Dependence of CO2 gas exchange and acid metabolism of the alpine CAM plant Sempervivum montanum on temperature and light.

The influence of light intensity and temperature on the diurnal course and magnitude of CO2 gas exchange and on acid metabolism was studied in the laboratory with rooted rosettes of Sempervivum montanum collected at 2,200 m above sea level in the Central Alps. Under a temperature regime having a cool dark period and warm light period, S. montanum exhibited the time course of CO2 gas exchange typical of a CAM plant; the response was very distinct even when the plants were well-watered. At day temperatures of less than 10° C and at night temperatures greater than 35° C, S. montanum behaved like a C3 plant. Characteristic for S. montanum are a broad temperature optimum and a wide range of temperatures in which CO2 uptake in light is possible (-2° to 45° C). Dark fixation of CO2 is evident between-2° and 35° C, an apparent uptake of external CO2, on the other hand, only as high as 20° C. Light saturation of CO2 uptake is reached at 60-80 W m-2 while the rate of deacidification is nearly maximal at 40 W m-2. These results show that, due to their specific metabolism, CAM plants can be favored not only in xeric habitats, but also in heat stressed mountain habitats where the daily variation in temperature may be extreme.