Zamioculcas zamiifolia (Araceae), a terrestrial East African aroid, with two defining attributes of crassulacean acid metabolism (CAM) (net CO2 uptake in the dark and diel fluctuations of titratable acidity) is the only CAM plant described within the Araceae, a mainly tropical taxon that contains the second largest number of epiphytes of any vascular plant family. Within the Alismatales, the order to which the Araceae belong, Z. zamiifolia is the only documented nonaquatic CAM species. Zamioculcas zamiifolia has weak CAM that is upregulated in response to water stress. In well-watered plants, day-night fluctuations in titratable acidity were 2.5 lmol H_(g fresh mass)1, and net CO2 uptake in the dark contributed less than 1% to daily carbon gain. Following 10 d of water stress, net CO2 uptake in the light fell 94% and net CO2 uptake in the dark increased 7.5-fold, such that its contribution increased to 19% of daily carbon gain. Following rewatering, dark CO2 uptake returned to within 5% of prestressed levels. We postulate that CAM assists survival of Z. zamiifolia by reducing water loss and maintaining carbon gain during seasonal droughts characteristic of its natural habitat.
