Toxicity of copper, folpet and their combination on duckweed (Lemna minor L.)

Copper and folpet are widely used as agrochemicals to control mildew and other fungal diseases in grape. Alone or in combination, these two fungicides present potential risks for the aquatic environment. Especially, since runoff is very important on the slopes of septentrional wine growing areas and copper and folpet may end up in surface waters either dissolved in runoff water or adsorbed to soil particles. Their toxicity was therefore evaluated on duckweed (Lemna minor L.), a sensitive model regularly used for (eco)toxicological studies. Toxicity assessments were based on inhibition of growth and chlorophyll content of L. minor cultures after 7 days. IC1O, IC50, and IC90 were determined for both compounds alone, they were respectively 0.03, 0.16, and 0.95 mg per litre for copper and 1.20, 7.50, and more than 40 mg per litre for folpet. When both compounds were combined, the response of L. minor depended on the initial folpet concentration. Indeed, a slight synergy was observed between 0 et 10 mg per litre folpet while at folpet concentrations of 10 to 35 mg per litre, the two fungicides were antagonists. The antagonism was positively correlated with folpet concentration. Concentration of free copper in the mixture did not depend on the presence of folpet, meaning that antagonism was not simply due to a reduced bioasavailability of the metal through complexation by folpet, but may have a physiological explanation. A physiological defence response elicited by copper in plants is the increase of catalase activity, that we were able to confirm in our experimental conditions. Similarly, folpet stimulated catalase activity and changes in the activity of the enzyme could not account for the synergy but possibly for the antagonism. Nevertheless, increase of catalase activity is very sensitive since 25 microg per litre of copper were sufficient to induce an observable effect after a 24 h-incubation. This property could be used as a rapid and sensitive biomarker to monitor the toxicity of xenobiotics alone or in combination and of environmental water, since a good number of these compounds are known to induce oxidative stress in plants.