Characterizing Distinct Glyphosate-Resistant Giant Ragweed (Ambrosia trifida) Biotypes and the Novel Rapid Response Antagonism of Selective Herbicides
Giant ragweed is among the most competitive summer annual weeds in corn and soybean production. Difficulty in controlling giant ragweed is exacerbated by the evolution of herbicide-resistant (HR) biotypes to both ALS-inhibiting herbicides and glyphosate. Glyphosate resistance (GR) in giant ragweed is unique as there exist two distinct phenotypic responses following application. The rapid response (RR) biotype displays a lesion-causing oxidative burst in mature leaves in as little as two hours after treatment and quickly results in leaf desiccation, thus restricting glyphosate translocation. The non-rapid response (NRR) biotype of giant ragweed exhibits slightly chlorotic leaves and stunted plant growth. Both biotypes resume normal growth within a week. This work first examines the distribution of HR giant ragweed in Indiana and further characterizes differences between the two GR biotypes. The effect of the RR on the efficacy of co-applied selective herbicides and how environmental variables moderate this interaction is also investigated. Lastly, the involvement of antioxidant enzyme safening associated with the oxidative burst in the RR biotype is studied. Glyphosate-resistant giant ragweed was found in 36 counties in Indiana compared to only 14 counties in a 2006 survey. Uniform populations containing the RR biotype were rare as GR populations largely consisted of solely NRR individuals or a mixture of RR and NRR individuals. A TaqMan® SNP genotyping assay was developed and used to identify ALS-resistant giant ragweed in 19 counties. Based on glyphosate dose-response experiments, the magnitude of resistance for the NRR biotype was 4.6 and for the RR biotype was 7.8 to 9.2 according to GR50 values. The GR biotypes displayed different photooxidative responses in that H2O2 generation in mature leaves of the RR biotype occurs sooner, and accelerates more rapidly. Furthermore, the speed at which H2O2 was generated was positively associated with glyphosate dose in the RR biotype. The RR to glyphosate results in foliage loss and reduced glyphosate translocation. Therefore, experiments were performed to determine how this influences the efficacy of five co-applied selective herbicides. In a field trial, combinations of glyphosate plus dicamba or topramezone were antagonistic on the RR biotype across multiple years. For both biotypes, the greatest efficacy was achieved with the tank-mixture of glyphosate plus dicamba. In the greenhouse, 1,680 g ae ha–1 glyphosate was antagonistic when applied with all five selective herbicides to the RR biotype, while at the 420 g ha –1 rate, glyphosate plus cloransulam was the only combination resulting in antagonism. Applying the selective herbicide one to two days prior to glyphosate improved control of the RR biotype and avoided any antagonistic interaction compared to the tank-mixture application. The RR biotype was exposed to transient air temperature, soil moisture, and light intensity regimes surrounding the period of glyphosate and glyphosate plus selective herbicide application. Induction of the RR, determined by H 2O2 generation, was more rapid at 30 C versus 10 C and at full pot water capacity versus 1/3 pot water capacity. Despite the initial response, there were no differences in final glyphosate efficacy between levels of each environmental factor. The magnitude of glyphosate-induced antagonism tended to be greater at 30 C versus 10 C and at full pot water capacity versus 1/3 pot water capacity. Antagonism was similar at both full and reduced light settings. Antioxidant enzyme activities were measured in mature and juvenile leaves of glyphosate-susceptible (GS) and RR biotypes following glyphosate treatment. Lipid peroxidation rose sharply in mature leaves of the RR biotype from 4 to 16 h after treatment (HAT). During this period, antioxidant enzyme activity in RR juvenile leaves increased compared to the GS biotype and were most evident in enzymes involved in the ascorbate-glutathione cycle. By 32 HAT, enzyme activities in the RR biotype declined, indicating a transient response to glyphosate exposure. Both GS and RR biotypes did not differ in response to the strong oxidant, paraquat. Thus, the oxidative protection observed in the RR biotype appears to be induced solely by glyphosate and is the first report of enhanced antioxidant enzyme activity in a GR weed. Collectively, this work indicates under continued glyphosate use, GR giant ragweed will continue to spread and the RR biotype may become more prevalent due to a greater magnitude of resistance compared to the NRR biotype. An ancillary consequence of the RR to glyphosate is the propensity to antagonize selective herbicides, particularly those which are phloem-mobile. Under optimum plant growing conditions, the degree of antagonism intensifies. Despite an antagonistic interaction between glyphosate and dicamba on the RR biotype, results show this herbicide combination to be highly effective for the control of both GR giant ragweed biotypes, and thus remains an effective management option in glyphosate- and dicamba-resistant cropping systems. The RR glyphosate resistance mechanism has eluded scientists thus far and continues to be an ongoing area of study. This research fills a knowledge gap by documenting increased antioxidant enzyme activity coinciding with the RR that may be responsible for safening young tissue from the oxidative burst.
Young, Purdue University.
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