No abstract

There is an increasing ecological interest in understanding the gradients in H(2)(18)O enrichment in leaf water (i.e. a Péclet effect), because an appreciation of the significance of the Péclet effect is important for improving our understanding of the mechanistic processes affecting the (18)O composition of leaf water and plant organic material. In data sets where both source water and leaf water (18)O data are available, we can evaluate the potential contribution of a Péclet effect. As an example, we recalculate data published earlier by Roden and Ehleringer (1999, Oecologia 121:467-477) as enrichments in leaf water (Delta(L)) and cellulose (Delta(cell)) above source water. Based on these recalculations, we present support for the relevance of a Péclet effect in leaves. Further, we demonstrate that the subtle variations in Delta(L) and Delta(cell) caused by a Péclet effect may be masked in experimental systems in which variation in the source water oxygen isotope ratio is considerable.

The Craig-Gordon evaporative enrichment model of the hydrogen (␦D) and oxygen (␦ 18 O) isotopes of water was tested in a controlled-environment gas exchange cuvette over a wide range (400‰ ␦D and 40‰ ␦ 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 ␦D or ␦ 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 ␦D and ␦18 O values of water at the site of evaporation. The isotopic composition of atmospheric vapor was shown to have profound effects on the ␦D and ␦ 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 ␦D and ␦ 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.The stable isotopes of hydrogen (␦D) and oxygen (␦ 18 O) in meteoric water vary in both space and time. When incorporated into the organic matter of plant tissues, analyses of these isotopes can provide valuable environmental information regarding patterns of plant water use (Dawson, 1993) and climatic variation (Schiegl, 1974;Gray and Thompson, 1976;Epstein and Krishnamurthy, 1990). One of the first steps in understanding how the stable isotopes of water are incorporated into plant organic matter is to model how leaf water is altered as a result of transpiration . (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.)A freely evaporating surface tends to enrich leaf water in heavy isotopes, since the lighter isotopes of hydrogen and oxygen in water vapor escape from liquid surfaces more readily that the isotopically heavy water molecules. Craig and Gordon (1965) were the first to model this isotopicfractionation effect for evaporation from ...