Abstract
Quizalofop-resistant wheat is the core component of the recently commercialized CoAXium™ Wheat Production System. As with other herbicides, quizalofop provides better weed control at early growth stages and under optimum temperature. However, in regions with winter wheat production, quizalofop application may be affected by unpredictable, rapid temperature decreases. Temperature shifts can cause crop injury or impact weed control efficacy. In the following study, we examine the effect of reduced temperature on quizalofop content and metabolism in CoAXium™ winter wheat and three winter weed species: downy brome (Bromus tectorum L.), feral rye (Secale cereale L.), and jointed goatgrass (Aegilops cylindrica Host). Temperature conditions include either 19 or 4.5°C daytime temperatures with tissue sampling over 5 timepoints (1–16 or 18 days after treatment, DAT). Analysis features liquid chromatography coupled to tandem mass spectrometry detection of the active form of quizalofop, quizalofop acid. Quizalofop content trends reveal delayed metabolism under cooler conditions for wheat and weeds. Quizalofop content peaks within 1–2 DAT in the warmer temperatures for all species and decreases thereafter. In contrast, content peaks between 8 and 9 DAT at cooler temperatures except for downy brome. Minimal decreases in content over time generally follow cooler temperature peaks. Further, the absence of differences in maximum quizalofop content in all species suggests absorption and/or de-esterification of quizalofop proherbicide to the active form is not reduced at cooler temperatures. Final dry shoot tissue biomass does not necessarily correspond to differences in metabolism, as biomass of wheat treated with a field rate of quizalofop does not differ between temperatures. Weeds were treated with sublethal doses of quizalofop in order to monitor herbicide metabolism without causing plant death. Under this condition, weed biomass only differs for jointed goatgrass, which has a greater biomass in the cooler temperature.
Highlights
The CoAXiumTM Wheat Production System centers on wheat with the AXigenTM trait for quizalofop resistance (Bough et al, 2021)
Systematic difference in absolute amounts of quizalofop acid (QZA) detected is related to variations in ultra-pure liquid chromatographic (UPLC) and mass spectrometer operating conditions between years, but overall trends in QZA metabolism are consistent between replications in both years for wheat and weeds
Anecdotal field reports of CoAXiumTM injury following AggressorTM application at low temperatures may be attributable to inconsequential visual injury
Summary
The CoAXiumTM Wheat Production System centers on wheat with the AXigenTM trait for quizalofop resistance (Bough et al, 2021). The AXigenTM trait is an alanine to valine mutation at position 2004 of wheat acetyl-CoA carboxylase (ACCase) (Ostlie et al, 2015) causing an enzyme conformational change that imparts resistance to quizalofop acid (QZA) (Bough and Dayan, 2021). The original QZA-resistant winter wheat mutant accessions were discovered at Colorado State University. Homozygous resistance mutations in two of three wheat sub-genomes were introgressed into a susceptible elite genetic background. The system provides effective post-emergence control of annual weedy grasses in winter wheat where control options are limited, and it is paired with a stewardship program to mitigate development of herbicide-resistant weeds
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