of Ni. Similarly, in field samples that contained very high concentrations of Ni (collected from Widgiemooltha, W A), there was insufficient citric acid to complex a significant portion of the Ni. The cellular and sub-cellular distribution of Ni in the two Ni hyperaccumulators was investigated using x-ray microanalysis. In leaves of Hybanthus floribundus, Ni was predominantly localised in the vacuoles of epidermal cells, averaging 128 lnmol kg- l of embedded tissue. Ni was also detected on the outside of the cell walls throughout the leaf cross sections, suggesting that apoplastic compartmentation is also involved in Ni tolerance and accumulation in this species. In leaves of Stackhousia tryonii, Ni was predominantly localised in the cuticle on the surface of the leaf and was also present in epidermal vacuoles. In both species, there was no indication that Ni was associated with phosphates or phytochelatins, as concentrations of P and S were insufficient to suggest an involvement in sequestration of Ni. Thirdly, a protocol for the micropropagation of Hybanthus floribundus was developed, as the germination requirements of seed of this species are unknown and propagation from cuttings is slow and has low efficiency. Large quantities of plants were successfully grown in culture from sterilised pieces of stem or leaf tissue. Importantly, cloned plants retained their ability to hyperaccumulate Ni. Micropropagation provides many opportunities for studying metal tolerance and hyperaccumulation of Ni in this species in the future. Moreover, the protocol presented in this thesis may be used for cultivation of Hybanthus floribundus for rehabilitation of mine sites or perhaps for phytoextraction. Finally, the ability of Hybanthus floribundus ssp. floribundus growing on non-metalliferous soil in Bendigo, Victoria, to hyperaccumulate Ni was investigated. Plants hyperaccumulated Ni when grown on Ni-enriched potting mix, with no adverse effect on growth or nutrient status. This finding suggests that hyperaccumulation of Ni in this species may not be restricted to the populations that grow on serpentine in Western Australia.