The Responses of Stomatal Density to CO2Partial Pressure

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.