Responses of C3 And C4 Weed Species to Tembotrione and Dicamba under Elevated Atmospheric CO2 and Water Deficit

The current atmospheric CO2 concentration of 400 ppm will increase to 700-1000 ppm by the year 2100, according to the IPCC. Since chemical control is the most common method to control weeds in cropping systems, and climate change can influence weed growth and herbicide efficacy, its effect on agricultural production will likely be critical. It is also known that C3 and C4 species react differently to changing climate, with C3 species having an advantage over C4 species. Therefore, to evaluate changes in weed responses and herbicide efficacy between photosynthetic systems, two C3 species, velvetleaf and weedy rice, and two C4 species, Johnsongrass and Palmer amaranth were studied. Greenhouse studies were conducted to evaluate herbicide efficacy under elevated CO2 concentration and water deficit on these species. Firstly, tembotrione, an HPPD herbicide, was applied at 0, ½, and ¾ the recommended rate for weedy rice, Johnsongrass, and Palmer amaranth. Velvetleaf plants received it at 0, 1 ⁄3, and ½ rates. Secondly, dicamba, an auxin herbicide, was applied on velvetleaf and Palmer amaranth at similar rates. In both studies, herbicide efficacy was evaluated at 10 and 21 DAT. At 10 DAT, carbon assimilation rate (A) and stomatal conductance (gs) measurements were taken on plants with non-necrosed leaves. At 21 DAT, total biomass and root to shoot ratio were measured. Results showed tembotrione application under elevated CO2 provided efficient weed control in weedy rice, Johnsongrass, and Palmer amaranth. However, lower herbicide efficacy was seen when velvetleaf plants were sprayed with tembotrione. Although lower herbicide rates were applied on velvetleaf, it showed increased tolerance to tembotrione, which could be explained by plant anatomical and physiological features. Dicamba showed higher efficacy under ambient CO2 concentration in both, velvetleaf and Palmer amaranth. In addition, the application of ½ dicamba rate increased the tolerance of velvetleaf under elevated CO2, which exemplifies the need for full herbicide rate to completely control this species and avoid resistance. Therefore, these findings suggest that under future climate scenarios, plant specific features can have a bigger interference on herbicide efficacy than a generalization on photosynthetic pathway