What was done
The author grew C3 (potato, Solanum tuberosum L. cv. Atlantic) and C4 (sorghum, Sorghum bicolor L.) crops in the field within open-top chambers exposed to ambient and twice-ambient atmospheric CO2 concentrations for at least two years to determine how high vapor pressure deficits (VPDs) might influence leaf gas exchange responses to elevated CO2 in these important agricultural species.

What was learned
Elevated CO2 did not significantly change the slope of the leaf net assimilation response to air VPD in either species.  However, it significantly increased rates of net assimilation in sorghum by about 13%, regardless of air VPD.  In potato, on the other hand, elevated levels of atmospheric CO2 increased rates of net assimilation in a VPD-dependent manner, raising them by 36% at a VPD of 0.5 kPa but by 70% at a VPD of 3.5 kPa.

What it means
As the CO2 concentration of the air continues to rise, C3 and C4 crops will likely exhibit enhanced rates of net photosynthesis, with C3 crops being more responsive on a percentage basis than C4 crops, especially at higher atmospheric vapor pressure deficits.