Stomatal responses to humidity in Opuntia inermis in relation to control of CO2 and H2O exchange patterns

Summary At constant cladode temperature the stomatal resistance of O. inermis increased when the cladode-air vapor pressure difference was increased and stomatal resistance decreased when the cladode-air vapor pressure difference was lowered. Net CO₂ fixation in the dark was very responsive to these humidity dependent changes in stomatal resistance. Net CO₂ fixation and stomatal resistance in the light did not respond to changes in cladode-air vapor pressure differences in the light under the conditions tested. When temperature response functions for dark CO₂ fixation were examined at constant ambient humidity, the reduction in dark CO₂ fixation at higher temperatures was largely due to stomatal closure in response to the increased vapor pressure difference. The water requirement for net CO₂ fixation in the dark at typical nocturnal vapor pressure differences was about 10 times lower than that of net CO₂ fixation in the light at vapor pressure differences typical of the late afternoon. The role of the stomatal responses to humidity in determining the patterns and rates of net CO₂ exchange in the light or dark, and its possible ecological significance is discussed.