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Effect of Elevated CO2 on Photosynthesis of Soybeans Exposed to Elevated Ozone, Air Temperature and Water Stress
Bernacchi, C.J., Leakey, A.D.B., Heady, L.E., Morgan, P.B., Dohleman, F.G., McGrath, J.M., Gillespie, K.M., Wittig, V.E., Rogers, A., Long, S.P. and Ort, D.R. 2006. Hourly and seasonal variation in photosynthesis and stomatal conductance of soybean grown at future CO2 and ozone concentrations for 3 years under fully open-air field conditions. Plant, Cell and Environment 10.1111/j.1365-3040.2006.01581.x

What was done
Soybeans (Glycine max (L.) Merr.) were grown for three years at the SoyFACE facility of the University of Illinois at Urbana-Champaign, Illinois (USA) at atmospheric CO2 concentrations of either 375 or 550 ppm under natural field conditions with and without a 23% increase in ambient atmospheric ozone concentration, while a number of weather and plant physiological parameters were measured from pre-dawn to post-dusk on several days during the three growing seasons

What was learned
The mean daily integral of leaf-level net photosynthesis (A) was enhanced by nearly 25% in the CO2-enriched air under ambient ozone concentrations, but by a slightly smaller 20% in the high-ozone air. In addition, to quote the authors, "there was a strong positive correlation between daytime maximum temperatures and mean daily integrated A at elevated CO2." From their graphical representation of this relationship, we estimate that at a daily maximum temperature of approximately 26.5C, A was stimulated by about 14%, while at a daily maximum temperature of approximately 34.5C, it was stimulated by about 35%. Likewise, the team of eleven researchers determined that "the effect of elevated CO2 on photosynthesis also tended to be greater under water stress conditions," rising from an approximate 17% enhancement of A at the most favorable soil moisture condition encountered to an enhancement close to 30% under the driest of the conditions experienced by the crop.

What it means
The results of this study are very encouraging. For the standard 300-ppm increase in atmospheric CO2 concentration we use to express the effects of elevated CO2 on plant photosynthesis, they indicate that this important plant process is stimulated by close to 42% in ambient air and 34% in ozone-polluted air for typical growing-season air temperature and soil moisture conditions experienced at the SoyFACE facility, while for both warmer and drier conditions, the CO2-induced photosynthetic enhancements are greater still.

Reviewed 3 January 2007