Submitted to: Global Change Biology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/4/2003
Publication Date: 2/1/2004
Citation: Leakey, A.D., Bernacchi, C.J., Dohleman, F.G., Ort, D.R. 2004. Will photosynthesis of maize (zea mays) in the U.S. corn belt increase in future [CO2] rich atmospheres? an analysis of diurnal courses of CO2 uptake under free-air concentration enrichment (face). Global Change Biology. 10:951-962. Interpretive Summary: Carbon dioxide in the world's atmosphere is on the rise. In the last century it has risen 20% and this geologically unprecedented pace will continue resulting in an atmosphere in 2050 containing 50% more CO2 than it did in 1900. While this enrichment of the atmosphere CO2 portends certain hazards of which global warming is the most publicized, it also offers the potential to increase plant production as carbon dioxide is in effect a fertilizer. However, in the case of corn, the third most important food crop globally, the effects of increasing CO2 are not adequately understood to allow accurate predictions of future crop production or allow varietal selection for improved productivity in a changing climate. Using technology that enables us to mimic the atmosphere of 2050 under natural field conditions, we found, contrary to predication, that the growth of corn was substantially stimulated by CO2 fertilization. This result calls for reassessment of the widely held view among crop research scientists that corn and other C4 plants will be unaffected by rising CO2.
Technical Abstract: 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. However, the effects of rising [CO2] upon C4 plants, and Z. mays specifically, are not sufficiently understood to allow accurate predictions of future crop production. A rainfed, field experiment utilizing FACE technology in the primary area of global corn production (U.S. Corn Belt) was undertaken to determine the effects of elevated [CO2] 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 at ambient [CO2] (354 µmol mol-1) was compared with the elevated [CO2] (549 µmol mol-1) predicted for 2050. Previous laboratory studies suggest that under favorable growing conditions C4 photosynthesis is not typically enhanced by elevated [CO2]. 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 U.S. Corn Belt, it was predicted that photosynthesis would not be enhanced by elevated [CO2]. 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 [CO2] significantly increased leaf photosynthetic CO2 uptake rate (A) by up to 41 % and 10 % on average. Greater A was associated with greater intercellular [CO2], lower stomatal conductance and lower evapotranspiration. 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 C4 photosynthesis is insensitive to elevated [CO2] 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 [CO2].