What was done
The authors collected seven C4 grass species from South Africa differing in photosynthetic sub-type (three NADP-ME, two NAD-ME and two PCK) and exposed them to atmospheric CO2 concentrations of 360 and 660 ppm for about six months to study the effects of elevated CO2 on their photosynthetic and growth responses.

What was learned
Elevated CO2 increased rates of net photosynthesis in five of the seven C4 grass species by 6 to 71%, while it decreased stomatal conductances in all of them by 10 to 61%.  Thus, the 300-ppm increase in the air's CO2 concentration enhanced plant water-use efficiency in all species by 68 to 118%.  In addition, elevated CO2 increased total shoot dry mass in six of the seven species by 10 to 43%.  Interestingly, there were no significant interactions between photosynthetic sub-type and CO2 response for any of these physiological parameters.

What it means
The data in this paper suggest that future increases in the air's CO2 concentration will enhance photosynthesis and biomass production in many C4 grass species, regardless of their photosynthetic sub-type.  In addition, atmospheric CO2 enrichment should enhance C4 grass water-use efficiency, perhaps allowing C4 grasses to expand their ranges into more arid habitats where their survival is currently limited by insufficient soil moisture availability.