What was done
Working with barley (Hordeum vulgare L. cv. Iranis) seedlings growing in 2.5-L pots filled with a 3:1 mixture of perlite:vermiculite located within controlled-environment chambers under a 14-hour photoperiod with an average day/night temperature of 24/20°C, a corresponding relative humidity of 60/80% and atmospheric CO2 concentrations of either 350 or 700 ppm, the authors -- after watering the plants twice a week with a complete Hoagland solution containing 20 mM nitrogen in the form of nitrate (with an application of deionized water between each Hoagland solution watering) -- subjected the plants to drought stress for 9, 13 or 16 days, during and after which periods they measured a number of plant physiological and biochemical properties and processes.

What was learned
Robredo et al. report that the "elevated CO2 concentration led to reduced water consumption, delayed onset of drought stress, and improved plant water status." In addition, they say that "in irrigated plants, elevated CO2 produced marked changes in plant nitrogen metabolism," noting that "nitrate reduction and ammonia assimilation were higher at elevated than at ambient CO2, which in turn yielded higher protein content." And in the drought-stressed plants, they report that "elevated CO2 reduced the water stress effect on both nitrate reduction and ammonia assimilation coincident with a less-steep decrease in midday leaf water potential." Last of all, they indicate that "recovery was always faster and slightly higher in plants grown under elevated CO2 conditions compared to those grown in ambient CO2."

What it means
After discussing their results in detail, the six Spanish scientists conclude that "elevated CO2 mitigates drought effects on nitrogen metabolism by improvement of photosynthesis and mitigation of water deficit," although they say that "other factors could be involved in the higher assimilation of nitrogen under elevated CO2," concluding that elevated CO2 "accelerates and enhances recovery of nitrogen metabolism after re-irrigation, in parallel with the recovery of water status and carbon assimilation."