Phytotoxicity of diuron alone and in combination with copper or folpet on duckweed (Lemna minor).

The photosystem II-herbicide diuron is widely used for weed control in Champagne's vineyards. Its important use and its relative persistence make it of particular interest for ecotoxicological studies. Toxicity of diuron was assessed on Lemna minor L., a representative aquatic macrophyte regularly used for toxicological studies. Toxicity assessments were based on inhibition of growth and total chlorophyll content of L. minor cultures after 7 days. Growth was inhibited and IC(50) and IC(90) were, respectively, 25 and 60 microg l(-1), but chlorophyll content of L. minor increased in response to the herbicide. When diuron was combined with copper, growth inhibition of L. minor depended on the concentrations of both chemicals. For some concentrations, combination of these chemicals resulted in a slight (but non-significant) antagonism. Additivity was observed for all other mixtures. When diuron was combined with folpet, growth and chlorophyll content of L. minor only depended on the concentration of the herbicide. Diuron was also found to prevent the copper-induced decrease of chlorophyll content when it was combined with this metal. A multifactorial model was found more appropriate to characterize interactions between pesticides than Abott's model.

Ascorbate and glutathione contents in duckweed, Lemna minor, as biomarkers of the stress generated by copper, folpet and diuron.

Glutathione and ascorbate are essential components of the general antioxidative strategy to overcome oxidative stress due to environmental constraints such as pollution. The variation of glutathione and ascorbate contents in duckweed (Lemna minor) was investigated after a 48 h exposure to copper, diuron and folpet under laboratory conditions in order to determine whether changes in their level could serve as suitable and early biomarkers of pollution. One could observe that diuron and folpet caused the glutathione level to increase, its redox status remaining unchanged, while copper led to a depletion of this antioxidant and to an increase in its oxidation rate. When duckweed was contaminated by folpet and the metal, an increase of the ascorbate pool size occurred from concentrations as low as 1 mg l(-1) and 50 µg l(-1) respectively. While the ascorbate pool became more oxidized because of exposure to copper concentrations ≤ 200 µg l(-1), folpet caused an increase in its reduction rate. Diuron was responsible for depletion of ascorbate, the redox status of which remained unchanged. Because it is an adaptation to stress and a defence process, the increase in the antioxidant pool size was proposed as a biomarker of exposure to an unsafe environment. Since depletion of antioxidant and an increase in its oxidation rate weakened cellular defences and indicated a precarious state, they could constitute early indicators of toxicity. So they were proposed as potential biomarkers of toxicity. It was concluded that the antioxidant content in duckweed might serve as a useful biomarker for monitoring water quality.

Toxic responses and catalase activity of Lemna minor L. Exposed to folpet, copper, and their combination.

Toxicity of copper and folpet--two fungicides widely used on grape--was evaluated on Lemna minor L., a sensitive aquatic weed regularly used for (eco)toxicological studies. Toxicity assessments were based on inhibition of growth and chlorophyll content of L. minor cultures after 7 days. IC10, IC50, and IC90 were determined for both compounds alone and were respectively, 0.03, 0.16, and 0.95 mg liter-1 for copper and 1.20, 7.50, and > 40 mg liter-1 for folpet. When both compounds were combined, the response of L. minor depended on the initial folpet concentration. Indeed, a slight synergy was observed for 5 mg liter-1 folpet, while at folpet concentrations of 20 to 35 mg liter-1, the two fungicides were antagonists. The antagonism was positively correlated with folpet concentration. Antagonism between Cu and folpet could not be explained by a reduced bioavailability of Cu since concentration of free copper in the mixture did not depend on the presence of folpet. One physiological defense response elicited by copper in plants is an increase in catalase activity. Copper and folpet stimulated catalase activity and changes in the activity of the enzyme could not account for the synergy but possibly for the antagonism. Nevertheless, catalase activity increase significantly after a 24-h exposure to 25 micrograms liter-1 of copper. The use of this property as a rapid and sensitive biomarker to monitor the toxicity of xenobiotics alone or in combination and of environmental water is discussed.