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Phytotoxicity of isoproturon to non-targeted aquatic plant Lemna minor L. - a comparison of pulsed and continuous exposure

Pesticides are usually repeatedly applied to agricultural surfaces and due to surface runoff and drainage may enter water environment. Therefore, pesticide contamination of water ecosystem mainly occurs in pulses, varying in concentration and duration. Standard toxicity tests used for essential risk assessment aim to maintain a constant concentration of tested substance through exposure period. On the other hand, tests that include experimental pulse scenarios alongside continuous exposure may provide information on latent herbicide effects and the potential of plants to recover or develop resistance to harmful herbicide effects. Therefore, the objective of the present study was to investigate the physiological changes in non- targeted aquatic plant Lemna minor L. caused by continuous and pulse isoproturon exposure. Isoproturon is one of the most frequently used phenylurea herbicides in conventional agriculture production, and it is often detected in surface water samples. To compare the effects of continuous exposure with pulse related effects different experimental designs were arranged. For continuous exposure treatment, a seven-day semi- static standard toxicity test was performed. Plants were treated with 0.05, 0.1, 0.15 and 0.2 mg L-1 of isoproturon added to Steinberg's nutrient solution. For the pulse exposure treatment, plants were treated with equivalent isoproturon concentration for three days followed by four days of the recovery period. Herbicide concentrations for pulse exposure were calculated as time-weighted averages so that each pulse exposure scenario delivered a dose per week equivalent to that of the continuous exposure. Isoproturon caused a decrease in photosynthetic pigments content and protein concentration which in turn resulted in inhibited growth and significantly lowered growing and multiplication rate of plants. The growth rate of L. minor was considerably lower under continuous exposure compared with plants exposed to pulse herbicide treatment which may be due to a fast recovery in pulse exposure. Nevertheless, a concentration- depended delay of growth rate in pulsed exposure was noted. Results from studies that include pulse exposure and recovery period may contribute to the furder improvement of standard Lemna-test as a relevant form of environmental impact assessment.

Isoproturon ; Lemna minor ; Experimental pulse scenario ; Recovery ; Realistic risk assessment

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