Effects of abscisic acid on CAM in Portulacaria afra.

Water stress induces Crassulacean acid metabolism (CAM)(†) in Portulacaria afra as manifested by day stomatal closure, organic acid fluctuation, and night CO2 uptake. We now have evidence that abscisic acid treatment of leaves causes partial stomatal closure that is accompanied by the induction of CAM in a manner similar to water stress. There appears to be an inverse relationship between exogenous CO2 uptake and decarboxylation of organic acids in that organic acids remain high during the day providing stomata are open. When stomata close, there is consumption of organic acids by decarboxylation. The hypothesis is that stomatal opening controls CAM in this species.