On the Controls of Leaf-Water Oxygen Isotope Ratios in the Atmospheric Crassulacean Acid Metabolism Epiphyte Tillandsia usneoides

Previous theoretical work showed that leaf-water isotope ratio (delta O-18(L)) of Crassulacean acid metabolism epiphytes was controlled by the delta O-18 of atmospheric water vapor (delta O-18(a)), and observed delta O-18(L) could be explained by both a non-steady-state model and a maximum enrichment steady-state model (delta O-18(L-M)), the latter requiring only delta O-18(a) and relative humidity (h) as inputs. delta O-18(L), therefore, should contain an extractable record of delta O-18(a). Previous empirical work supported this hypothesis but raised many questions. How does changing delta O-18(a) and h affect delta O-18(L)? Do hygroscopic trichomes affect observed delta O-18(L)? Are observations of changes in water content required for the prediction of delta O-18(L)? Does the leaf need to be at full isotopic steady state for observed delta O-18(L) to equal delta O-18(L-M)? These questions were examined with a climate-controlled experimental system capable of holding delta O-18(a) constant for several weeks. Water adsorbed to trichomes required a correction ranging from 0.5% to 1%. delta O-18(L) could be predicted using constant values of water content and even total conductance. Tissue rehydration caused a transitory change in delta O-18(L), but the consequent increase in total conductance led to a tighter coupling with delta O-18(a). The non-steady-state leaf water models explained observed delta O-18(L) (y = 0.93*x - 0.07; r(2) = 0.98) over a wide range of delta O-18(a) and h. Predictions of delta O-18(L-M) agreed with observations of delta O-18(L) (y = 0.87*x - 0.99; r(2) = 0.92), and when h > 0.9, the leaf did not need to be at isotopic steady state for the delta O-18(L-M) model to predict delta O-18(L) in the Crassulacean acid metabolism epiphyte Tillandsia usneoides.