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Published in: Plant Physiology, vol. 76, no.3, 1984.


Portulacaria afra (L.) Jacq., a perennial facultative Crassulacean acid metabolism (CAM) species, was studied under natural photoperiods and temperatures in San Diego, California. The plants were irrigated every fourth day throughout the study period. Measurements of 14CO2 uptake, stomatal resistance, and titratable acidity were made periodically from July 1981 through May 1982. P. afra maintained C3 photosynthesis during the winter and the spring. Diurnal acid fluctuations were low and maximal 14CO2 uptake occurred during the day. The day/night ratio of carbon uptake varied from 5 to 10 and indicated little nocturnal CO2 uptake. CAM photosynthesis occurred during the summer and a mixture of both C3 and CAM during the fall. Large acid fluctuations of 100 to 200 microequivalents per gram fresh weight were observed and maximal 14CO2 uptake shifted to the late night and early morning hours. Daytime stomatal closure was evident. A reduction in the day/night ratio of carbon uptake to 2 indicated a significant contribution of nocturnal CO2 uptake to the overall carbon gain of the plant. The seasonal shift from C3 to CAM was facilitated by increasing daytime temperature and accompanied by reduced daytime CO2 uptake despite irrigation.