Oxygen isotope composition of CAM and C3Clusia species: Non-steady-state dynamics control leaf water 18O enrichment in succulent leaves

Leaf gas exchange and leaf water 18O enrichment (?18OL) were measured in three Clusia species under field conditions during dry and wet seasons and in Miconia argentea during the dry season in the Republic of Panama. During the dry season, all three Clusia species used crassulacean acid metabolism (CAM); during the wet season Clusia pratensis operated in the C3 mode, while Clusia uvitana and Clusia rosea used CAM. Large departures from isotopic steady state were observed in daytime ?18OL of the Clusia species, especially during the dry season. In contrast, daytime ?18OL was near isotopic steady state in the C3 tree M. argentea. Across the full data set, non-steady-state predictions explained 49% of variation in observed ?18OL, whereas steady-state predictions explained only 14%. During the wet season, when ?18O L could be compared with Clusia individuals operating in both C 3 and CAM modes, steady-state and non-steady-state models gave contrasting predictions with respect to interspecific variation in daytime ?18OL. The observed ?18O L pattern matched that predicted for the non-steady state. The results provided a clear example of how non-steady-state control of leaf water 18O dynamics can shift the slope of the relationship between transpiration rate and daytime ?18OL from negative to positive. � 2008 The Authors.