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

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.