The effect of CO(2) enrichment on leaf photosynthetic rates and instantaneous water use efficiency of Andropogon gerardii in the tallgrass prairie.

Open-top chambers were used to study the effects of CO(2) enrichment on leaf-level photosynthetic rates of the C(4) grass Andropogon gerardii in the native tallgrass prairie ecosystem near Manhattan, Kansas. Measurements were made during a year with abundant rainfall (1993) and a year with below-normal rainfall (1994). Treatments included: No chamber, ambient CO(2) (A); chamber with ambient CO(2) (CA); and chamber with twice-ambient CO(2) (CE). Measurements of photosynthesis were made at 2-hour intervals, or at midday, on cloudless days throughout the growing season using an open-flow gas-exchange system. No significant differences in midday rates of photosynthesis or in daily carbon accumulation as a result of CO(2) enrichment were found in the year with abundant precipitation. In the dry year, midday rates of photosynthesis were significantly higher in the CE treatment than in the CA or A treatments throughout the season. Estimates of daily carbon accumulation also indicated that CO(2) enrichment allowed plants to maximize carbon acquisition on a diurnal basis. The increased carbon accumulation was accounted for by greater rates of photosynthesis in the CE plots during midday. During the wet year, CO(2) enrichment decreased stomatal conductance, which allowed plants to decrease transpiration while still photosynthesizing at rates similar to plants in ambient conditions. During the dry year, CO(2) enrichment allowed plants to maintain photosynthetic rates even though stomatal conductance and transpiration had been reduced in all treatments due to stress. Estimates of instantaneous water-use efficiency were reduced under CO(2) enrichment for both years.