
%0 Journal Article
%A Spalding, MH 
%A Stumpf, DK 
%A Ku, MSB 
%A Burris, RH 
%A Edwards, GE 
%T Crassulacean Acid Metabolism and Diurnal Variations of Internal CO<sub>2</sub> and O<sub>2</sub> Concentrations in <I>Sedum praealtum</I> DC
%J Functional Plant Biology
%D 1979/08/01
%V 6
%N 4
%P 557-567
%K 
%R https://doi.org/10.1071/PP9790557
%U https://www.publish.csiro.au/paper/PP9790557 
%X 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. 
