Hyperaccumulation of metals in Thlapsi geosingense and Thlapsi rotundifolium ssp. Cepaeifolim

Metal-hyperaccumulating plant species have been proposed as starting point and model system for the development of emerging phytoremediation technologies. This project aims to improve our understanding of rhizospheric and physiological processes responsible for the hyperaccumulation of metals in two species of Thlaspi (T. cepaeifolium, Pb/Zn/Cd accumulator, and T. goesingense, a Ni/Cr accumulator). Both species are indigenously growing in Austria, with T. goesingense endemically restricted to few sites in East Austria. We will address physiological processes involved in metal uptake and translocation by these plants as influenced by pH, metal species translocated in the plants, and sites of metal accumulation in plant organs. Focus will also be centered on associated processes in the rhizosphere soil that influence the bioavailability of metals. Pot experiments will be performed to study the response (plant growth, metal uptake and accumulation) of both Thlaspi when grown on soils differing in the pattern of metal contamination from the original location, a situation which is of major concern in phytoremediation. The project is partitioned in three major sections: (1) investigation of processes involved in metal mobilization and uptake in the rhizosphere, including (a) the identification of exudates released by the Thlaspi species during hydroponic experiments at different pH, and (b) the study of root-soil interactions in rhizoboxes equipped with microlysimeters; (2) Experiments on metal uptake, using three different approaches: (a) hydroponic experiments at different pH, (b) transplant experiments, and (c) co-transplant experiments using pot studies; and (3) Experiments to assess metal translocation and compartmentalization in both Thlaspi species, focusing on (a) the analysis of the xylem sap to study translocation processes involved in hyperaccumulation, and (b) X- ray microanalysis (EDAX) to assess compartmentalization of metals in shoots. From the above studies we expect to elucidate fundamental physiological processes that regulate the hyperaccumulation of metals in Thlaspi species, i.e. metal uptake (in both hydroponics and soil), translocation and compartmentalization of metals in plant organs. The interaction of rhizosphere processes and metal mobilization, such as the effect of pH changes and release of organic acids will be studied as well. The study of potential synergistic influences of these species when jointly grown in different soils will improve our understanding of rhizospheric interactions. Moreover, the transplant experiments will provide evidence as to whether the two Thlaspi species can extract other metals than those present in their indigenous environments. The expected advances in our understanding of fundamental processes involved in hyperaccumulation of metals by Thlaspi species will provide useful information for further development of phytoextraction technologies.

New insights into the hyperaccumulation of heavy metals were the results of this project. Specific objects were to elucidate processes in soil and root zone (rhizosphere). Influence of plant roots on heavy metal mobility and thus bioavailability was demonstrated. Starting point in the investigation of hyperaccumulation processes in soil and plant was a field survey conducted on a serpentine site. Here, a specific influence of metal-accumulating and metal-excluding plants on rhizosphere soil properties was found. The mobilisation of water-soluble Ni and additionally a high concentration of dissolved organic carbon was determined in the rhizosphere of the Ni hyperaccumulator Thlaspi goesingense. In cooperation with a project funded by the University of Agricultural Sciences Vienna (BOKU), a new rhizobox design has been developed. This new concept allows the determination of concentration gradients of heavy metals or nutrients in the rhizosphere of plants. In an experiment with Thlaspi goesingense it has been shown that water- soluble Ni is increased in the rhizosphere, whereas a decrease was found for the exchangable Ni. These findings may be related to plant-induced mobilisation processes. However, the depletion of exchangable Ni may be directly related to the excessive Ni uptake by the plant. The results of this experiment were used to create a mathematical model to describe and calculate rhizosphere processes, which was done in another cooperation with a project funded by the Austrian Ministry for Education, Science and Culture. With the help of this model it could be demonstrated that Ni replenishement from less soluble Ni fractions in the rhizosphere is a fundamental process determining concentration gradients of Ni. Interestingly, pH changes play a minor role in Thlaspi goesingense rhizosphere. However, the release of root exduates has a significant influence on mobilisation of heavy metals. The analysis of root exudates could only be achieved to a limited extent due to some problems in collection and chromatographic separation and detection. However, in a current project, also funded by FWF (P 15357, P 15215), intensive research focuses on the determination of quantity and quality of T. goesingense root exudates. The results of this project have a significant impact on the development of phytoremediation technologies (soil remediation with plants). Mobilisation of heavy metals and other pollutants is the key process determining the success of phytoremediation. This project emphasises the role of root exudates in metal mobilisation and thus metal bioavailability. The plant induced metal depletion and metal replenisment processes in soil could be demonstrated. Most of the hyperaccumulating plants are too small to be used in phytoremediation, but they could be used in intercroppings in order do induce mobilisation of heavy metals and thus improve the plant uptake of heavy metals.