Net CO2 output by CAM plants in the light: the role of leaf conductance

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Abstract
The reasons for CO2 release by some CAM plants during the light period were investigated by comparative gas‐exchange studies with three leaf‐succulent species, Kalanchoë tubiflora Hamet, Sedum morganianum E. Walth. and Sempervivum tectorum L. Sempervivum showed net CO2 output during the day at constant day‐night temperatures of both 15 and 25°C, whereas Kalanchoë and Sedum showed no CO2 release at this time at either temperature. Net nocturnal CO2 uptake was markedly reduced at 25°C compared with 15°C in Kalanchoë and Sempervivum, while CO2 was released at night at higher temperature by Sedum if leaf/air vapour pressure difference was also increased.Values for intercellular partial pressure of CO2 (piCO2) were calculated from leaf conductance and were characteristically lowest in the dark and highest in the light period. Maximum values of piCO2 ranged from 264 Pa in Sedum to 87 Pa in Kalanchoë, but were somewhat lower than those reported from direct measurements of the gas phase in other CAM plants. Leaf conductance declined with increasing piCO2 in all three species. Minimum values of approximately 0.2 mmol m−2 s−1 were observed at piCO2= 33 Pa in Kalanchoë and Sedum, with no further decrease at higher piCO2, implying that this largely represented the conductance of the cuticle. In Sempervivum, however, leaf conductance declined more gradually, and was as much as 0.8 mmol m−2 s−1 at the highest observed values of piCO2. The cause of net CO2 output in the light by Sempervivum thus appeared to be lower stomatal sensitivity to CO2, resulting in relatively high leaf conductance even at times when piCO2 greatly exceeded that of ambient air.