Will photosynthesis of maize (Zea mays) in the US Corn Belt increase in future CO2 rich atmospheres? An analysis of diurnal courses of CO2 uptake under free‐air concentration enrichment (FACE)

Abstract The C 4 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. However, the effects of rising CO 2 upon C 4 plants, and Z. mays specifically, are not sufficiently understood to allow accurate predictions of future crop production. A rainfed, field experiment utilizing free‐air concentration enrichment (FACE) technology in the primary area of global corn production (US Corn Belt) was undertaken to determine the effects of elevated CO 2 on corn. FACE technology allows experimental treatments to be imposed upon a complete soil–plant–atmosphere continuum with none of the effects of experimental enclosures on plant microclimate. Crop performance was compared at ambient CO 2 (354 μ mol mol −1 ) and the elevated CO 2 (549 μmol mol −1 ) predicted for 2050. Previous laboratory studies suggest that under favorable growing conditions C 4 photosynthesis is not typically enhanced by elevated CO 2 . However, stomatal conductance and transpiration are decreased, which can indirectly increase photosynthesis in dry climates. Given the deep soils and relatively high rainfall of the US Corn Belt, it was predicted that photosynthesis would not be enhanced by elevated CO 2 . The diurnal course of gas exchange of upper canopy leaves was measured in situ across the growing season of 2002. Contrary to the prediction, growth at elevated CO 2 significantly increased leaf photosynthetic CO 2 uptake rate ( A ) by up to 41%, and 10% on average. Greater A was associated with greater intercellular CO 2 , lower stomatal conductance and lower transpiration. Summer rainfall during 2002 was very close to the 50‐year average for this site, indicating that the year was not atypical or a drought year. The results call for a reassessment of the established view that C 4 photosynthesis is insensitive to elevated CO 2 under favorable growing conditions and that the production potential of corn in the US Corn Belt will not be affected by the global rise in CO 2 .