Title: Global warming can negate the expected CO2 stimulation in photosynthesis and productivity for soybean grown in the Midwest United States Authors
Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 17, 2013
Publication Date: May 1, 2013
Citation: Ruiz-Vera, U.M., Siebers, M., Gray, S., Rosenthal, D.M., Kimball, B.A., Ort, D.R., Bernacchi, C.J. 2013. Global warming can negate the expected CO2 stimulation in photosynthesis and productivity for soybean grown in the Midwest United States. Plant Physiology. 162:410-423. Interpretive Summary: Rising CO2 and increasing temperature are likely to have a significant impact on plant production, particularly for major crops such as soybean. Temperature is likely to decrease whereas rising CO2 is shown to increase photosynthesis and productivity. Much of what is understood regarding the responses of crop to combined increases in CO2 and temperature rely on theory (models) or measurements made in artificial growth environments. This experiment grows soybean under field conditions over two years and uses in field CO2 fumigation and heating technology to impose elevated CO2 and warming treatments separately and together. The results show the elevated CO2 stimulated photosynthesis and elevated temperature decreases photosynthesis. These photosynthetic rates did not translate into similar growth and yield responses. Elevated temperature resulted in slightly higher productivity one year and slightly lower productivity another. Elevated CO2 resulted in higher photosynthetic rates for both years which resulted in higher growth both years but increased yields for only one year. The combined treatment also resulted in higher photosynthesis for two years, but the biomass and seed yield increased for only one year, the cooler of the two years. These results indicate a much more complex response to increases in CO2 and temperature than were originally predicted; much of this has to do with the growing season conditions. The first year was cooler and the additional heat and CO2 appears to have a stimulatory effect whereas the second year was warmer than average so the additional heat, even with elevated CO2, caused growth and yields to decline. These finding suggest that the initial prediction of elevated CO2 and temperature yielding significant increases in biomass and yields may not necessarily occur in the future.
Technical Abstract: The combined influence of temperature and rising CO2 has a complex influence on leaf photosynthetic rates. While CO2 almost universally increases C3 photosynthesis, the impact of rising temperatures can have a varied response based on a range of other factors including physiological adjustment and meteorological and climatic conditions. This research addresses the influence of rising CO2 and increasing temperature, together and in combination, on the photosynthesis, stomatal conductance, growth, and yields of soybean grown in a Temperature by Free Air CO2 Enrichment (T-FACE) experiment at the Soybean Free Air CO2 Enrichment (SoyFACE) experiment in Champaign, IL. Soybean was grown over two growing seasons using standard agronomic practices except that plants were fumigated with additional CO2 manner (atmospheric + 200 ìmol mol-1), increasing temperature (canopy temperature + 3.5°C) or a combination of these two treatments. Results show that elevated temperature over both growing seasons showed lower photosynthetic rates and lower stomatal conductance. Growth increased in one year but decreased in the second whereas yields were consistently lower. Elevated CO2 increased biomass both years but yields only for one year. The combined treatment showed no differences in yield relative to the elevated CO2 only plots for either year and in one year the biomass and seed yield in the combined treatment was lower than the control. The differences between the years are likely due to one of the years, in which the influence of temperature on photosynthesis, stomatal conductance, and yields were greatest, being a much hotter than normal growing season. These results suggest that the influence of interacting global change factors of CO2 and temperature may drive lower productivity than originally predicted.