Date of Award
Master of Science
TITLE: EFFECT OF PPO-INHIBITING HERBICIDES ON MALE-TO-FEMALE SEX RATIO OF AMARANTHUS PALMERI Background: Amaranthus palmeri S. Wats. (Palmer amaranth) is a dicotyledonous, dioecious species having separate male and female plants that forces outcrossing, ensures genetic diversity, and is recognized as one of the most noxious, invasive agricultural weed pests in the Mid-West. It can be characterized by extended emergence periodicity, aggressive growth habit, high fecundity, and high water use efficiency as well as high competitive ability. Fisher (1930) predicted 1: 1 primary sex ratios after the period of parental investment, but initial field studies indicated that A. palmeri populations were female-biased, departing from the expected 1:1 sex ratio. Therefore, managing population sex ratios would be an important consideration for controlling A. palmeri populations, as this species has become resistant to several herbicide modes of action. Objective: This study was conducted to investigate the male-to-female sex ratio of Amaranthus palmeri following exposure to PPO-inhibiting herbicides, to gain a better understanding of potential effects of herbicide application on the population sex ratio. Methods: A greenhouse experiment and a two-year field experiment were conducted at the Horticultural Research Center, Southern Illinois University Carbondale in spring 2016 and Collinsville, Illinois in the summer season of 2015 and 2016, respectively. The greenhouse experiment was conducted by applying two protoporphyrinogen oxidase (PPO)-inhibiting herbicide treatments of either lactofen (Cobra) or fomesafen (Flexstar) on four different Illinois populations (Cahokia, Collinsville, Rend Lake, and Massac). The field experiment was conducted for two years in a soybean field throughout the growing season of 2015 and 2016 in Collinsville, Illinois, USA. This study included 12 pre- and post-emergence PPO-inhibiting herbicide treatments of 10 herbicides with 3 replicates to investigate the variation among sex ratios by treatment. Results: For the greenhouse experiment, depending on the population, herbicide treatments expressed a male-to-female sex ratio of either 1:1 or male-biased in contrast to the female-biased field observations. This study also suggested that these PPO-inhibiting herbicide treatments may have an influence on the growth and sex ratio of A. palmeri populations. The field experiment indicated that A. palmeri populations have a female-biased sex ratio in untreated controls. The pre-emergence application of sulfentrazone (Spartan) at rates of 226.8 to 340.19 g a.i./ha provided the highest control efficacy, as compared to other treatments. Post-emergence-only applications provided limited control over the population. Fomesafen (Flexstar) was the only PPO-inhibiting herbicide which led to a male-based population in both years. In the future, and with increased understanding of the mechanism behind sex expression in A. palmeri, knowledge of plant-environment relationships such as these could provide an opportunity to reduce seed production in populations by favoring the production of males.
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