Seasonal response to drought and rewatering in Portulacaria afra (L.) Jacq.

Gas exchange characteristics of droughted and rewatered Portulacaria afra were studied during the seasonal shift from CAM to C3 photosynthesis. 14CO2 uptake, stomatal conductance, and total titratable acidity were determined for both irrigated and 2, 4, and 7.5 month waterstressed plants from summer 1984 to summer 1985. Irrigated P. afra plants were utilizing the CAM pathway throughout the summer and shifted to C3 during the winter and spring. Beginning in September, P. afra plants shifted from CAM to CAM-idling after 2 months of water-stress. When water-stress was initiated later in the fall, exogenous CO2 uptake was still measurable after 4 months of drought. After 7.5 months of stress, exogenous CO2 uptake was absent. The shift from CAM to CAM-idling or C3 in the fall and winter was related to when water stress was initiated and not to the duration of the stress. Gas exchange resumed within 24 h of rewatering regardless of the duration of the drought. In the winter and spring, rewatering resulted in a full resumption of daytime CO2 uptake. Whereas during the summer, rewatering quickly resulted in early morning CO2 uptake, but nocturnal CO2 uptake through the CAM pathway was observed after 7 days. Gas exchange measurements, rewatering characteristics, and transpirational water loss support the hypothesis that the C3 pathway was favored during the winter and spring. The CAM pathway was functional during the summer when potential for water loss was greater. Our investigations indicate that P. afra has a flexible photosynthetic system that can withstand long-term drought and has a rapid response to rewatering.