PARENT SESSION
PS9 - Plant Toxicology
Sunday, 17 November 2002
8:00 AM to 6:30 PM
Exhibit Hall

(P114) Reduction of growth rates by glyphosate, chlorsulfuron, and atrazine on Pithophora oedogonia .

Kish, Peter^*,1,2, ¹ Oklahoma State Universtiy, Stillwater, Oklahoma, USA² Oklahoma School of Science and Mathematics, Oklahoma City, Oklahoma, USA

ABSTRACT- Pre-/post-emergent herbicides, Roundup®, Glean®, and Aatrex® are used to control weed populations in sorghum and winter wheat fields in West Central Oklahoma. These herbicides have the potential to runoff into local streams and affect the growth of indigenous algae. The active ingredients are Roundup® glyphosate (N-(Phosphoonomethyl)glycine(acid)), Glean®, chlorsulfuron (2-Chloro-N-[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)aminocarboyl] benzenesulfonamide), and Aatrex®, atrazine (2-chloro-4-ethylamino-6-isopropyl-amino-s-triazine). Technical formulations of these herbicides were used to determine their effects upon the growth rates of Pithophora oedogonia. P. oedogonia was selected as a model to simulate the herbicides overall effects upon periphyton in streams of West Central Oklahoma and aquatic ecosystems in general. Glyphosate, chlorsulfuron, and atrazine inhibit growth rates of P. oedogonia. Under low light conditions (76 mmol/m²-s) growth rate EC ₅₀ values of glyphosate, chlorsulfuron, and atrazine were determined to be 300.00, 28.00, and 0.06 mg/l respectively. In addition, tandem herbicide mixtures inhibited growth rates of P. oedogonia greater than did exposures to individual herbicides. Corrected Sum-Additive Index (CSAI) were used to determine less-than-additive or greater-than-additive effect of the tandem herbicide mixtures. CSAI for atrazine + chlorsulfuron mixture is ⁻3.4 indicating a less-than-additive effect, for atrazine + glyphosate ⁺0.277 indicating a greater-than-additive effect, and for glyphosate + chlorsulfuron, ⁺0.639 indicating a higher greater-than-additive effect. These CSAI values indicate that some of herbicide mixtures can have stronger inhibitory effects upon algal growth than would be predicted based upon exposure to individual herbicides.

Key words: Pithophora oedogonia, herbicide, corrected sum-additive index, growth rate