Photosynthetic pathways in a midwestern rock outcrop succulent, Sedum nuttallianum Raf. (Crassulaceae).

Shoots of Sedum nuttallianum exhibited CAM(*) acid fluctuations in the field. These nocturnal acid accumulations persisted in the laboratory under well-watered and water-stressed conditions. Simultaneous measurements of transpiration, however, indicated daytime stomatal opening and nocturnal stomatal closure. Measurements of CO2 and H2O vapor exchange continuously for six days after watering substantiated these results in part: the majority of CO2 uptake occurred during the day early in the experiment; however, after several days without water, nighttime CO2 uptake was stimulated and eventually was greater than the drastically reduced daytime CO2 uptake. This nighttime uptake was never quite sufficient to account for all estimated increases in tissue acidity. Thus, a combination of CAM and CAM-cycling occurred early in the desiccation experiment. Evidence for CAM and a form of CAM-idling was found later in the experiment. Though nighttime CO2 uptake occurred and persisted after only one day without water, rates were too low to alter the tissue δ(13)C/(12)C value from a C3-like number (-30‰). Thus, although CAM and CAM-idling may have survival value during extended droughts, shoots of S. nuttallianum apparently utilize the C3 pathway to obtain most of their carbon.