The Pathway of Carbon Dioxide Fixation in Crassulacean Plants

Abstract
Combined gas chromatography-mass spectrometry of malic acid derivatives has been used to show unequivocally that malic acid, synthesized during active acid accumulation in the dark by Kalanchoë daigremontiana Hammet et Perrier in the presence of 13CO2 is produced by a pathway involving a single carboxylation. The significance of the finding that crassulacean malate synthesized in the dark and in the presence of 14CO2 often contains 66% of the total carboxyl label in carbon atom 4, which has previously been taken to indicate the operation of a double carboxylation pathway or has been dismissed as an artefact, is discussed.