Poster: Global Change

Abs # 64: Will photosynthesis of maize (Zea mays) in the US Corn Belt increase in future [CO₂] rich atmospheres? An analysis of diurnal courses of gas exchange under Free-Air Concentration Enrichment (FACE).

Presenter:	Leakey, Andrew , leakey@life.uiuc.edu
Authors	Leakey, Andrew  (A)   Bernacchi, Carl  (A) (B)  Long, Stephen  (A)   Ort, Donald  (A) (B)
Affiliations:	(A): Department of Plant Biology, University of Illinois (B): USDA Agricultural Research Service

The C₄ grass Z. mays (corn) is the third most important food crop globally in terms of production. Nonetheless, the effect of rising [CO₂] upon corn is not sufficiently understood to allow accurate predictions of future crop production. In the primary area of global corn production (U.S. Corn Belt), a rainfed, field experiment, utilised FACE technology to grow crops at elevated CO₂. CO₂ fumigation was imposed from sowing to harvest across 20-m diameter plots, with none of the effects of experimental enclosures on plant microclimate. The in situ diurnal course of gas exchange of upper canopy leaves was compared under ambient (370 ppm) and elevated [CO₂] (550 ppm). Previous laboratory studies suggest that C₄ photosynthesis is CO₂ saturated under ambient conditions and, therefore, should not respond to elevated [CO₂]. However, stomatal conductance and transpiration are typically decreased, which can indirectly increase photosynthesis in dry climates. Given the deep soils and good rainfall of the Corn Belt, it was predicted that photosynthesis would not be enhanced by elevated [CO₂]. However, by contrast, growth at elevated [CO₂] did significantly increase leaf photosynthetic CO₂ uptake rate (A) by 10 % on average, and up to 41 % at one stage of development. The increases in A were associated with greater intercellular [CO₂] and greater water use efficiency. Summer rainfall during 2002 was very close to the 50-year average for this site, indicating that the year was typical and not a drought year. The results call for a reassessment of the established view that C₄ photosynthesis is saturated at the current atmospheric [CO₂] and that the production potential of corn in the US Corn Belt will not be affected by rising [CO₂].