What was done
The authors collected honey mesquite seeds from several trees growing in the southwestern United States in regions that receive approximately 200 and 890 mm of annual rainfall.  Seeds were germinated and seedlings were grown in glasshouses receiving atmospheric CO2 concentrations of 370 and 700 ppm for a total of 66 or 82 days in experiment one and two, respectively.  In both experiments, plants were subjected to water stress by withholding supplemental irrigation after initial seedling establishment.  Thus, the authors were able to study the interactive effects of elevated CO2 and water stress on growth, water relations and survivorship of seedlings of different genotypes.

What was learned
Because the two experiments yielded nearly identical results, we will focus our discussion on experiment two, since it was of greater duration.  Elevated CO2 apparently reduced rates of transpiration in all seedlings shortly after the start of the experiment.  After 30 days of differential CO2 exposure, seedlings growing at 700 ppm displayed shoot water potential values that were nearly 40% higher (less negative and thus less stressful) than those measured in seedlings growing at ambient CO2.  At the end of the 82-day dry-down, elevated CO2 had significantly increased seedling root and shoot biomass by 37 and 46%, respectively, regardless of the region from where the seeds they sprouted originated.  Moreover, atmospheric CO2 enrichment more than doubled seedling survivorship under drought conditions without significantly favoring one genotype over another.

What it means
As the CO2 content of the air increases, honey mesquite seedlings will likely experience improved water relations and exhibit increased growth and biomass production, regardless of genotype.  In addition, as more CO2 enters the atmosphere, mesquite seedlings will experience an enhanced ability to survive drought, which is the largest selective factor delineating their range.  And because the authors found no significant interaction of elevated CO2 with genotype, the increasing atmospheric CO2 concentration should not act as a selective agent among genetically different mesquite plants.  Thus, the genetic diversity that exists within the honey mesquite population of the American southwest should be maintained as the air's CO2 concentration continues to rise.