As irradiance changes, the stomatal aperature is altered, which in turn affects the degree to which the stomata limit photosynthesis through decreased CO2 availability (stomatal limitation, see Long and Bernacchi, 2003). This stomatal limitation (l) to intercellular [CO2] (Ci) can be compensated for by increasing the [CO2] outside the leaf. By maintaining a constant Ci, photosynthesis is altered by only changing irradiance and not confounded by changing Ci.
Photosynthetic response to incident irradiance in Zea mays (corn). The confounding effect of changing intercellular [CO2] (Ci) was removed by maintaining a constant Ci, by increasing the [CO2] outside the leaf (Ca).