title: Differential response of Amaranthus tuberculatus (Moq ex DC) JD Sauer to glyphosate.
abstract: Midwest USA farmers have reported inconsistent control of Amaranthus tuberculatus (= rudis) (Moq ex DC) JD Sauer by glyphosate in glyphosate-resistant crops. The potential of selection for decreased A tuberculatus sensitivity to glyphosate was therefore investigated in a reportedly resistant Everly, IA population (P0-EV). Evaluation of six A tuberculatus populations from the Midwest USA estimated a seedling baseline sensitivity of 2.15 mM glyphosate. Based on these results, three generations of divergent recurrent selection were implemented on P0-EV to isolate resistant and susceptible populations. A seedling assay was developed to screen large amounts of seeds and thus expedite the selection process. Whole-plant and seedling rate responses of P0-EV and a known pristine A tuberculatus population from Paint Creek, OH (P0-WT) identified no significant difference in response to glyphosate; however, greater phenotypic variance was ostensibly evident in P0-EV. The first recurrent generation selected for resistance at 3.2 mM glyphosate (RS1-R) had a 5.9- and 1.7-fold resistance increase at the seedling and whole-plant levels, respectively, compared with the susceptible generation selected at 32 microM glyphosate. After three cycles of recurrent selection, 14.6-fold difference in resistance at the seedling level and 3.1-fold difference at the whole-plant level were observed when comparing the populations selected for resistance (RS3-R) and susceptibility (RS3-S). Overall, recurrent selection increased the frequency of resistant individuals and decreased the variability to glyphosate at the population level. Nevertheless, variability for glyphosate resistance was still evident in RS3-R. Results herein suggested that A tuberculatus is inherently variable to glyphosate and that selection decreased the sensitivity to glyphosate. We purport that evolved glyphosate resistance in A tuberculatus may require multiple cycles of selection under field conditions. Historic estimated use of glyphosate alludes to the evolution of tolerant weed populations.
authors.affiliation: Department of Agronomy, Iowa State University, Ames IA 50011-1011, USA. iazelaya@iastate.edu
authors.forename: Ian A
authors.initials: IA
authors.lastname: Zelaya
authors.forename: Micheal D K
authors.initials: MD
authors.lastname: Owen
mesh_terms: D027721:Amaranthus / Q000187:drug effects*; D004305:Dose-Response Relationship, Drug; D004351:Drug Resistance; D005998:Glycine / Q000031:analogs & derivatives* / Q000494:pharmacology; D006540:Herbicides / Q000494:pharmacology*; D036226:Seedlings / Q000187:drug effects; D012641:Selection, Genetic; D000097797:Glyphosate
chemical_list: D005998:Glycine; D006540:Herbicides; D000097797:Glyphosate