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Induction of Acid Metabolism in Portulacaria afra¹

^a Department of Biology, University of California, Riverside, California 92502

Portulacaria afra, a succulent plant, shifts from a predominantly C₃ mode of gas exchange to a typical Crassulacean acid metabolism type CO₂ uptake in response to water or NaCl stress. Control plants in the absence of water stress assimilated CO₂ during the light (about 7-8 mg CO₂ dm^–2 hr^–1), transpiration (about 1.5 g dm^–2 hr^–1) was predominantly during the day, stomates were open during the day, and there was little diurnal organic acid fluctuation. Stressed plants showed only dark CO₂ uptake and dark water loss, nocturnal stomatal opening, and an increased diurnal fluctuation of titratable acidity. Within 2 weeks after rewatering, stressed plants returned to the control acid fluctuation levels indicating that the response to stress was reversible.