Abstract
Amaranthus palmeri S. Watson (Palmer amaranth) is a fast-growing, dioecious, highly competitive agricultural weed species, which is spreading across the US Midwest. Population sex ratios are an important consideration in the management of A. palmeri populations as this species has become resistant to several herbicide sites of action, and there is need to minimize seed production by female plants. Environmental conditions, particularly stressors, may influence sex ratios, and herbicides act as major stressors and evolutionary filters in agricultural fields. Amaranthus spp. have shown a tendency for rapid evolution of herbicide resistance, with the frequency of protoporphyrinogen oxidase (PPO)-inhibitor resistance increasing across the Midwestern US. A greenhouse experiment was conducted to investigate the effect of two PPO-inhibiting herbicide treatments of either lactofen or fomesafen on four different Illinois populations (Cahokia, Collinsville, Rend Lake, and Massac). Plants raised from seed from the Massac population were tallest, and both males and females from this population also had the highest vegetative biomass. Female plants from the Collinsville population had more reproductive biomass than male plants. Control populations were male-biased (Cahokia, Collinsville), female-biased (Masaac), and 1:1 (Rend Lake). Lactofen shifted the male-biased populations to female-biased or 1:1 and the female-biased population to 1:1. Fomesafen-treated populations were male-biased or 1:1. This study suggests that PPO-inhibiting herbicide treatments may influence the growth and sex ratio of A. palmeri populations, which is an underlying factor in the rate of herbicide evolution in this species. An understanding of the underlying mechanisms of how external factors influence sex ratios may eventually provide an opportunity to reduce seed production in populations by shifting sex ratios towards a male bias.
Highlights
In some flowering species, dioecy is a sexually dimorphic phenomenon [1]
The experiment investigated sex ratios among four different A. palmeri seed sources treated with post-emergence applications of protoporphyrinogen oxidase (PPO)-inhibiting herbicides
Horticultural Research Center (HRC); and three weeks after herbicide application, they were transferred to a greenhouse at the Southern Illinois University Carbondale (SIUC) Tree Improvement Center (TIC) on three benches, which act as replicates in the experiment
Summary
Dioecy is a sexually dimorphic phenomenon [1]. In these species, male-to-female sex ratios are expected to be 1:1 because of parental investment due to the effect of deleterious incidence, and reliance on selection to maintain unbiased sex ratios [2]. With environmental stress or when there are limited resources, higher female reproductive costs can lead to vulnerability to stress, and result in male-biased populations in dioecious species [17]. These differences in resource allocation result in dissimilarities in the rate of recurrence of flowering, which causes sex-biased mortality. Dioecious plant populations exhibit male- or female-biased sex ratios if the cost of production or reproductive fitness between male and female populations are not equal over time, and these altered or biased sex ratios can impact population growth rates over time [17]. PPO-inhibiting herbicides differentially affect the growth characteristics of male and female plants
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