COMPARATIVE SENSITIVITY OF SELENASTRUM-CAPRICORNUTUM AND LEMNA-MINOR TO 16 HERBICIDES

Aquatic plant toxicity tests are frequently conducted in environmental risk assessments to determine the potential impacts of contaminants o n primary producers. An examination of published plant toxicity data d emonstrates that wide differences in sensitivity can occur across phyl ogenetic groups of plants. Yet relatively few studies have been conduc ted with the specific intent to compare the relative sensitivity of va rious aquatic plant species to contaminants. We compared the relative sensitivity of the algae Selenastrum capricornutum and the floating va scular plant Lemna minor to 16 herbicides (atrazine, metribuzin, simaz ine, cyanazine, alachlor, metolachlor, chlorsulfuron, metsulfuron, tri allate, EPTC, trifluralin, diquat, paraquat, dicamba, bromoxynil, and 2,4-D). The herbicides studied represented nine chemical classes and s everal modes of action and were chosen to represent major current uses in the United States. Both plant species were generally sensitive to the triazines (atrazine, metribuzin, simazine, and cyanazine), sulfonu reas (metsulfuron and chlorsulfuron), pyridines (diquat and paraquat), dinitroaniline (trifluralin), and acetanilide (alachlor and metolachl or) herbicides. Neither plant species was uniformly more sensitive tha n the other across the broad range of herbicides tested. Lemna was mor e sensitive to the sulfonureas (metsulfuron and chlorsulfuron) and the pyridines (diquat and paraquat) than Selenastrum. However Selenastrum was more sensitive than Lemna to one of two thiocarbamates (triallate ) and one of the triazines (cyanazine). Neither species was sensitive to selective broadleaf herbicides including bromoxynil, EPTC, dicamba, or 2,4-D. Results were not always predictable in spite of obvious dif ferences in herbicide modes of action and plant phylogeny. Major depar tures in sensitivity of Selenastrum occurred between chemicals within individual classes of the triazine, acetanilide, and thiocarbamate her bicides. Results indicate that neither species is predictively most se nsitive, and that a number of species including a dicot species such a s Myriophyllum are needed to perform accurate risk assessments of herb icides.