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

The physiological effects of thallium(I)-acetate on the duckweed Lemna minor after 1-, 4-, 7- and 14-d exposure were analyzed. High bioaccumulation of Tl (221 mg kg(-1) dry wt at 2.0 mu M TI-acetate) caused an inhibition of plant growth. After 14-d exposure, 0.2, 0.5, 1.0 and 2.0 mu M TI-acetate reduced the frond-number growth rate by 21.1%, 39.4%, 66% and 83.1%, respectively. TI-acetate also induced a modulation of the antioxidative response by depleting the ascorbate content and affecting the antioxidative enzymes activities. Superoxide dismutase showed a continuous increase of activity (31-67%) after TI-acetate exposure. Other antioxidative enzymes displayed a biphasic response to both the concentration and the exposure period. Exposure up to 7 d decreased the catalase activity (up to 40%) in plants treated with higher TI-acetate concentrations. In contrast, 14-d exposure increased the activity of the enzyme (>= 90%). Short-term exposure increased ascorbate peroxidase activity (13-41%), except in plants exposed to the highest TI-acetate concentration. However, 14-d exposure decreased the enzyme activity at all concentrations tested (38-60%). Although pyrogallol peroxidase activity increased (up to 26%) during 4-d exposure, longer exposures to the highest two concentrations decreased the activity of the enzyme (25-48%). In general, short-term exposure to TI-acetate activated the antioxidant capacity, which resulted in recovery of the frond-number growth rates in TI-treated plants. In spite of the activation of the antioxidative response during short-term exposure, higher TI-acetate concentrations increased the hydrogen peroxide level (up to 45%) and induced marked oxidative damage to lipids, proteins and DNA. Longer exposure induced a decline of the antioxidative response, and plants showed the symptoms of oxidative damage even at lower TI-acetate concentrations. The genotoxic effect was evaluated by an alkaline version of the cellular and acellular Comet assay, which revealed an indirect genotoxic effect of TI-acetate, suggesting oxidatively induced damage to DNA. (C) 2009 Elsevier B.V. All rights reserved.