Bioassay of photosynthetic inhibitors in water and aqueous soil extracts with Eurasian watermilfoil (Myriophyllum spicatum)

Eurasian watermilfoil, an aquatic flowering plant, was found to be a suitable bioassay plant for the detection of photosynthetic inhibitor herbicides in water and aqueous extracts of soil. Stock cultures of Eurasian watermilfoil were maintained in an aigal-free medium under constant environmental conditions. Oxygen evolu- tion from three lateral shoots 3 to 5 cm in length was measured before and after herbicide treatment. Inhibition of photosynthesis was detected within 10 min of treat- ment. Concentrations as low as 5.9 x 1048M terbacil and 6.4 x 1048M diuron were detected in water. Simazine, atrazine, and metribuzin were detected in water at a concentration of 10-7M. Eurasian watermilfoil bioassays in lake water spiked with various concentrations of terbacil, metribuzin, atrazine, and simazine indicated that the bioresidual activity of these chemicals can be estimated in samples of natural water without extraction or purification. Using aqueous extracts taken from herbicide-treated soil, the Eurasian watermilfoil bioassay detected minimum levels of 0.047 and 0.07 kg/ha terbacil and atrazine, respectively. Neither soybean nor oat showed visible injury symptoms at these concentrations. The Eurasian watermilfoil bioassay has the advantage of being more rapid and/or more sensitive than methods using algae, oat seedlings, or leaf discs. Nomenclature: Atrazine, 6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-tria- zin-2,4-diamine, diuron, N-(3,4-dichlorophenyl)-N,N-di- methylurea; metribuzin, 4-amino-6-(1,1-dimethylethyl)- 3-(methylthio)-1,2,4-triazin-5(4H)-one; simazine, 6-chloro- N,N-diethyl-1,3,5-triazine-2,4-diamine; terbacil, 5-chloro-