On the Controls of Leaf-Water Oxygen Isotope Ratios in the Atmospheric Crassulacean Acid Metabolism Epiphyte Tillandsia usneoides
2011
Helliker, Brent R.
Previous theoretical work showed that leaf-water isotope ratio (δ¹⁸OL) of Crassulacean acid metabolism epiphytes was controlled by the δ¹⁸O of atmospheric water vapor (δ¹⁸Oa), and observed δ¹⁸OL could be explained by both a non-steady-state model and a "maximum enrichment" steady-state model (δ¹⁸OL₋M), the latter requiring only δ¹⁸Oa and relative humidity (h) as inputs. δ¹⁸OL, therefore, should contain an extractable record of δ¹⁸Oa. Previous empirical work supported this hypothesis but raised many questions. How does changing δ¹⁸Oa and h affect δ¹⁸OL? Do hygroscopic trichomes affect observed δ¹⁸OL? Are observations of changes in water content required for the prediction of δ¹⁸OL? Does the leaf need to be at full isotopic steady state for observed δ¹⁸OL to equal δ¹⁸OL₋M? These questions were examined with a climate-controlled experimental system capable of holding δ¹⁸Oa constant for several weeks. Water adsorbed to trichomes required a correction ranging from 0.5[per thousand] to 1[per thousand]. δ¹⁸OL could be predicted using constant values of water content and even total conductance. Tissue rehydration caused a transitory change in δ¹⁸OL, but the consequent increase in total conductance led to a tighter coupling with δ¹⁸Oa. The non-steady-state leaf water models explained observed δ¹⁸OL (y = 0.93*x - 0.07; r² = 0.98) over a wide range of δ¹⁸Oa and h. Predictions of δ¹⁸OL₋M agreed with observations of δ¹⁸OL (y = 0.87*x - 0.99; r² = 0.92), and when h > 0.9, the leaf did not need to be at isotopic steady state for the δ¹⁸OL₋M model to predict δ¹⁸OL in the Crassulacean acid metabolism epiphyte Tillandsia usneoides.
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