What was done
Cherry seedlings were grown from germination for two full years in open-top chambers with ambient (350 ppm) or twice-ambient (700 ppm) levels of atmospheric CO2.  During the first growing season, half of the seedlings were exposed to a gradually-developing water stress treatment by withholding irrigation for a 6-week period.  These same seedlings were subsequently exposed to a rapid water stress treatment during the height of the next year's growing season.  These manipulations allowed the authors to study the interactive effects of elevated CO2 and water stress on cherry seedling photosynthesis and water relations.

What was learned
Elevated CO2 was found to have little impact on bulk water relations in well-watered and water-stressed cherry seedlings, probably because there were no significant differences in soluble sugar concentrations between CO2 treatments.  Elevated CO2 did, however, significantly reduce stomatal conductance in well-watered, but not water-stressed, seedlings throughout the entire experiment.  Atmospheric CO2 enrichment stimulated rates of photosynthesis by more than 44% in both water regimes, causing significant increases in instantaneous transpiration efficiency.  In addition, photosynthetic rates of well-watered plants did not acclimate to elevated CO2 at any time during the study, even though their rubisco activities were decreased relative to those measured in ambiently-grown controls.

What it means
As the CO2 content of the air rises, cherry seedlings will likely display significantly enhanced rates of net photosynthesis, even during times of water stress.  During such unfavorable growing conditions, even if stomatal conductance rates remain constant, seedlings should exhibit increased efficiencies of water use.  All of these phenomena should allow cherry seedlings to increase their growth and biomass in the future, even if soil moisture supplies become less than adequate at certain times.