engleski

Antioxidative response of Lemna minor plants exposed to thallium(I) acetate

Thallium (Tl) is a heavy metal included in priority pollutant list due to its high toxicity. In this study, we assess toxicity of monovalent thallium, which is highly soluble and easily absorbed by water macrophytes. We exposed Lemna minor plants to Tl(I)-acetate, the effect of which was monitored after 1-, 4-, 7- and 14-d exposure. After 14-d exposure, 0.2, 0.5, 1.0 and 2.0 μ M Tl-acetate reduced the frond-number growth rate by 21.1%, 39.4%, 66% and 83.1%, respectively. In order to rule out possible effects of the acetate group, plants were also exposed to potassium(I)-acetate. An ICP-MS analysis showed that the Tl content in plant tissue after 7-d exposure followed the Tl-acetate concentration in tested solutions. Tl-acetate induced a modulation of the antioxidative response by depleting the ascorbate content and affecting the activities of antioxidative enzymes. Superoxide dismutase showed a continuous increase of activity after Tl exposure. Unlike superoxide dismutase, other antioxidative enzymes displayed a biphasic response to both the concentration and the period of the exposure. During 7-d exposure, a decreased catalase activity in plants exposed to 1.0 and/or 2.0 μ M Tl-acetate was observed. In contrast, 14-d exposure increased the catalase activity already at the concentration of 0.2 μ M. Exposure of up to 7 d increased the ascorbate peroxidase activity except in plants exposed to 2.0 μ M Tl-acetate, while 14-d exposure decreased the ascorbate peroxidase activity already at the concentration of 0.5 μ M. The pyrogallol peroxidase activity rised after 1- and 4-d exposure to Tl-acetate. Longer exposure (7 and 14 d) decreased the pyrogallol peroxidase activity in plants exposed to the highest two concentrations. In general, short-term exposure to Tl-acetate activated the antioxidant protective mechanisms, resulting in a development of adaptive mechanisms to highly toxic Tl. The adaptation was confirmed by a growth analysis, which showed a recovery of the frond-number growth rates in Tl-treated plants after the initial toxic phase. In spite of the activation of the antioxidative response, higher concentrations of Tl-acetate induced a hydrogen peroxide accumulation as well as an oxidative damage to lipids, proteins and DNA. Long-term exposure to Tl-acetate induced an impairment of some components of the antioxidative defense and a severe oxidative damage to biomolecules. The genotoxic effect was evaluated by an alkaline version of Comet assay. Results of the cellular and acellular Comet assays revealed an indirect genotoxic effect of Tl-acetate, suggesting an oxidatively induced damage to DNA.

Lemna minor L.; thallium; oxidative stress; genotoxicity

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