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

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 (14)CO(2) uptake, stomatal resistance, and titratable acidity were made periodically from July 1981 through May 1982. P. afra maintained C(3) photosynthesis during the winter and the spring. Diurnal acid fluctuations were low and maximal (14)CO(2) uptake occurred during the day. The day/night ratio of carbon uptake varied from 5 to 10 and indicated little nocturnal CO(2) uptake. CAM photosynthesis occurred during the summer and a mixture of both C(3) and CAM during the fall. Large acid fluctuations of 100 to 200 microequivalents per gram fresh weight were observed and maximal (14)CO(2) 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 CO(2) uptake to the overall carbon gain of the plant. The seasonal shift from C(3) to CAM was facilitated by increasing daytime temperature and accompanied by reduced daytime CO(2) uptake despite irrigation.