What was done
Four C4 plants were grown in controlled environment chambers receiving diurnal atmospheric CO2 concentrations of 350 and 700 ppm or just a nocturnal CO2 concentration of 700 ppm (and 350 ppm CO2 during the day) for about three weeks to determine the effects of nocturnal atmospheric CO2 enrichment on C4 plants.

What was learned
Diurnal CO2 enrichment caused a significant increase in the photosynthesis (+13%) and total dry mass (+21%) of only one C4 species, Amaranthus retroflexus, relative to that observed for control plants exposed diurnally to ambient CO2 concentrations.  However, there were no significant effects of nocturnal CO2 enrichment on this species, indicating that the observed increase in biomass, resulting from diurnal atmospheric CO2 enrichment, was not facilitated by greater carbon conservation stemming from reductions in dark respiration rates.  Similarly, plants exposed to diurnal CO2 enrichment did not increase their biomass due to more efficient internal water balancing, as leaf water potentials were not significantly different among plants of any CO2 treatment.

In this study, however, three C4 species did not exhibit any positive growth response to diurnal atmospheric CO2 enrichment, most likely due to the short duration of CO2 exposure, as both corn and sorghum have been observed to increase their growth in response to elevated CO2.  The other species, which was unresponsive to atmospheric CO2 enrichment, was Amaranthus hypochondriacus.

What it means
As the CO2 content of the air continues to rise, C4 plants can, and will, continue to exhibit increases in growth, as nearly all C3 plants do, regardless of the fact that C4 plants are thought to be enzymatically-saturated for CO2 under current atmospheric CO2 concentrations.  In addition, such growth increases cannot result completely from better internal water balancing and reduced rates of dark respiration, as determined from measurements made on plants exposed only to nocturnal atmospheric CO2 enrichment.  Thus, the complete mechanisms in C4 plants, which allow them to exhibit greater photosynthetic rates and biomass increases in response to elevated CO2, are yet to be elucidated.