Stomatal Sensitivity to Carbon Dioxide and Humidity

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Abstract
The sensitivity of stomatal conductance to changes of CO2 concentration and leaf-air vapor pressure difference (VPD) was compared between two C3 and two C4 grass species. There was no evidence that stomata of the C4 species were more sensitive to CO2 than stomata of the C3 species. The sensitivity of stomatal conductance to CO2 change was linearly proportional to the magnitude of stomatal conductance, as determined by the VPD, the same slope fitting the data for all four species. Similarly, the sensitivity of stomatal conductance to VPD was linearly proportional to the magnitude of stomatal conductance. At small VPD, the ratio of intercellular to ambient CO2 concentration, Ci/Ca, was similar in all species (0.8-0.9) but declined with increasing VPD, so that, at large VPD, Ci/Ca was 0.7 and 0.5 (approximately) in C3 and C4 species, respectively. Transpiration efficiency (net CO2 assimilation rate/transpiration rate) was larger in the C4 species than in the C3 species at current atmospheric CO2 concentrations, but the relative increase due to high CO2 was larger in the C3 than in the C4 species.