Comparative Environmental Risk Assessment of Herbicides Used in Wheat and Barley Fields by Duckweed, Lemna minor, Test in Aquatic Ecosystems

Nowadays, the pollution in aquatic ecosystems caused by the application of pesticides has become a serious problem. To evaluate the phytotoxicity of ten herbicides used in wheat and barley fields by duckweed (L. minor) test, a dose-response experiment was carried out in completely randomized design with four replications. The treatments consisted of eight different concentrations of various active ingredients of herbicides including clodinafop-propargyl, pinoxaden, pinoxaden + clodinafop-propargyl, diclofop- methyl, fenoxaprop-p-ethyl, mesosulfuron-methyl + iodosulfuron-methyl sodium, metsulfuron-methyl+ sulfosulfuron, 2,4-D and bromoxynil + MCPA with herbicide-free control treatment. Toxicity assessment index was based on inhibition of the relative leaf surface growth rates (RGR) of the plants after seven days. The EC50 values which were derived by fitting the log-logistic curves, showed that bromoxynil + MCPA with EC50 equal to 0.51 microgram/l had the highest effect in reduction of the relative leaf surface growth rate of Lemna compared with other herbicides. This high toxicity probably is attributed to its physicochemical properties and rapid uptake by the plant cells. 2,4-D with EC50 equal to 87.97 µg/l showed the lowest toxicity on Lemna. The phytotoxicity of the nine herbicides tested could be ranked as follows: Bromoxynil +MCPA > Mesosulfuron-methyl + iodosulfuron-methyl sodium > Pinoxaden > Pinoxaden + Clodinafop-propargyl > Diclofop- methyl > Metsulfuron-methyl+ sulfosulfuron > Clodinafop-propargyl > Fenoxaprop-P-ethyl > 2,4-D.  The information obtained from the results of the present study can be useful for adopting the necessary criteria for aquatic ecosystem management.