Measurements of transpiration isotopologues and leaf water to assess enrichment models in cotton.

The two-pool and Péclet effect models represent two theories describing mechanistic controls underlying leaf water oxygen isotope composition at the whole-leaf level (δ(18) OL ). To test these models, we used a laser spectrometer coupled to a gas-exchange cuvette to make online measurements of δ(18) O of transpiration (δ(18) Otrans ) and transpiration rate (E) in 61 cotton (Gossypium hirsutum) leaves. δ(18) Otrans measurements permitted direct calculation of δ(18) O at the sites of evaporation (δ(18) Oe ) which, combined with values of δ(18) OL from the same leaves, allowed unbiased estimation of the proportional deviation of enrichment of δ(18) OL from that of δ(18) Oe (f) under both steady-state (SS) and non-steady-state (NSS) conditions. Among all leaves measured, f expressed relative to both δ(18) O of transpired water (ftrans ) and source water (fsw ) remained relatively constant with a mean ± SD of 0.11 ± 0.05 and 0.13 ± 0.05, respectively, regardless of variation in E spanning 0.8-9.1 mmol m(-2)  s(-1) . Neither ftrans nor fsw exhibited a significant difference between the SS and NSS leaves at the P < 0.05 level. Our results suggest that the simpler two-pool model is adequate for predicting cotton leaf water enrichment at the whole-leaf level. We discuss the implications of adopting a two-pool concept for isotopic applications in ecological studies.