THIFENSULFURON AND IMAZETHAPYR INTERACTION AT THE ALS ENZYME IN SULFONYLUREA TOLERANT SOYBEAN

Authors: SIMPSON, DAVID - UNIV OF ILLINOIS; STOLLER, EDWARD

Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/22/1996
Publication Date: N/A
Citation: N/A

Interpretive Summary: A new variety of sulfonylurea-tolerant soybean (STS soybean TM) is now available to growers, apparently enough to seed about 1,000,000 acres in 1995. STS soybeans resist injury from applications of thifensulfuron (Pinnacle TM), but are severely injured from applications of the tank mixture of thifensulfuron and (Pursuit TM), a mixture used widely for broad-spectrum weed control. We conducted experiments to determine the physiological reasons for the injury from the mixture, concentrating primarily on the ALS (acetolactase synthase) enzyme, the site where these herbicides exert their toxic effect in plants. We found that thifensulfuron did not inhibit the ALS enzyme in STS soybean (Asgrow 3200) as much as it did in the non-STS soybean (Williams 82). The STS soybean resisted plant injury and ALS inhibition from 15X the normal application rates of thifensulfuron, indicating an enzyme highly resistant to thifensulfuron. However, the STS soybean was injured and its ALS was inhibited from field rates of imazethapyr. Soybean injury form the tank mixture of these two herbicides was due to the sensitivity of its ALS enzyme to imazethapyr. This information will be useful to other weed scientists, extension workers and industry personnel in weed science.

Technical Abstract: Greenhouse and laboratory experiments were conducted to determine if the synergistic injury to sulfonylurea tolerant soybean (STS soybean) caused by the thifensulfuron plus imazethapyr tank mix involves an interaction between the herbicides at the ALS enzyme. In vitro studies predicted and subsequent in vivo studies affirmed that thifensulfuron inhibits ALS in Asgrow 3200, a STS soybean, but to a lesser extent than in Williams 82, a non-STS soybean. Foliar application of thifensulfuron (4.4 g ha-1) caused 100% inhibition of ALS in Williams 82 and only 70% inhibition in Asgrow 3200 at 6 HAT. Increasing the thifensulfuron rate from 4.4 to 141 g ha-1 did not result in increased ALS inhibition in Asgrow 3200 indicative of a highly resistant ALS isozyme. The STS trait did not affect visual injury, in vitro ALS inhibition, or in vivo ALS inhibition caused by imazethapyr. Soybean injury and in vivo ALS inhibition increased with imazethapyr rate. The relationship between imazethapyr inhibition of ALS in vivo and soybean injury was curvelinear. In vitro ALS assays revealed no synergistic interaction between thifensulfuron and imazethapyr at the enzyme level. In vivo ALS activity was reduced to 7 and 10% of the controls at 24 and 48 HAT by the tank mix compared to 56 and 84% for thifensulfuron alone and 20 and 28% for imazethapyr alone. These data show that the synergistic increase in soybean injury caused by tank mixing thifensulfuron with imazethapyr results from the cumulative inhibition of ALS and not a synergistic inhibition.