What was done
The authors analyzed the peer-reviewed literature of the past decade to determine how elevated atmospheric CO2 concentrations affect plant water use.  In their review, they compiled over 150 individual plant water use responses to atmospheric CO2 enrichment.

What was learned
Elevated CO2 increased rates of net photosynthesis in about 85% of the reported studies, while reducing stomatal conductances and rates of transpiration in approximately 75% of the cases analyzed.  Consequently, atmospheric CO2 enrichment increased plant water-use efficiency in more than 90% of the experiments that were conducted.  In addition, elevated CO2 reduced total water uptake in more than 50% of the studies, while slowing the development of water stress as indicated by plant water potential data.

What it means
As the air's CO2 content continues to rise, the vast majority of earth's plants will likely display increases in water-use efficiency, which may actually be a more common response to atmospheric CO2 enrichment than enhanced rates of photosynthesis.  Thus, as the authors conclude, plants growing in future atmospheres of higher CO2 concentration "will probably survive eventual higher drought stress and some species may even be able to extend their biotope into less favourable sites."