The effect of climate change on glyphosate control of Avena fatua, Brassica napus, and Echinochloa crus-galli.

Atmospheric CO2 concentrations and temperatures have significantly increased, accompanied by substantial changes in precipitation patterns. These changes are anticipated to intensify in the future. In Nordic regions, increasing temperatures can improve growing conditions for some crops by extending the growing season and expanding cultivation northward. Climate changes may also favour some weed species, potentially reducing crop yield and affecting herbicide efficacy. To assess glyphosate efficacy under future climate conditions, we conducted two dose-response experiments on barnyard grass (Echinochloa crus-galli- C4 plant), oilseed rape (Brassica napus- C3 plant) and wild oat (Avena fatua- C3 plant). Plants were grown under ambient conditions (400 ppm CO2 at 18/12 °C (day/night)) and predicted future conditions (800 ppm CO2 at 20.5/14.5 °C (day/night)). Glyphosate was applied at 3-4 - leaf-stage in doses of 0, 8.75, 17.5, 35, 70, 140, 420, 1260, and 2520 g active ingredient (a.i.) ha-1, with the highest dose only included in the second experiment. Chlorophyll fluorescence was measured 48 h after spraying. Two days after spraying, oilseed rape exhibited stress symptoms under both growing conditions, while barnyard grass showed symptoms only under future conditions and doses exceeding 6 g a.i. ha-1. Plants were harvested 72 h after spraying for transcriptome analysis and two weeks after spraying to determine dry weight, C%, N% and C/N ratio. The ED50 and ED90 values did not significantly differ between the two environments for each grass species. However, oilseed rape required significantly higher glyphosate doses to reduce dry weight by 50 and 90 % at ambient growing conditions, likely due to the faster translocation of glyphosate. This suggests that glyphosate doses can be reduced in a warmer climate with an elevated CO2 level. No apparent differences in the C%, N%, or C/N ratio were observed between environments for any species. Transcriptome analysis indicated that all species respond differently to glyphosate and climate change.