The Responses of Stomatal Density to CO2Partial Pressure

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Abstract
Experiments on a range of species of tree, shrub and herb have shown that stomatal density and stomatal index increase as the partial pressure of CO2 decreases over the range from the current level of 34 Pa to 22.5 Pa. Stomatal density responds to the reduced partial pressure of CO2 in a simulation of high altitude (3000 m), when the CO2 mole fraction is unchanged. When the partial pressure of CO2 is increased from 35 to 70 Pa stomatal density decreases slightly, with a response to unit change in CO2 which is about 10% of that below 34 Pa. Measurements of gas exchange on leaves which had developed in different CO2 partial pressures, but at low saturation vapour pressure deficits in the range of 0.7 to 0.9 kPa, indicated lower photosynthetic rates but higher stomatal conductances at reduced CO2 partial pressures. Experiments on populations of Nardus stricta originating from altitudes of 366 m and 810 m in Scotland, indicated genetic differences in the responses of stomatal density to CO2 in pressures simulating altitudes of sea level and 2 000 m. Plants from the higher altitude showed greater declines in stomatal density when the CO2 partial pressure was increased.