What was done
The authors grew potato (Solanum tuberosum cv. Kennebec) plants from "seed tubers" in soil-plant-atmosphere research (SPAR) chambers maintained at daytime atmospheric CO2 concentrations of either 370 or 740 ppm in a 75/25 mix of coarse sand and vermiculite at well-watered and progressively water-stressed conditions until they were harvested when canopy photosynthetic rates dropped to below 50% of their seasonal peak values, before and after which they measured a variety of plant physical properties and physiological parameters.

What was learned
Fleisher et al. report that "elevated CO2 plants maintained a higher daily net assimilation rate throughout most of the growing season," and that "at moderate and more severe levels of water stress, CO2 enrichment appeared to encourage shifting of assimilate into tubers as opposed to additional vegetative growth." Hence, they say that "total biomass, yield and water use efficiency increased under elevated CO2, with the largest percent increases occurring at irrigations that induced the most water stress," and that "water use efficiency was nearly doubled under enriched CO2 when expressed on a tuber fresh weight basis."

What it means
"Overall," in the words of the three researchers, "the results indicate that increases in potato gas exchange, dry matter production and yield with elevated CO2 are consistent at various levels of water stress as compared with ambient CO2." And, of course, they provide what we so desperately need in today's world, and what we will need even more as the world's population continues to grow: significantly enhanced food production per unit of water used.