Roden, J.S., and J.R. Ehleringer. 1999. Leaf water dD and d18O observations confirm robustness of Craig-Gordon model under wide ranging environmental conditions. Plant Physiology 120: 1165-1174.
The Craig-Gordon evaporative enrichment model of the hydrogen
( dD) and oxygen ( d 18 O) isotopes of water was tested in a controlled-environment
gas exchange cuvette over a wide range (400 dD and 40 d 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 dD ord 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 dD and d 18 O values of water at
the site of evaporation. The isotopic composition of atmospheric
vapor was shown to have profound effects on the dD and d 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 dD
and d 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.