Photosynthesis, Productivity, and Yield of Maize Are Not Affected by Open-Air Elevation of CO2 Concentration in the Absence of Drought

While increasing temperatures and altered soil moisture arising from climate change in the next 50 years are projected to decrease yield of food crops, elevated CO2 concentration (CO2) is predicted to enhance yield and offset these detrimental factors. However, C4 photosynthesis is usually saturated at current CO2 and theoretically should not be stimulated under elevated CO2. Nevertheless, some controlled environment studies have reported direct stimulation of C4 photosynthesis and productivity, as well as physiological acclimation, under elevated CO2. To test if these effects occur in the open air and within the Corn Belt, maize (Zea mays) was grown in ambient CO2 (376 micromol mol(-1)) and elevated CO2 (550 micromol mol(-1)) using Free-Air Concentration Enrichment technology. The 2004 season had ideal growing conditions in which the crop did not experience water stress. In the absence of water stress, growth at elevated CO2 did not stimulate photosynthesis, biomass, or yield. Nor was there any CO2 effect on the activity of key photosynthetic enzymes, or metabolic markers of carbon and nitrogen status. Stomatal conductance was lower (-34%) and soil moisture was higher (up to 31%), consistent with reduced crop water use. The results provide unique field evidence that photosynthesis and production of maize may be unaffected by rising CO2 in the absence of drought. This suggests that rising CO2 may not provide the full dividend to North American maize production anticipated in projections of future global food supply.