Physiological mechanisms in the synergism between thifensulfuron and imazethapyr in sulfonylurea-tolerant soybean (Glycine max)

Greenhouse and laboratory experiments were conducted to determine if the synergistic interaction between imazethapyr and thifensulfuron in sulfonylurea-tolerant (STS) soybean involved enhanced absorption and translocation or reduced metabolism of one or both herbicides. Thifensulfuron at 4.4 g ha(-1) and imazethapyr at 70 g ha(-1) caused 0 and 28% injury to STS soybean 7 DAT, respectively, while the combination of both herbicides caused 50% injury 7 DAT. Imazethapyr had no effect on absorption of C-14-thifensulfuron into the first trifoliolate, Imazethapyr did not affect absorption, translocation, or metabolism of C-14-thifensulfuron, Metabolism of C-14-thifensulfuron was rapid, with less than 10% remaining after 24 h, and was not affected by imazethapyr. Likewise, thifensulfuron did not affect the absorption, translocation, or metabolism of 14C-imazethapyr. Therefore, the synergism between thifensulfuron and imazethapyr does not involve changes in the absorption, translocation, or metabolism of either herbicide, Nomenclature: Imazethapyr, 2-[4,5-dihydro-4-methyl-4-(1-methyethyl)-5-oxo- 1H-imidazol-2-yl]-5-ethyl-3-pyridine-carboxylic acid; thifensulfuron, 3-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino]carbonyl]amino]sulfonyl]-2-phenecarboxylic acid; soybean, Glycine max (L.) Merr.