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Wheat, Field Pea, and Canola Response to Glyphosate and AMPA Soil Residues
- Robert E. Blackshaw, K. Neil Harker
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- Journal:
- Weed Technology / Volume 30 / Issue 4 / December 2016
- Published online by Cambridge University Press:
- 23 February 2017, pp. 985-991
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The tripling of glyphosate use in the Canadian prairies during the past decade has raised concerns over the possible accumulation of glyphosate and its main metabolite AMPA in soil over time and whether there could be any detrimental effects on crop production. A controlled environment study was conducted at two locations in Alberta, Canada, to determine glyphosate and AMPA soil concentrations that would injure wheat, field pea, and canola. Treatments included glyphosate acid or AMPA applied at 0, 10, 25, 100, 250, and 500 mg kg−1 soil. Shoot and root biomass determinations 8 wk after emergence (WAE) indicated that shoot and root biomass of all crops progressively declined with increasing soil concentrations of glyphosate at both locations. In contrast, AMPA reduced crop shoot and root biomass at only one of two sites. Estimated soil concentrations of glyphosate causing 20% reductions in shoot and root biomass ranged from 80 to 190, 90 to 350, and 120 to 320 mg kg−1 for field pea, canola, and wheat, respectively. Soil concentrations of AMPA causing 20% crop biomass reductions ranged from 40 to 70, 20 to 30, and 80 to 120 mg kg−1 for field pea, canola, and wheat, respectively. Although substantial crop injury occurred in this study, it must be noted that these rates are very high in terms of field application rates that would be required to achieve these soil concentrations. Doses causing crop injury would convert to estimated glyphosate field rates ranging from 17.6 to 77 kg ha−1. Overall results indicate that even with frequent high-dose glyphosate applications over several years, the likelihood of wheat, field pea, and canola injury from soil residues is low. Nevertheless, there may be merit in greater monitoring of glyphosate and AMPA soil residues in cropping systems with high glyphosate utilization over a long time period.
Physiological Mechanisms of Glyphosate Resistance
- Wendy Pline-Srnic
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- Journal:
- Weed Technology / Volume 20 / Issue 2 / June 2006
- Published online by Cambridge University Press:
- 20 January 2017, pp. 290-300
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Glyphosate, a nonselective herbicide and also the world's most widely used herbicide, inhibits 5-enol-pyruvylshikimate-3-phosphate synthase (EPSPS), an enzyme in the aromatic amino acid biosynthetic pathway. Because of its broad-spectrum and potent weed control and favorable environmental characteristics, attempts to engineer glyphosate resistance have been intensive in the past few decades. The use of at least three different mechanisms has conferred glyphosate resistance in normally sensitive crop species. Early work focused on progressive adaptation of cultured plant cells to stepwise increases in glyphosate concentrations. The resulting cells were resistant to glyphosate because of EPSPS overexpression, EPSPS gene amplification, or increased enzyme stability. Further work aimed to achieve resistance by transforming plants with glyphosate metabolism genes. An enzyme from a soil microorganism, glyphosate oxidoreductase (GOX), cleaves the nitrogen– carbon bond in glyphosate yielding aminomethylphosphonic acid. Another metabolism gene, glyphosate N-acetyl transferase (gat), acetylates and deactivates glyphosate. A third mechanism, and the one found in all currently commercial glyphosate-resistant crops, is the insertion of a glyphosate-resistant form of the EPSPS enzyme. Several researchers have used site-directed mutagenesis or amino acid substitutions of EPSPS. However, the most glyphosate-resistant EPSPS enzyme to date has been isolated from Agrobacterium spp. strain CP4 and gives high levels of resistance in planta. Weeds resistant to glyphosate have offered further physiological mechanisms for glyphosate resistance. Resistant field bindweed had higher levels of 3-deoxy-d-arbino-heptulosonate 7-phosphate synthase, the first enzyme in the shikimate pathway, suggesting that increased carbon flow through the shikimate pathway can provide glyphosate resistance. Resistant goosegrass has reduced translocation of glyphosate out of the treated area. Although glyphosate resistance has been achieved by numerous mechanisms, currently the only independent physiological mechanism to give adequate and stable resistance to glyphosate for commercialization of glyphosate-resistant crops has been glyphosate-resistant forms of EPSPS.