What was done
The authors grew Brassica juncea plants for an entire season in open-top chambers with ambient or enriched (600 ppm) atmospheric CO2 concentrations and adequate or inadequate soil moisture levels to study the interactive effects of these variables on photosynthesis and growth.

What was learned
Under well-watered conditions, plants grown in elevated CO2 exhibited net photosynthetic rates that were 34% greater than those displayed by plants grown in ambient CO2.  With the onset of water-stress, however, net photosynthesis was depressed by about 40% for plants grown in ambient CO2, while it was only depressed by 30% for plants grown in elevated CO2, indicating that even under water-stress conditions, elevated CO2 allowed for greater carbohydrate production.  This phenomenon may have contributed to a better overall water status of plants grown in elevated CO2 during water stress, as indicated by higher leaf water potentials and greater relative leaf water contents compared to those measured in plants grown in ambient CO2.

In addition, elevated CO2 reduced leaf dark respiration rates by about 25%, regardless of soil moisture, suggesting that a greater proportion of the increased carbohydrate pool in the CO2-enriched plants remained within them to facilitate increases in growth and development.  This phenomenon was found to be much more important for water-stressed than for well-watered plants.  Whereas well-watered CO2-enriched plants exhibited root, leaf, and stem dry masses that were 28, 38, and 40% greater than their ambient counterparts, water-stressed CO2-enriched plants displayed values that were 53, 59, and 80% greater than their respective counterparts.  Moreover, the percentage increase in seed yield caused by elevated CO2 was more than four-times greater for water-stressed plants than it was for those that were well-watered (88 vs. 21%).

What it means
As the CO2 content of the air increases, Brassica juncea plants will likely exhibit increased rates of photosynthesis and decreased rates of dark respiration, thereby increasing their biomass and the amount of carbon that is sequestered within their tissues.  This response will undoubtedly be greater on a percentage basis if water-stress in experienced by such plants during their growth cycle.  Thus, insufficient levels of soil moisture will not reduce the growth-promoting effects brought about by the aerial fertilization effect of rising atmospheric CO2 levels.