Heavy metal tolerance, uptake, accumulation, and translocation in Salix and its application in phytoremediation

In the environment, heavy metals are major stressors for plants, and since plants are immobile, they cannot avoid the stress but must adapt. Therefore, plants may develop avoidance and tolerance mechanisms to reduce susceptibility to metal toxicity. Salix is often found at metal-contaminated sites and, thus, is likely to have evolved mechanisms to adapt with heavy metals. The aim of this thesis was to identify heavy metal-related traits like tolerance, uptake, accumulation, and root-shoot translocation in Salix and to find potential physiological processes that explain those traits. In addition, Salix’s potential for removing heavy metals from polluted soil, i.e., for phytoremediation, was to be investigated. About 130 clones of Salix were analysed and characterized regarding tolerance to, uptake of, accumulation, and translocation of cadmium, copper, and zinc. The heredity of these traits, as well as potential tolerance mechanisms such as the induction of phytochelatin production to bind metals, the production of stress-related signalling substances like salicylic acid, antioxidant defence mechanisms, reactions to treatment with nicotinic acid or nicotinamide, preventive uptake or efflux of metals and possible metal-metal interactions, were investigated. Furthermore, the potential for phytoremediation was studied in the field, i.e., in agricultural and industrial polluted soil. The results showed a significant variation in all the analysed traits for all metals. The correlations between the traits were that all traits had high heredity was high for all traits. Phytochelatins were not found. The activity of ascorbate peroxidase, guaiacol peroxidase, and superoxide dismutase increased in response to metal treatments with no differences between tolerant and sensitive clones. Catalase activity decreased in response to all heavy metals examined. Salicylic acid levels were not affected by heavy metals. Glutathione levels increased to Cu and Zn treatments but not to Cd. Lipid peroxidation (measured as thiobarbituric acid-reactive materials) increased the response to all treatments, more in roots of tolerant clones and in shoots of sensitive clones. Nicotinic acid and nicotinamide treatments decreased the toxic effects of heavy metals in sensitive, but not tolerant clones. Zinc net uptake increased in the presence of Cu and Cd in all clones, while Cd uptake was not affected by interactions with Cu or Zn. Net uptake of Cu decreased in the presence of Cd and Zn in highly tolerant clones. Cd uptake was lower in sensitive low-accumulating and higher in tolerant high-accumulating clones. Zn uptake did not differ between clones. Leakage was similar in untreated and pre-treated plants for Zn and Cd but faster in tolerant low accumulators of Cd and in tolerant high accumulators of Zn. Salix plants from polluted areas showed high accumulation in root and low translocation to shoots, combined with higher internal tolerance than clones from unpolluted sites. Salix cultivation led to significant soil decontamination from heavy metals and organic pollutants already after two years. After ten years of Salix cultivation, contaminant levels had decreased by 20% to 87%, depending on the contaminant. In agricultural soil, wheat cultivated after four years of Salix had up to 33% lower Cd levels in the grains, and Cd levels in the soil were reduced by up to 27%. These results show that Salix has no dominant mechanisms for the heavy metal-related traits investigated. In any case, the ability of Salix to accumulate high levels of heavy metals can be used in phytoremediatio