The effects of CO2 concentration and temperature on growth and control of barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.]susceptible and resistant to imazethapyr were evaluated. The treatments were doses of imazethapyr (zero to 848 g ha-1) applied in imidazolinone-susceptible (SUSSP01 and MOSTS01) and -resistant (ARRGR01 and PALMS01) biotypes, grown at ambient and elevated temperature (24/20°C or 30/26°C (day/night)) and CO2 concentrations (400 or 700 ppm). The herbicide effect on resistant biotypes was lower with the increase of CO2 concentration and temperature. Temperature increase from 24/20°C to 30/26°C enhanced the resistance factor (RF) in plants grown under 400 ppm from 3.58 to 31.73 in the biotype ARRGR01 and from 3.87 to 11.07 in the biotype PALMS01. When grown at 700 ppm, the increase in the RF was from 6.85 to 12.67 (ARRGR01) and from 7.95 to 27.11 (PALMS01) in response to increasing the temperature. The greater resistance in climate change scenarios can be explained by physiological and growth parameters. Plants cultivated at higher temperatures and CO2 concentration had a higher number of tillers per plant, shoot and root dry mass. The relative chlorophyll content was lower under high temperature, as a consequence of increased growth and demand for nutrients. The electron transport rate was severely reduced by increasing the CO2 concentration in plants grown at a temperature of 24/20°C. In summary, increases in CO2 concentration and temperature make resistant barnyardgrass even more insensitive to imazethapyr.
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