Date of Award


Degree Name

Doctor of Philosophy


Plant Biology

First Advisor

Young, Bryan

Second Advisor

Lightfoot, David


Weed management options in agronomic crop production have been severely limited by widespread populations of weeds resistant to herbicides, including waterhemp [Amaranthus tuberculatus (Moq.) Sauer (syn. rudis)] resistant to foliar applications of herbicides that inhibit protoporphyrinogen oxidase (PPO; EC activity (PPO-R). Herbicides within this site of action (WSSA site of action #14) remain efficacious when soil-applied to PPO-R waterhemp populations. Therefore, the continued use of these herbicides for soil-residual control of PPO-R waterhemp, especially in soybean production, is paramount with limited postemergence herbicides that remain effective. An improved understanding of the selection for PPO-R waterhemp would provide information to help minimize future loss of residual PPO-inhibiting herbicide activity. Five studies, consisting of 14 experiments, were conducted to improve our understanding of the selection for herbicide-resistant individuals. Soil-residual herbicides have been suspected to select for herbicide-resistant individuals; however, this phenomenon has never been observed experimentally in field conditions. This dissertation provides direct evidence from greenhouse and field experiments that significant selection pressure can occur from soil-residual herbicides; however, this selection for resistance could be delayed when using full commercial herbicide rates and effective herbicides from multiple sites of action. Also, the frequency of heterozygous individuals (RS) and PPO-inhibiting herbicide efficacy on RS individuals is a factor in the selection for herbicide resistance; however, current information on the these topics is limited. To provide additional information on RS individuals, a large-scale genotypic and phenotypic screen of multiple PPO-R waterhemp populations was conducted. It was determined that RS individuals were less frequent than expected and PPO-inhibiting herbicide efficacy on RS individuals was population-dependent. Finally, the hormetic effects of soil-residual herbicides have been paradoxically implicated as a means of both mitigating and exacerbating the selection for herbicide resistant biotypes; however, limited information was available on the hormetic effects of soil-residual PPO inhibitors. Greenhouse and growth chamber experiments were conducted to improve understanding of hormesis and soil-residual PPO-inhibiting herbicides. Experiments indicated that PPO-inhibiting herbicides may exert a limited hormetic effect on waterhemp germination below doses that cause a phytotoxic effect of the emerging seedling, indicating this effect may exacerbate the issue of selection for PPO-R waterhemp. Overall, data presented in this dissertation provides important information on the under-studied interaction between PPO-inhibiting herbicides and PPO-R waterhemp to safeguard the sustained efficacy of herbicides within this site of action.




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