ARS | Publication request: ELEVATED CO2 DOES NOT STIMULATE C4 PHOTOSYNTHESIS DIRECTLY, BUT IMPACTS WATER RELATIONS AND INDIRECTLY ENHANCES CARBON GAIN DURING DROUGHT STRESS IN MAIZE (ZEA MAYS) GROWN UNDER FREE-AIR CO2 ENRICHMENT (FACE).

Submitted to: Meeting Proceedings
Publication Type: Proceedings/Symposium
Publication Acceptance Date: April 15, 2005
Publication Date: N/A

Technical Abstract: The potential for, and mechanism of, [CO2] effects on C4 plants has received considerable interest but remains poorly understood. In 2002 and 2004, a rain-fed field experiment utilizing FACE technology was undertaken, in the U.S. Corn Belt, to determine the effects of elevated [CO2] on Zea mays. FACE allows experimental treatments to be imposed on an undisturbed soil-plant-atmosphere continuum without the effects of experimental enclosures on plant microclimate. Each year, crop performance was compared at ambient [CO2] (~370 ppm) and the elevated [CO2] (~550 ppm)predicted for 2050, within a fully replicated design. The diurnal couse of gas exchange of upper canopy leaves was measured across the growing season of 2002. This was repeated in 2004 along with analysis of carbon and nitrogen metabolism, water relations, growth, and yield. This tested if elevated {CO2} would directly: (1) stimulate C4 photosynthesis, and (2) reduce stomatal conductance and, therefore, crop water use. The experiments also tested if altered water relations under elevated {CO2} could feedback to enhance carbon gain during water stress. 2004 was unusual climatically in that at no time in the growing season was there any soil water deficit. In this year, there was no [CO2} effect on photosynthesis, carbon metabolism, growth or yield. Nevertheless, elevated [CO2] reduced stomatal conductance, crop evapo-transpiration and soil moisture depletion. 2002 was a "typical" year in which plants experienced episodic water stress. During these dry periods, photosynthesis was greater under elevated [CO2]. We conclude, elevated CO2 can only indirectly enhance carbon gain during drought.