Crassulacean Acid Metabolism and Diurnal Variations of Internal CO₂ and O₂ Concentrations in Sedum praealtum DC

Abstract

Internal CO₂ and O₂ concentrations in Sedum praealtum DC. were determined by gas chromatography of 200-µl gas samples. Day-night monitoring showed that internal CO₂ varied from a high of approximately 4000 µl/l during periods of daytime stomatal closure to a low of 270-280 µl/l during the dark period (stomata open). Internal O₂ concentrations varied from a high of approximately 26 % at midday to a low of 20.8 % during the dark period. The calculated internal O₂/CO₂ ratio varied about 12-15-fold from 50-60 near midday to approximately 750 during the dark period (ratio in normal air is roughly 600). Day-night patterns of CO₂ exchange and malic acid concentration were typical for a plant with crassulacean acid metabolism (CAM). Influx of CO₂ during the late light period was inhibited by O₂, but dark CO₂ influx was O₂-insensitive. Gas samples taken near midday from several CAM plants all showed elevated internal CO₂ and O2 concentrations. Ratios of O₂/CO₂ in these plants ranged from 81 in Sedum praealtum to 285 in Hoya carnosa. The highest internal O₂ concentration observed was 41.5% in Kalanchoe gastonis-bonnieri. The high CO₂ concentration in leaves of CAM plants during daytime stomatal closure should provide a near- saturating level of this substrate for photosynthesis. In comparison to C₃ plants, the relatively low O₂/CO₂ ratio in the CAM leaf during malic acid decarboxylation should be favourable for photosynthesis and unfavourable for O₂ inhibition of photosynthesis.