Effects of elevated CO2 on the response of glyphosate resistant and susceptible Palmer amaranth (Amaranthus palmeri S. Wats.) to varying rates of glyphosate

Authors: Runion, George; Prior, Stephen - Steve; DURSTOCK, MARY - Auburn University; SANZ-SAEZ, ALVARO - Auburn University; Price, Andrew

Submitted to: Archives of Agronomy and Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/24/2023
Publication Date: 3/9/2023
Citation: Runion, G.B., Prior, S.A., Durstock, M., Sanz-Saez, A., Price, A.J. 2023. Effects of elevated CO2 on the response of glyphosate resistant and susceptible Palmer amaranth (Amaranthus palmeri S. Wats.) to varying rates of glyphosate. Archives of Agronomy and Soil Science. 69(13):2739-2752. https://doi.org/10.1080/03650340.2023.2173741.
DOI: https://doi.org/10.1080/03650340.2023.2173741

Interpretive Summary: Overuse of Roundup® has led to herbicide resistant weed population. Since plants need CO2 to grow, the increasing level of CO2 in the Earth’s atmosphere often increases plant growth. This includes many weed species, which could make weed control more difficult. However, we know little on how rising atmospheric CO2 will affect herbicide resistant weeds. We looked at how increasing rates of Roundup® affected normal and resistant pigweed following growth in normal air and air with higher CO2. Roundup® controlled normal pigweed plants but not Roundup® resistant weeds and atmospheric CO2 level had little impact on weed growth or weed control. This means that normal pigweed should continue to be well controlled by Roundup®, while farmers will continue to experience problems with the rapid spread of Roundup® resistant pigweed. The future will undoubtedly require a more integrated pest management approach utilizing a variety of practices (e.g., other herbicides, cover crops, crop rotations, periodic tillage, etc.) for effective control of herbicide resistant weeds.

Technical Abstract: Repeated application of herbicides has led to herbicide resistant populations for many weed species. Elevated CO2 often increases plant growth (including many weeds) which can exacerbate problems associated with these plant pests, including impacts on crop growth and yield, as well as on weed control. How rising levels of atmospheric CO2 affect herbicide resistant weeds remains largely unknown. Seed of Palmer amaranth (Amaranthus palmeri S. Wats.) populations known to be resistant or susceptible to glyphosate were germinated in a glasshouse in 3L containers, thinned to one plant per container, and exposed to either ambient or elevated (ambient +200 µmol mol-1) concentrations of atmospheric CO2 in open top field chambers. After two weeks of CO2 exposure, plants from each population were sprayed with glyphosate at 0, 0.5x, 1.0x, or 1.5x the recommended label rate. Afterwards, CO2 exposure continued for an additional two weeks during which visual herbicide injury and leaf gas exchange were assessed. Plants were then destructively harvested for determination of treatments effects on growth parameters. Elevated CO2 had little effect on most variables except for decreasing stomatal conductance and increasing water use efficiency. The glyphosate resistant ecotype showed decreases in height, damage, and stomatal conductance, and increased root dry weight, photosynthesis, and water use efficiency. In general, increasing rates of glyphosate increased growth and decreased gas exchange variables; this was frequently due to effects only in the susceptible ecotype. Data from this study suggest that future levels of atmospheric CO2 are unlikely to affect Palmer amaranth, including the ability to control glyphosate resistant populations of this weed.