Background
Leakey et al. note that "the C4 grass Zea mays (maize or corn) is the third most important food crop globally in terms of production, and demand is predicted to increase 45% from 1997 to 2020 (Young and Long, 2000; Rosegrant et al., 2001)."  Hence, they further note that "any effects of elevated CO2 on crop productivity will have significant economic and social consequences," but that "there are currently no experimental data on the response of Z. mays to growth at elevated CO2 under standard agricultural practice in the field."

What was done
In an experiment designed to rectify this situation, the five University of Illinois scientists grew corn (Zea mays L. cv. 34B43) in the field at their SoyFACE facility in the heart of the US Corn Belt while exposing different sections of the field to atmospheric CO2 concentrations of 354 and 549 ppm using cultural practices deemed "typical for this region of Illinois" during a year that turned out to have experienced summer rainfall that was "very close to the 50-year average for this site, indicating that the year was not atypical or a drought year."  On five different days during the growing season (11 and 22 July, 9 and 21 August, and 5 September), they also measured diurnal patterns of photosynthesis, stomatal conductance and microclimatic conditions.

What was learned
Contrary to what many people had long assumed would be the case for a C4 crop such as corn growing under the best of natural conditions, Leakey et al. found that "growth at elevated CO2 significantly increased leaf photosynthetic CO2 uptake rate by up to 41%."  The highest whole-day increase was 21% (11 July) followed by 11% (22 July), during a period of low rainfall.  Thereafter, however, during a period of greater rainfall, there were no significant differences between the photosynthetic rates of the plants in the two CO2 treatments, so that over the entire growing season, the CO2-induced increase in leaf photosynthetic rate averaged 10%.

Additionally, on all but the first day of measurements, stomatal conductance (g_S) was significantly lower (-23% on average) under elevated CO2 compared with ambient CO2, which led to reduced transpiration (E) rates in the CO2-enriched plants on those days as well; and since "low soil water availability and high evaporative demand can both generate water stress and inhibit leaf net CO2 assimilation in C4 plants," in the words of the authors, they say that the lower g_S and E they observed under elevated CO2 "may have counteracted the development of water stress under elevated CO2 and prevented the inhibition of leaf net CO2 assimilation observed under ambient CO2."

What it means
In the words of the researchers, "contrary to expectations, this US Corn Belt summer climate appeared to cause sufficient water stress under ambient CO2 to allow the ameliorating effects of elevated CO2 to significantly enhance leaf net CO2 assimilation."  Hence, they conclude that "this response of Z. mays to elevated CO2 indicates the potential for greater future crop biomass and harvestable yield across the US Corn Belt."

References
Rosegrant, M.W., Paisner, M.S., Meijer, S. et al.  2001.  Global Food Projections to 2020: Emerging Trends and Alternative Futures.  International Food Policy Research Institute.

Young, K.J. and Long, S.P.  2000.  Crop ecosystem responses to climatic change: maize and sorghum.  In: Reddy, K.R. and Hodges, H.F. (Eds.).  Climate Change and Global Crop Productivity.  CABI International, Oxon, UK, pp. 107-131.