RELATIONSHIPS BETWEEN STOMATAL BEHAVIOR AND INTERNAL CARBON-DIOXIDE CONCENTRATION IN CRASSULACEAN ACID METABOLISM PLANTS

Measurements of internal gas phase CO2 concentration, stomatal resistance and acid content were made in Crassulacean acid metabolism plants growing under natural conditions. High CO2 concentrations, sometimes in excess of 2%, were observed during the day in a range of taxonomically widely separated plants (Opuntia ficus-indica L., O. basilaris Engelm. and Bigel., Agave desertii Engelm., Yucca schidigera Roezl. ex Ortiges, Anamas comosus [L.] Merr., Aloe vera L., Cattleya sp. and Phalanopsis sp.) and below ambient air concentrations were observed at night. Stomatal resistance was always high when CO2 concentration was high and experiments in which attempts were made to manipulate internal CO2 concentrations gave data consistent with stomatal behavior in Crassulacean acid metabolism being controlled by internal CO2 concentration. Exogenous CO2 applied in darkness at a concentration similar to those observed in the light caused an increase in stomatal resistance. In pads of O. basilaris Engelm. and Bigel. subjected to severe water stress, internal gas phase CO2 concentrations exhibited fluctuations opposite in phase of fluctuations in acid content. Stomatal resistance remained high and the opening response to low CO2 concentration was almost entirely eliminated.