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

Summary: The two-pool and Péclet effect models represent two theories describing mechanistic controls underlying leaf water oxygen isotope composition at the whole-leaf level (δ¹⁸O_L). To test these models, we used a laser spectrometer coupled to a gas-exchange cuvette to make online measurements of δ¹⁸O of transpiration (δ¹⁸O_trans) and transpiration rate (E) in 61 cotton (Gossypium hirsutum) leaves. δ¹⁸O_trans measurements permitted direct calculation of δ¹⁸O at the sites of evaporation (δ¹⁸O_e) which, combined with values of δ¹⁸O_L from the same leaves, allowed unbiased estimation of the proportional deviation of enrichment of δ¹⁸O_L from that of δ¹⁸O_e (f) under both steady-state (SS) and non-steady-state (NSS) conditions. Among all leaves measured, f expressed relative to both δ¹⁸O of transpired water (f_trans) and source water (f_sw) 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 f_trans nor f_sw 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.