What was done
The authors grew three C4 plant species, differing in CO2 leak rates from their bundle sheath cells, in controlled environment chambers receiving atmospheric CO2 concentrations of 350 and 700 ppm for approximately 40 days to determine the effects of atmospheric CO2 enrichment on their photosynthesis and growth.  Of the three plants utilized, Flaveria trinervia had CO2 leakage rates from bundle sheath cells that were 2.5 times greater than those characteristic of the other two species, Panicum maximum and Panicum miliaceum.  All plants were well-watered to ensure that any CO2-induced changes in photosynthesis or growth resulted directly from atmospheric CO2 enrichment of the plant and not from an indirect effect of elevated CO2 on water conservation in these species.

What was learned
On average, elevated CO2 stimulated single leaf photosynthesis in all three species to the same degree (+12%) under moderate light intensity.  Under high light intensity, however, elevated CO2 increased single leaf photosynthesis for Flaveria and Panicum miliaceum by an average of 14%, while high light increased it by 24% in Panicum maximum.  Thus, these observations indicate that none of these three C4 species are enzymatically CO2 saturated under ambient CO2 concentrations.

Upon closer examination of the photosynthetic apparatus, the authors determined that the observed CO2-induced increases in assimilation were not associated with significant increases in the carboxylation activity of either rubisco or PEP-carboxylase, which are the primary CO2-fixing enzymes of the C3 and C4 photosynthetic pathways, respectively.  Furthermore, enhanced photosynthetic rates led to a significant biomass increase only in Flaveria (+50%), which exhibited the greatest rate of CO2-leakage among the three tested species and consequently was anticipated to be the least responsive to atmospheric CO2 enrichment.  Thus, there was no direct correlation between the degree of bundle sheath cell CO2-leakiness and C4 plant growth response to elevated CO2.  Nonetheless, this paper demonstrates, once again, that C4 plants are photosynthetically responsive to atmospheric CO2 enrichment, and this phenomenon can lead to significant increases in growth, as was the case for Flaveria.

What it means
As the atmospheric CO2 concentration rises, it is likely that C4 plants will exhibit significant increases in photosynthetic carbon fixation under moderate to high light regimes, even under well-watered conditions.  Moreover, this phenomenon can lead to substantial increases in biomass production, as was the case for Flaveria.  And in an effort to punctuate this observation, the authors concluded, "it should not be assumed that C4 plants are incapable of responding directly to elevated CO2" (as do C3 plants).   Thus, it is likely that C4 plants will grow better and bigger in a future world containing greater concentrations of atmospheric CO2.