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|Title:||Physiological response of sulfonylurea-tolerant soybean to thifensulfuron and imazethapyr|
|Author(s):||Simpson, David Michael|
|Doctoral Committee Chair(s):||Stoller, Edward W.|
|Department / Program:||Crop Sciences|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
Biology, Plant Physiology
|Abstract:||Thifensulfuron, a sulfonylurea herbicide, and imazethapyr, an imidazolinone herbicide, inhibit acetolactate synthase (ALS) and used for weed management in soybean production. Tank mixing thifensulfuron with imazethapyr would provide an excellent weed control spectrum for soybean production in Illinois but soybean injury precludes its use. A sulfonylurea tolerant soybean (STS soybean) has been recently developed that have increased tolerance to thifensulfuron due to an insensitive ALS enzyme. This increase in thifensulfuron tolerance could allow the use of the imazethapyr plus thifensulfuron tank mix. However, an interaction between thifensulfuron and imazethapyr negates the STS trait and causes synergistic soybean injury.
Field and green house studies demonstrated that STS soybean has excellent tolerance to thifensulfuron alone. STS soybean does not have increased tolerance to imazethapyr. While the tank mix caused less injury to STS than non-STS, unacceptable injury still occurs in STS soybean. Reducing the rate of thifensulfuron in the tank mix did not reduce injury to STS soybean. Reducing imazethapyr rate in the tank mix did reduce injury in STS soybean. Yield was not affected by the injury caused by thifensulfuron and/or imazethapyr.
Greenhouse and laboratory experiments were conducted to determine the effect of the tank mix on the absorption, translocation, and metabolism of thifensulfuron and imazethapyr in STS soybean. The absorption of both imazethapyr and thifensulfuron was rapid in the first 8 h after application and was not affected by the tank mix. Tank mixing did not affect acropetal and basipetal translocation of imazethapyr and thifensulfuron from the treated leaflet. Metabolism of thifensulfuron and imazethapyr was not affected by the tank mix.
To determine if the mechanism of this interaction involved a physiological interaction at the active site, in vitro ALS assays were conducted. Thifensulfuron at 0.01 to 100 $\mu$M inhibited ALS from STS soybean less than ALS from non-STS soybean. The addition of thifensulfuron to various concentrations of imazethapyr did increase ALS inhibition but the increase was not synergistic. An in vivo ALS assay was developed to determine if thifensulfuron inhibits ALS in the whole plant. The in vivo ALS assay uses 1,1-cyclopropanedicarboxylic acid to cause the accumulation of acetolactate. In vivo ALS assays demonstrated that thifensulfuron does cause 70, 60, 42, and 15% and inhibition of ALS at 6, 12, 24, and 48 HAT. When combined with the inhibition caused by imazethapyr, ALS inhibition is increased to 98, 93, 94, and 92% at 6, 12, 24, and 24 HAT. Increasing the rate of thifensulfuron from 4.4 to 140 g/ha did not increase ALS inhibition thus indicating the presence of a ALS isozyme which is highly resistant to thifensulfuron. Since this resistant isozyme is not cross tolerant to imazethapyr, tank mixing imazethapyr with thifensulfuron results the inhibition of the resistant isozyme in addition to the sensitive isozyme.
|Rights Information:||Copyright 1995 Simpson, David Michael|
|Date Available in IDEALS:||2011-05-07|
|Identifier in Online Catalog:||AAI9543726|