What was done
The authors studied the effects of osmotic stress and abscisic acid on stomatal conductance in Arabidopsis thaliana plants subjected to different concentrations of atmospheric CO2.  Plants were grown hydroponically in controlled environment chambers, and received additions of polyethylene glycol and abscisic acid to their aerated nutrient solution to induce osmotic stress and stomatal closure, respectively.

What was learned
Leaf conductance and water loss increased substantially when plants were moved from ambient to half-ambient atmospheric CO2 concentrations.  In contrast, when exposed to twice-ambient levels of atmospheric CO2, values of stomatal conductance significantly declined.  Because elevated CO2 also stimulated photosynthesis, these phenomena increased the water-use efficiency of CO2-enriched plants by 41% relative to that observed in ambiently-grown plants.

When polyethylene glycol was added to induce water stress, atmospheric CO2 enrichment had an even stronger inhibitory effect on stomatal conductance.  Even though photosynthetic rates were somewhat reduced, due to this osmotic stress, the water-use efficiency of such CO2 enriched plants was now nearly 120% greater than that exhibited by their ambient controls.  When abscisic acid was added to induce stomatal closure, the CO2 response of Arabidopsis thaliana was similar to that observed under osmotic stress, with atmospheric CO2 enrichment causing significant gains in plant water-use efficiency.

What it means
As the air's CO2 content rises, Arabidopsis thaliana will likely increase its photosynthetic rates while decreasing its stomatal conductance.  Thus, the water-use efficiency of this species will likely increase with the rising CO2 concentration of the air.  In addition, if stressed due to inadequate soil moisture supply, this species should still exhibit significant reductions in stomatal conductance and tremendous gains in water-use efficiency.