Degree Type

Thesis

Date of Award

2012

Degree Name

Master of Science

Department

Agronomy

First Advisor

Micheal D. K. Owen

Abstract

Asiatic dayflower is a difficult weed to control with glyphosate, and has become a problem in glyphosate-tolerant soybean and corn for some Iowa farmers. Asiatic dayflower is an annual plant that is widely distributed in the temperate zones of the world. Also, this weed produces aerial dimorphic seeds, a common feature in other weeds belonging to the Commelinaceae. Asiatic dayflower dimorphic seeds can be described as truncated at one end and the other type described as being concave (i.e. non-truncated). Research investigating the biological and ecological characteristics that gives Asiatic dayflower an ecological advantage in glyphosate tolerant crops is lacking. Additionally, the tolerance mechanism of this weed to glyphosate is not fully understood. Therefore, this thesis is composed of three major experiments aiming to address ecology, biology and physiology characteristics that have allowed Asiatic dayflower to adapt to Iowa crop systems. The first experiment was designed to study emergence patterns and seedling emergence depths in two agronomic environments. In addition, seedling emergence depth was characterized by seed type in 2009 and 2010 to determine if dimorphic seeds represent an ecological advantage for Asiatic dayflower in different environments. Asiatic dayflower time of emergence varied by location and by year; emergence began early in May at Osceola and Vinton and continued until late July and early August at Osceola and Vinton fields, respectively. The number of Asiatic dayflower plants from truncated seeds exceeded those that emerged from non-truncated seeds. However, no differences were detected between seed types when seedling median depth of emergence was compared. Median depth of emergence ranged from 0.4 to 0.8 cm at the Osceola fields, and 2.2 to 2.4 cm at the Vinton field. A second experiment was established to study Asiatic dayflower emergence patterns using artificial seed banks. Artificial seed banks provided edaphic conditions that were similar to those found in agricultural fields but with known seed bank characteristics. Asiatic dayflower seeds were collected from natural infestations in fields near Osceola and Vinton, IA and seed collections were separated according to dimorphic seed type (i.e. truncated and non-truncated). Two artificial seed bank experiments were established at the Agronomy Farm and Curtiss Farm near Ames, IA during the fall 2009 and repeated fall 2010. Artificial seed banks consisted of PVC pipes buried in the ground, subsequently filled with soil mixed with a known quantity of Asiatic dayflower seeds; treatments consisted of seed origin and dimorphic type. After emergence, seedlings were counted and removed. During 2010, Asiatic dayflower seedlings started to emerge in early-May and emergence continued until mid-July at the Agronomy Farm, and from mid-April until mid-June at the Curtiss Farm. During 2011, seedling emergence started in April and continued until early-July and late-June at the Agronomy Farm and Curtiss Farm, respectively. Not all seeds in the artificial seed banks germinated. At both locations and in both years, total emerged seedlings from Osceola and Vinton truncated seeds planted in the artificial seed bank ranged from 35 to 65% and from 44 to 65%, respectively. However the total emerged seedlings from Osceola and Vinton non-truncated seeds ranged from 27 to 80% and 39 to 62%, respectively. Asiatic dayflower seedling emergence in artificial seed banks was likely affected by environmental conditions each year and the seed lot age used in the experiment. The third experiment addressed glyphosate uptake and translocation in Asiatic dayflower as a mechanism that may contribute to the tolerance observed in fields. Two experiments to investigate glyphosate uptake and translocation in Asiatic dayflower as well as the glyphosate rate response were conducted in the greenhouse. The first study characterized 14C-glyphosate uptake and translocation while the second study addressed the response of Asiatic dayflower to glyphosate dose. It was found that 14C-glyphosate uptake increased with time; glyphosate was primarily found in the treated leaf and translocated into the roots. At 2 and 48 hours after treatment, absorbed 14C-glyphosate in the treated leaf was 10 and 25% of the total 14C-glyphosate applied, respectively. Absorption of 14C-glyphosate in the roots at 2 and 48 hours after treatment was 2 and 9 % of the total 14C-glyphosate applied, respectively. Only small amounts of 14C-glyphosate were found in other plant parts (i.e. tissue above and below the treated leaf). Asiatic dayflower response to glyphosate doses was measured as plant growth relative to the untreated control. Asiatic dayflower growth varied across the repeated experiments. Asiatic dayflower growth exceeded that of the untreated control plants by 31 and 74 % in the first and second experiments, respectively, when glyphosate was applied at 0.63 kg a.e. ha-1. Conversely, growth relative to the untreated control plants in the third experiment was lower for plants treated with 0.63 kg a.e. ha-1 of glyphosate when compared to the untreated control plants. The Brain-Cousens model best described the data for experiments 1 and 2 because it accounts for the hormetic effect caused by sub-lethal doses; the log-logistic model best described the data for the third trial where no hormesis was detected. The base dose (i.e. 1.26 kg a.e. ha-1) and higher doses decreased Asiatic dayflower growth relative to the untreated control.

Copyright Owner

Jose Maria Gomez

Language

en

Date Available

2012-10-31

File Format

application/pdf

File Size

105 pages

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