What was done
Soybeans (Glycine max L. cv 'Pana') were grown from emergence to grain maturity in ambient and CO2-enriched air (372 and 552 ppm CO2, respectively) at the SoyFACE facility of the University of Illinois at Urbana-Champaign, Illinois, USA, while CO2 uptake and transpiration measurements were made from pre-dawn to post-dusk on seven days representative of different developmental stages of the crop.

What was learned
Across the growing season, the mean daily integral of leaf net photosynthesis increased by 24.6% in the elevated CO2 treatment, while average mid-day stomatal conductance decreased by 21.9%, in response to the 48% increase in atmospheric CO2.  In addition, the authors report "there was no evidence of any loss of stimulation toward the end of the growing season," noting that the largest stimulation of photosynthesis actually occurred during late seed filling.  Nevertheless, they state that the photosynthetic stimulation they observed was only "about half the 44.5% theoretical maximum increase calculated from Rubisco kinetics."

What it means
Although the photosynthetic response of this soybean variety to atmospheric CO2 enrichment was large and sustained (roughly equivalent to a 41% increase in assimilation in response to a 300-ppm increase in the air's CO2 concentration), there is apparently an opportunity for this important crop to do much better, i.e., to perhaps become twice as responsive to increases in atmospheric CO2 concentration as it is currently, which potential could well be realized as a result of future developments in the field of genetic engineering.