Metal contamination issues are becoming increasingly common in Australia and elsewhere, with many documented cases of metal toxicity including in mining and agriculture. Metals are a natural part of terrestrial systems occurring in soil, rock, air, water and organisms. A few metals, including Cu, Mn and Zn, are essential to plant metabolism in trace amounts. It is only when metals are present in bioavailable forms at excessive levels that they have the potential to become toxic to plants. If decisions are to be made about how to utilise potentially toxic substrates, an understanding is required of plant responses to these substrates. However, little rigorously designed and quantitative research has dealt with the responses of Australian plants to metals. This thesis outlines the responses of Acacia holosericea, Eucalyptus camaldulensis, Eucalyptus crebra and Melaleuca leucadendra (and Casuarina cunninghamiana) to excess Cu, Mn and Zn.
The species were grown in a series of dilute solution culture experiments utilising techniques such as Programmed Nutrient Addition and solution speciation to maximise the application to soil (solution), and to ensure that results were for the bioavailable metal. Results from these experiments suggested that the species were most sensitive to Cu, and less sensitive of Zn and Mn. Eucalyptus camaldulensis was highly tolerant of excess Zn, and especially excess Mn, supply. While all the species tested were sensitive to elevated Cu concentrations when compared to many crop and pasture species, A. holosericea was more tolerant of Cu than the other species tested. Critical internal and external concentrations (the concentration corresponding to a decrease in dry matter of 10 %) were determined for all species in the presence of all three metals. Critical tissue concentrations were compared with published data for toxicity in fauna. Tissue concentrations in E. camaldulensis grown at excess Mn were of particular concern for wildlife which might feed on the leaves of the species.
The youngest fully expanded leaf (YFEL) proved to be a good index tissue for testing Zn and Mn toxicity. However for Cu toxicity, better correlations were determined between toxicity indices and root metal concentration. Hence, it is suggested in cases of suspected Cu toxicity, at least with the species studied and possibly with other species as well, that use be made of the roots as the index tissue of choice.
Foliar symptoms were recorded throughout the experiments and provide a method of determining probable metal toxicities in the field. Many of the plants exhibited chlorosis similar to Fe deficiency under excess metal exposure. Some of these responded to leaf painting with Fe solution while others did not. A hypothesis is suggested whereby metal-induced Fe chlorosis may be metabolically mediated and may not respond to painting with Fe solution nor be evident as decreased tissue Fe concentrations.
Copper was found to be heavily detrimental to the roots of plants, while Zn was detrimental to both roots and shoots, and Mn tended to have a greater negative effect on shoots. This information has been gathered from solution culture experiments showing critical concentrations in roots vs. shoots, decreases in growth, root macro-symptoms and on SEM research into the effects of Cu, Mn and Zn on root ultra-structure. Symptoms noted in the SEM research included increased root tearing and cracking and root precipitates in plants grown at excess Zn and Cu. Roots of plants grown at excess Mn showed little difference from control plants.
The species responses to excess Cu, Mn and Zn were also tested in a glasshouse soil experiment using a Yellow Kurosol collected from Mt Cotton Queensland and contaminated with varying levels of Cu, Mn or Zn. Considerable agreement existed between toxic tissue and solution concentrations in both the solution culture and soil experiments. As well, there was considerable agreement between foliar symptoms in the solution culture and soil experiments. Limitations in the experimental design and methods for improving attainment of soil solution equilibrium conditions are discussed.