Observations of hydrogen and oxygen isotopes in leaf water confirm the Craig-Gordon model under wide-ranging environmental conditions

The Craig-Cordon evaporative enrichment model of the hydrogen (delta D) and oxygen (delta(18)O) isotopes of water was tested in a controlled-environme nt gas exchange cuvette over a wide range (400 parts per thousand delta D a nd 40 parts per thousand delta(18)O) of leaf waters. (Throughout this payer we use the term "leaf water" to describe the site of evaporation, which sh ould not be confused with "bulk leaf water" a term used exclusively for unc orrected measurements obtained from whole leaf water extractions.) Regardle ss of how the isotopic composition of leaf water was achieved (i.e. by chan ges in source water, atmospheric vapor delta D or delta(18)O, vapor pressur e gradients, or combinations of all three), a modified version of the Craig -Gordon model was shown to be sound in its ability to predict the delta D a nd delta(18)O values of water at the site of evaporation. The isotopic comp osition of atmospheric vapor was shown to have profound effects on the delt a D and delta(18)O of leaf water and its influence was dependent on vapor p ressure gradients. These results have implications for conditions in which the isotopic composition of atmospheric vapor is not in equilibrium with so urce water, such as experimental systems that grow plants under isotopicall y enriched water regimes. The assumptions of steady state were also tested and found not to be a major limitation for the utilization of the leaf wate r model under relatively stable environmental conditions. After a major per turbation in the delta D and delta(18)O of atmospheric vapor, the leaf reac hed steady state in approximately 2 h, depending on vapor pressure gradient s. Following a step change in source water, the leaf achieved steady state in 24 h, with the vast majority of changes occurring in the first 3 h. Ther efore, the Craig-Cordon model is a useful tool for understanding the enviro nmental factors that influence the hydrogen and oxygen isotopic composition of leaf water as well as the organic matter derived from leaf water.