Induction of Acid Metabolism in Portulacaria afra

Portulacaria afra, a succulent plant, shifts from a predominantly C(3) mode of gas exchange to a typical Crassulacean acid metabolism type CO(2) uptake in response to water or NaCl stress. Control plants in the absence of water stress assimilated CO(2) during the light (about 7-8 mg CO(2) 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 CO(2) 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.