LEAF PHOTOSYNTHESIS AND WATER-USE OF BIG BLUESTEM UNDER ELEVATED CARBON-DIOXIDE

With the atmospheric concentration of CO2 increasing, it is important to know how this will affect crop growth. The objective of the study was to determine the effect of elevated CO2 on big bluestem (Andropogon gerardii Vitman) growing in a tallgrass prairie on a Tully silty clay loam (fine, mixed, mesic Pachic Argiustoll) kept at a high water level (field capacity) or a low water level (half field capacity). Sixteen cylindrical plastic chambers were placed on the prairie to maintain the two levels of CO2 (mean +/- SD: 337 +/- 32 Ind 658 +/- 81-mu-mol mol-1) over a full growing season. Soil-water content was measured weekly with a neutron probe. Photosynthesis, transpiration, stomatal resistance, and intercellular CO2 concentration were determined with a portable leaf Photosynthetic system. Canopy temperature was monitored with an infrared thermometer. Elevated (doubled) CO2 reduced transpiration rate of big bluestem by 25 and 35% under the high- and low-water treatments, respectively. Under both watering regimes, stomatal resistance was greater by almost-equal-to 1.6 s cm-1 with doubled CO2 than with ambient CO2. Plants grown with doubled CO2 at high- and low-water levels had warmer canopy temperatures (average 1.15 and 0.70-degrees-C warmer, respectively) than plants grown at ambient CO2. Carbon-dioxide concentration did not affect the rate of photosynthesis, even though intercellular CO2 concentration was increased under high CO2. Elevated CO2 did not increase the height of plants grown at the high water level, but it did increase the height at the low water level by an average of 9 cm.