Is the “Diuron Effect” Due to a Herbicide Strengthening of Antioxidative Defenses of Lemna minor?

Abstract

In a previous study, we reported that diuron prevented the copper-induced decrease of the chlorophyll content of duckweed (Lemna minor) when it was combined with the metal. Among the possible hypotheses available to explain this “protective” effect, we suggested that diuron might stimulate antioxidative defenses, thus strengthening duckweed against the copper-induced oxidative stress. To test this hypothesis, we investigated the variation of ascorbate peroxidase, glutathione reductase, glutathione S-transferase, catalase, guaiacol peroxidase, and pyrogallol peroxidase during time- and concentration-dependent contamination of duckweed by the herbicide. Our results clearly showed that diuron weakly induced enzymatic antioxidative defenses of L. minor. Indeed, ascorbate peroxidase activity was constant in both experiments and glutathione reductase induction was slight and transient. Glutathione S-transferase, pyrogallol peroxidase, and guaiacol peroxidase were poorly stimulated by the herbicide and even inhibited, in the case of guaiacol peroxidase, when exposure exceeded 48 h. Likewise, catalase activity was inhibited from the lowest concentration (25 μg L−1) and the shortest time of exposure (6 h). So, these results suggest that the “diuron-effect” is not attributable to a herbicide strengthening of enzymatic antioxidative defenses of L. minor.