Seasonal Shifts of Photosynthesis in Portulacaria afra (L.) Jacq.

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Abstract
Portulacaria afra (L.) Jacq., a perennial facultative Crassulacean acid metabolism (CAM) species, was studied under natural photoperiods and temperatures in San Diego, California [USA]. The plants were irrigated every 4th day throughout the study period. Measurements of 14CO2 uptake, stomatal resistance and titratable acidity were made periodically July 1981-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 C 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 100-200 microequivalents/g fresh wt 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 C uptake to 2 indicated a significant contribution of nocturnal CO2 uptake to the overall C 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.