What was done
The authors reviewed the current scientific literature pertaining to C4 plant responses to atmospheric CO2 enrichment with the intent to determine what mechanisms could be responsible for the positive growth responses that have been observed in plants of this type.

What was learned
After reviewing the literature, the authors proposed two major mechanisms that may be responsible for increasing C4 plant growth under conditions of elevated CO2.  The first potential mechanism operates through CO2-induced increases in net photosynthetic rates.  Although it has often been thought that C4 species are enzymatically saturated with carbon under current atmospheric CO2 concentrations, a number of studies have shown that such is not always the case.  Thus, even though the processes associated with this mechanism are not completely understood, increases in rates of net photosynthesis in C4 species should result in biomass increases under atmospheric CO2 enrichment.

The second mechanism that may be responsible for increasing C4 plant growth under atmospheric CO2 enrichment deals with CO2-induced reductions in stomatal conductance.  Reductions in stomatal conductance invariably decrease transpirational water losses through plant stomata.  Hence, elevated CO2 exposure tends to enhance soil moisture conservation, which can improve overall plant water relations and facilitate greater biomass production.  In addition, reductions in transpirational water loss may slightly increase leaf temperature, thereby stimulating rates of photosynthesis and biomass production, which increase concomitantly with increases in air temperature and atmospheric CO2 concentration.

What it means
As the air's CO2 content continues to rise, C4 plants will likely exhibit enhanced rates of photosynthesis and biomass production, just as nearly all C3 plants do under elevated CO2 conditions.  Although the mechanisms responsible for such responses in C4 species are not fully understood, future increases in the air's CO2 concentration should nevertheless directly stimulate photosynthetic rates in these plants by increasing internal leaf CO2 concentrations at the same time that they indirectly stimulate growth by causing reductions in stomatal conductance that lead to improvements in plant water relations.