What was done
Soybeans were grown in glasshouses at ambient (360 ppm) or elevated (700 ppm) CO2 concentrations for 52 days.  Thereafter, three different stress treatments were imposed upon them for an additional eight days: (1) water deficit, (2) high temperature, and (3) water deficit with high temperature.  Photosynthesis was measured several times before, and several times after, the imposition of the stress treatments to determine if elevated CO2 had any influence on its recovery.

What was learned
At elevated CO2, there was less of a percentage reduction in photosynthesis for each of the three stress treatments.  On average, the three stresses caused a 48% decrease in photosynthesis at ambient CO2, compared to unstressed control plants, while at elevated CO2, an average decrease of only 39% was observed.  For the water deficit stress treatment, elevated CO2 restored photosynthesis to pre-stress control values after only six days, while plants grown at ambient CO2 never returned to control values.  For the high temperature stress, plants in both CO2 treatments recovered to about 86% of their respective control values, but plants grown in elevated CO2 reached this state of recovery earlier.  For the combined water deficit with high temperature stress treatment, neither CO2 treatment brought about the full recovery of photosynthesis.  However, plants grown in elevated CO2 attained 72% of their control value by harvest time, while plants grown in ambient CO2 only reached 52% of their control value.

What it means
As the CO2 content of the air rises ever higher, soybean plants should maintain higher rates of photosynthesis than they do now, even in the presence of environmental stresses including high temperature, water deficit or both of these growth-reducing phenomena.  Of all environmental stresses, water stress may be the most important, as water limits the growth of plants in many areas around the globe.  The results of this research suggest that, in the words of the authors, "the deleterious effects of short-term water deficit on the rate of leaf carbon assimilation in future climates of elevated CO2 could be rather less than at the present."  And it is just such a phenomenon that may allow plants of the future to recover quicker, following periods of water stress, and bring them back to an enhanced level of photosynthetic activity that allows them to sequester even greater quantities of carbon from the air than they do today.