Comparison of Modeled and Observed Environmental Influences on the Stable Oxygen and Hydrogen Isotope Composition of Leaf Water in Phaseolus vulgaris L.

Abstract

In this paper we describe how a model of stable isotope fractionation processes, originally developed by H. Craig and L.I. Gordon ([1965] in E Tongiorgi, ed, Proceedings of a Conference on Stable Isotopes in Oceanographic Studies and Paleotemperature, Spoleto, Italy, pp 9-130) for evaporation of water from the ocean, can be applied to leaf transpiration. The original model was modified to account for turbulent conditions in the leaf boundary layer. Experiments were conducted to test the factors influencing the stable isotopic composition of leaf water under controlled environment conditions. At steady state, the observed leaf water isotopic composition was enriched above that of stem water with the extent of the enrichment dependent on the leaf-air vapor pressure difference (VPD) and the isotopic composition of atmospheric water vapor (AWV). The higher the VPD, the larger was the observed heavy isotope content of leaf water. At a constant VPD, leaf water was relatively depleted in heavy isotopes when exposed to AWV with a low heavy isotope composition, and leaf water was relatively enriched in heavy isotopes when exposed to AWV with a large heavy isotope composition. However, the observed heavy isotope composition of leaf water was always less than that predicted by the model. The extent of the discrepancy between the modeled and observed leaf water isotopic composition was a strong linear function of the leaf transpiration rate.The stable isotopic composition of plant leafwater is altered during transpiration. Water vapor molecules containing the lighter isotopes of oxygen and hydrogen escape from the leaf more readily than do heavy isotope molecules, so that during transpiration, leaf water becomes enriched in heavy isotope molecules (19,24,28 photosynthetic gas exchange and plant water-use efficiency (10,11,20,24,28).Previous attempts to use the Craig and Gordon model in studies of leaf water isotopic enrichment have been complicated by two factors. First, there has been uncertainty about the value to use for the kinetic fractionation factor. Values for the relative rates of diffusion of water vapor molecules containing light and heavy isotopes of oxygen and hydrogen have been measured (16). These measured values for the kinetic fractionation factor are appropriate for molecular diffusion only, however, and need to be modified for turbulent conditions in a boundary layer. In previous studies there has been uncertain and inconsistent modification of the kinetic fractionation factor to account for turbulence in the water vapor diffusion pathway (3,7,11,14,20,22,23,25). Second, a major assumption in the derivation of the evaporative enrichment model is that isotopic steady state is reached. When leaf water is at isotopic steady state, the isotopic composition of transpiration water is the same as the source or stem water isotopic composition (24). The assumption of isotopic steady state has not been verified in most studies attempting to test the ability of the Craig and Gordon model to predict leaf wat...

Needle cell elongation and maturation timing derived from pine needle cellulose δ18O

“…Barbour et al (2004) suggest that the importance of a Péclet effect may be assessed by plotting the reciprocal of the fractional difference between measured and modelled leaf water against the transpiration rate. The Flanagan et al (1991) model estimates of these parameters yielded a positive slope with a significant correlation (r = 0.90, P < 0.01, N = 6), indicating that a Péclet effect must be considered when interpreting the leaf water d 18 O, and therefore the leaf cellulose d 18 O, at this site. Without the inclusion of a stomatal conductance estimate, as calculated from estimates of tree water deficit (Zweifel et al 2005), the relationship had a negative slope and was not significant (r = 0.63).…”

“…1974;Flanagan, Comstock & Ehleringer 1991;Farquhar & Lloyd 1993;Aucour, HillaireMarcel & Bonnefille 1996;Roden, Lin & Ehleringer 2000;Gan et al . 2003).…”

“…These assumptions are certainly inaccurate in terms of absolute stomatal conductance, but the estimates provide a stomatal conductance value for any particular day that is correct relative to the estimate for other days (higher or lower), and it is the relative stomatal conductance that is required to assess the influence of a Péclet effect. These estimates of stomatal conductance were used as input to the Flanagan et al (1991) model to estimate the transpiration rate. The mod-elled transpiration rate was then plotted against the fractional difference between measured leaf-water enrichment and the enrichment as predicted by the Craig and Gordon model (Barbour et al 2004).…”

“…The term e eq was calculated from the ) e e equations of Majoube (1971 ). For the term a kb , diffusion through the boundary layer, the value a k 2/3 was used (Flanagan et al 1991). The terms e i and e a were calculated using values for temperature and relative humidity estimated for the tree site from radiosonde data, following the equations of Ball (1987).…”

Dynamics of isotopic exchange of carbon dioxide in a Tennessee deciduous forest

“…A large proportion of CO2 that diffuses into a photosynthesizing leaf diffuses back out without being fixed by photosynthesis, after equilibrating isotopically with leaf water via the chloroplastic enzyme carbonic anhydrase [Williams et al, 1996]. Transpiration is an evaporative and thus fractionating process, and leaf water shows substantial diurnal enrichment in b180 [Flanagan et al, 1991;Wang and Yakir, 1995]. Thus CO2 associated with photosynthesizing leaves is labeled by the oxygen isotopic composition of enriched leaf water.…”