Impacts of global warming and rising atmospheric CO2 concentrations on physiology, development and productivity of Midwestern crops

High accumulation of greenhouse gases in the atmosphere, particularly CO2, drives increases in the global surface temperatures and is already impacting life on Earth. The impact of these changes on agro-ecosystem is particularly important as we rely on agriculture for food, fiber, and renewable energy, the demand for which is increasing as human population rises. Photosynthesis is the primary means by which plants sense rising atmospheric concentrations of CO2 ([CO2]) and all metabolic pathways are responsive to changes in temperature. In this thesis, I address, first, the effects of rising [CO2] and increasing temperatures on photosynthesis, growth and yields for the tenth most productive crop, soybean. Environmental conditions are shown to strongly influence the progression of crops through vegetative and reproductive growth stages. Therefore, I next evaluate the effects of elevated CO2 and high temperature on vegetative and reproductive development for soybean. Maize, the second most productive crop in the world, is expected to behave different than soybean under conditions of elevated CO2 and high temperatures because this crop differs in its photosynthetic pathway. Thus, I evaluate the response of photosynthesis, growth and yield in maize under the same conditions presented for soybean. My research was conducted over three years using the Temperature by Free Air CO2 Enrichment experiment (T-FACE) which simulated conditions predicted by mid-century in terms of CO2 and temperature, at the SoyFACE research facility in Champaign, IL. For soybean, photosynthesis, growth and yield were reduced with high temperatures and increased with elevated CO2. However, rising both CO2 and temperature produced a variable response that was dependent on the weather conditions during the growing season. Soybean under the combined treatment did not perform better than under only elevated [CO2] but elevated CO2 helped to mitigate the decreases in photosynthesis under high temperature conditions. Additionally, soybean presented a faster vegetative development under increased temperature which offset the delay of reproductive development caused by elevated CO2. For maize, it was found that the increase in temperature reduced photosynthesis during reproductive stages while elevated CO2 had no effect on photosynthesis. A stimulation of vegetative biomass and a reduction of yield were observed with increased temperature, resulting in no change for the total above-ground biomass. The results in this thesis add evidence for the negative effects of global warming to photosynthesis and productivity in soybean and maize. Additionally, these results are different from predictions based on laboratory or enclose experiments for the CO2 plus temperature treatment, emphasizing the necessity for in-field experiments in order to have more reliable predictions of the effects of climate change on agriculture. This thesis contributes toward improved predictions of key agronomic species grown in the Midwestern agriculture in the context of climate change.