Modelling advection and diffusion of water isotopologues in leaves

Abstract

We described advection and diffusion of water isotopologues in leaves in the non-steady state, applied specifically to amphistomatous leaves. This explains the isotopic enrichment of leaf water from the xylem to the mesophyll, and we showed how it relates to earlier models of leaf water enrichment in non-steady state. The effective length or tortuosity factor of isotopologue movement in leaves is unknown and, therefore, is a fitted parameter in the model. We compared the advection-diffusion model to previously published data sets for Lupinus angustifolius and Eucalyptus globulus. Night-time stomatal conductance was not measured in either data set and is therefore another fitted parameter. The model compared very well with the observations of bulk mesophyll water during the whole diel cycle. It compared well with the enrichment at the evaporative sites during the day but showed some deviations at night for E. globulus. It became clear from our analysis that night-time stomatal conductance should be measured in the future and that the temperature dependence of the tracer diffusivities should be accounted for. However, varying mesophyll water volume did not seem critical for obtaining a good prediction of leaf water enrichment, at least in our data sets. In addition, observations of single diurnal cycles do not seem to constrain the effective length that relates to the tortuosity of the water path in the mesophyll. Finally, we showed when simpler models of leaf water enrichment were suitable for applications of leaf water isotopes once weighted with the appropriate gas exchange flux. We showed that taking an unsuitable leaf water enrichment model could lead to large biases when cumulated over only 1 day.

HESS Opinions "A perspective on isotope versus non-isotope approaches to determine the contribution of transpiration to total evaporation"

“…Several studies (Yepez et al, 2005;Lai et al, 2006;Lee et al, 2007;Cuntz et al, 2007;Ogée et al, 2007) have shown that both fluxes (from non-fractionating vegetation and fractionating soils and open waters) show a strong seasonal and diurnal variability. For instance, in a dry season, vegetation with deep roots has access to water from deeper soil layers or groundwater (Moreira et al, 1997;Wang and Yakir, 2000;Lee et al, 2007).…”

The patterns and implications of diurnal variations in the d-excess of plant water, shallow soil water and air moisture

“…Leaf water isotope enrichment is conventionally described by the steady-state Craig-Gordon equation (Craig and Gordon, 1965), and non-steady-state forms have also been proposed to account for the less enriched leaf water condition predicted by the steady-state model (e.g., Cuntz et al, 2007;Farquhar et al, 2007). To test whether we could use the current understanding of leaf water enrichment to reproduce the observed d-excess variations in leaf water, we used the steady-state Craig-Gordon model to estimate leaf water δ 18 O and δD values, and then calculated d-excess values using dexcess = δD-8.0 × δ 18 O (Dansgaard, 1964).…”

Progress and challenges in using stable isotopes to trace plant carbon and water relations across scales

“…Leaf water enrichment The enrichment of the leaf water has been modelled with approaches of increasing complexity (e.g. Cuntz et al, 2007). Steady-state isotopic enrichment of oxygen or hydrogen over source water at the site of evaporation in the leaf ( e ) can be calculated by the Craig & Gordon model (Craig and Gordon, 1965;Dongmann et al, 1974) by Eq.…”

“…(9) is a simplification of the advection-diffusion description of leaf water enrichment ( LnAD ), as given by Cuntz et al (2007) and Ogée et al (2007) in Eq. (10).…”