ASHS 2015 Annual Conference

Atmospheric concentrations of CO₂ have been increasing since the onset of the industrial revolution. Regardless of the debate on the effects of this rise on climate, most plants exhibit a positive growth response to elevated CO₂ due to increased photosynthesis, resource use efficiency, and/or allocation to belowground structures and weeds are no exception. In fact, the CO₂-induced growth stimulation of several invasive weeds was greater than for any previously examined plant species and many weeds are predicted to become more problematic as CO₂ continues to rise. This raises concerns about how weed control strategies in plant production systems will change in a future, higher CO₂ world. Further complicating this situation is recent evidence suggesting that elevated CO₂may increase herbicide tolerance in some weeds. This tolerance may be due to a herbicide dilution effect caused by increased growth. Also, changes in weed morphology (e.g., increased leaf thickness) and/or physiology (e.g., decreased stomatal conductance) can alter herbicide uptake, translocation, and overall efficacy. Weed control strategies will be further complicated by the fact that repeated application of herbicides, particularly those with the same modes of action, results in the development of weed populations resistant to herbicidal control and herbicide resistant weed populations are being identified. To date, approximately 235 different weed species have developed resistance to 22 of the 25 known herbicide sites of action and to 155 different herbicides; herbicide resistant weeds have been reported in 82 crops in 65 countries. Determining how weed species will respond to increased levels of CO₂ and the subsequent effect of increased CO₂ on herbicide efficacy on these weeds is critical to our ability to control important weeds and increase productivity and profitability as atmospheric CO₂ continues to rise.