Variations in the Phases of Crassulacean Acid Metabolism and Regulation of Carboxylation Patterns Determined by Carbon-Isotope-Discrimination Techniques

Abstract

The four phases of gas exchange, defined for CAM plants by Osmond in 1978, have provided the framework for othe interpretation of physiological and biochemical studies on the regulation of this pathway in both constitutive and facultative CAM plants. Whilst the main CAM processes of dark uptake of CO2 and organic-acid decarboxylation occur during Phases I and III, respectively, the daytime Phases II and IV are often considered merely as transitions between C3 and C4 carboxylation. However, uptake of external CO2 during the day, particularly during Phase IV, contributes a substantial proportion of the assimilates used for the growth of consitutive CAM plants. In addition, daytime photosynthesis is essential for maintaining a positive carbon balance during the C3-CAM transition in facultative plants.