Reference
Bunce, J.A. 2005. Response of respiration of soybean leaves grown at ambient and elevated carbon dioxide concentrations to day-to-day variation in light and temperature under field conditions. Annals of Botany 95: 1059-1066.
What was done
Soybeans (Glycine max Merrill cv Kent) were grown in the field in open-top chambers maintained at atmospheric CO2 concentrations of ambient and ambient +350 ppm at the Beltsville Agricultural Research Center in Maryland, USA, where net carbon dioxide exchange rate measurements were performed on a total of 16 days between 18 July and 11 September of 2000 and 2003 during the flowering to early pod-filling stages of the growing season, after which the data were analyzed to see what effect, if any, elevated CO2 concentrations had on photosynthesis and respiration rates of fully expanded leaves.
What was learned
Averaged over the course of the study, daytime net photosynthesis per unit leaf area was 48% greater in the soybean leaves growing in the CO2-enriched air, while nighttime respiration per unit leaf area was unaffected by elevated CO2. However, because the elevated CO2 increased leaf dry mass per unit area by an average of 23%, respiration per unit of mass was significantly lower for the leaves of the soybeans growing in the CO2-enriched air.
What it means
According to the author, "these results do not support predictions that elevated CO2 would increase respiration per unit leaf area by increasing photosynthesis or by increasing leaf mass per unit of area." Consequently, what we have here is a win-win situation, where higher atmospheric CO2 concentrations increase photosynthesis while actually reducing respiration on a unit mass basis, which is a sure recipe for accelerated growth and higher soybean seed yields.