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

Abstract

The Craig-Gordon evaporative enrichment model of the hydrogen (deltaD) and oxygen (delta(18)O) isotopes of water was tested in a controlled-environment gas exchange cuvette over a wide range (400 per thousand deltaD and 40 per thousand delta(18)O) of leaf waters. (Throughout this paper we use the term "leaf water" to describe the site of evaporation, which should not be confused with "bulk leaf water" a term used exclusively for uncorrected measurements obtained from whole leaf water extractions.) Regardless of how the isotopic composition of leaf water was achieved (i.e. by changes in source water, atmospheric vapor deltaD or delta(18)O, vapor pressure 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 deltaD and delta(18)O values of water at the site of evaporation. The isotopic composition of atmospheric vapor was shown to have profound effects on the deltaD and delta(18)O of leaf water and its influence was dependent on vapor pressure gradients. These results have implications for conditions in which the isotopic composition of atmospheric vapor is not in equilibrium with source water, such as experimental systems that grow plants under isotopically 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 water model under relatively stable environmental conditions. After a major perturbation in the deltaD and delta(18)O of atmospheric vapor, the leaf reached steady state in approximately 2 h, depending on vapor pressure gradients. 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. Therefore, the Craig-Gordon model is a useful tool for understanding the environmental factors that influence the hydrogen and oxygen isotopic composition of leaf water as well as the organic matter derived from leaf water.