What was done
The authors grew the drought-resistant perennial C4 grass, Panicum coloratum, in controlled environmental chambers having atmospheric CO2 concentrations of 350 or 1000 ppm and different vapor pressure deficits (VPD), which were maintained by keeping the relative humidity of the air at either 50 or 80%, for approximately five weeks.  In addition, plants were watered daily to 65, 80, or 100% of their potting soils' field capacity to study the growth response of this C4 grass to elevated CO2 under environmental conditions that promote plant water stress.

What was learned
Under favorable environmental conditions, characterized by a low VPD and high soil moisture (100% field capacity), atmospheric CO2 enrichment failed to cause any significant increases in leaf or stem dry weight.  However, when water-stressed conditions prevailed, due to either a high VPD, low field capacities of 65 or 80%, or combinations of both parameters, elevated CO2 caused large significant increases in growth.  At the high VPD, for example, the percentage increases in leaf dry weight at field capacities of 65 and 80% were 117 and 112%, respectively, while the growth responses for stems under these conditions were 50 and 57%.

Such increases in growth resulted, in part, from the ability of elevated CO2 to ameliorate the negative effects of water stress on growth.  Under the most extreme water-stressed condition, for example, leaf water potential values were about 3.5 times more negative, i.e., more stressful, for plants grown in air of 350 ppm CO2 than for plants grown in air of 1000 ppm.  Moreover, transpirational water loss was always less for plants grown in elevated CO2.  In fact, for the most water-stressed condition, which resulted from a high VPD and a field capacity of 65%, plants grown in ambient CO2 exhibited transpiration rates that were about 2.5 times greater than those observed for plants grown in elevated CO2.  Thus, elevated CO2 increased dry matter production of water-stressed plants and reduced their water losses via transpiration, causing dramatic increases in their water-use efficiency, which increased when the water content of the soil was low or when the VPD of the air was high, relative to ideal conditions.

What it means
As the CO2 content of the air continues to rise, C4 grasses, such as this Panicum species, should exhibit increased growth, particularly in environments that experience water stress caused by high temperatures, low soil water contents, or both of these growth-reducing conditions.  Under any of these unfavorable circumstances, therefore, greater concentrations of atmospheric CO2 will likely allow C4 grasses to maintain better internal water relations by reducing transpirational water losses, which will result in greater water-use efficiencies and the likely persistence or expansion of the plants into hot, arid regions that are commonly subjected to drought.