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

The two-pool and Peclet effect models represent two theories describing mechanistic controls underlying leaf water oxygen isotope composition at the whole-leaf level (O-18(L)). To test these models, we used a laser spectrometer coupled to a gas-exchange cuvette to make online measurements of O-18 of transpiration (O-18(trans)) and transpiration rate (E) in 61 cotton (Gossypium hirsutum) leaves. O-18(trans) measurements permitted direct calculation of O-18 at the sites of evaporation (O-18(e)) which, combined with values of O-18(L) from the same leaves, allowed unbiased estimation of the proportional deviation of enrichment of O-18(L) from that of O-18(e) (f) under both steady-state (SS) and non-steady-state (NSS) conditions. Among all leaves measured, f expressed relative to both O-18 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 0.8-9.1mmolm(-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.