What was done
The authors grew well watered and fertilized barley (Hordeum vulgare L.) seedlings (seven per each 2.5-liter pot filled with perlite and vermiculite) in controlled-environment chambers maintained at atmospheric CO2 concentrations of either 350 or 700 ppm. At the conclusion of the 18th day after seedling emergence, the treatments were split, with one treatment continuing to be watered three times a week, but with the other treatment receiving no further water additions. At that time, and on several following dates, a number of soil and plant water parameters were measured, along with rates of leaf transpiration and net photosynthesis.

What was learned
Robredo et al. report that "during the period of drought, elevated CO2 delayed by 3-4 days the depletion of soil water content," due to "the lower rates of transpiration in plants grown under CO2 enrichment." As a result, they found that "under elevated CO2, plant water stress developed more slowly," due to "a slower rate of soil water depletion," and as a result of this phenomenon, they report that "the stimulation of carbon assimilation by elevated CO2 was even greater in droughted compared to well-watered plants," in spite of the fact that "elevated CO2 caused stomata closure."

What it means
In summing up their findings and explaining their significance, the seven Spanish researchers say that "exposure to high carbon dioxide concentration resulted in an increase in photosynthesis and in a reduction in whole plant transpiration, contributing to an increase in water use efficiency that was more noticeable when plants were subjected to elevated CO2 in conjunction with drought." Consequently, they concluded that "growing plants under [an] elevated CO2 environment mitigates or delays the effects of water stress in barley," which, obviously, is a very favorable phenomenon with positive real-world economic consequences.