Toxic Responses and Catalase Activity ofLemna minorL. Exposed to Folpet, Copper, and Their Combination,

Toxicity of copper and folpet—two fungicides widely used on grape—was evaluated onLemna minorL., a sensitive aquatic weed regularly used for (eco)toxicological studies. Toxicity assessments were based on inhibition of growth and chlorophyll content ofL. minorcultures after 7 days. IC10, IC50, and IC90were determined for both compounds alone and were respectively, 0.03, 0.16, and 0.95 mg liter−1for copper and 1.20, 7.50, and >40 mg liter−1for folpet. When both compounds were combined, the response ofL. minordepended on the initial folpet concentration. Indeed, a slight synergy was observed for 5 mg liter−1folpet, 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 increased significantly after a 24-h exposure to 25 μg liter−1of 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.