Abstract
A Palmer amaranth population (seeds collected in the year 2000; Washington Co., MS) suspected to be susceptible to glyphosate was examined as a population and as individual plants and found to exhibit varying tolerance or resistance to glyphosate. Whole plant spraying of glyphosate (0.84 kg·ha?1) to the population revealed that approximately 40% of this population were resistant to glyphosate and an LD50 of 0.75 kg·ha?1 was determined. Spray application of glyphosate indicated that some plants displayed varying degrees of resistance 14 days after treatment. Initial tests using leaf disc bioassays on 10 individual plants selected randomly from the population, allowed characterization of glyphosate resistance using both visual ratings of injury and quantitative measurement via chlorophyll content analysis. After initial bioassays and spray application, five plants with a range of tolerance to glyphosate were selected for cloning so that further studies could be accomplished on these individuals. Q-PCR analysis of these clones showed that resistance was not due to elevated EPSPS gene copy number. Shikimate levels were lower in the resistant and higher in the susceptible clones which correlated with varying degrees of resistance demonstrated in bioassays and spray application of glyphosate of these clones. Results demonstrate that individuals in a population can vary widely with respect to herbicide resistance and suggest that uptake, translocation, sequestration, metabolism or altered target site may contribute to the resistance in some individuals of this population.
Highlights
The broad-spectrum, systemic herbicide glyphosate (Nphosphono-methylglycine) was introduced in the early 1970s and about twenty years later (1990s) genetically engineered glyphosate-resistant crops became available
Glyphosate-resistance in Palmer amaranth (Amaranthus palmeri) was first reported in Georgia [3] and following this event, glyphosate resistant Palmer amaranth was discovered in several other southern states [4,5]
Palmer amaranth has been shown to cross with waterhemp (Amaranthus rudis), thereby transferring herbicide resistance traits [33]. These results are important because they demonstrate a wide diversity of individuals within a given weed population that can obfuscate the characterization of herbicide resistance mechanisms
Summary
The broad-spectrum, systemic herbicide glyphosate (Nphosphono-methylglycine) was introduced in the early 1970s and about twenty years later (1990s) genetically engineered glyphosate-resistant crops became available. The glyphosate resistance problem has been exacerbated by its increased use for weed control in genetically modified crops such as corn, cotton and soybeans [2]. These resistant weed biotypes exist in most countries of the world creating major problems in rangelands, natural settings and rightsof-way. Palmer amaranth [dioecious plant (either male or female)] can grow tall, has a rapid growth rate [7] and produces abundant seeds [8,9] These traits render it extremely competitive with crop plants and the management of glyphosate-resistant Palmer amaranth is very challenging with the herbicide technology presently available [10,11,12]
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