The enigma of effective path length for ¹⁸O enrichment in leaf water of conifers

Abstract

The Péclet correction is often used to predict leaf evaporative enrichment and requires an estimate of effective path length (L). Studies to estimate L in conifer needles have produced unexpected patterns based on Péclet theory and leaf anatomy. We exposed seedlings of six conifer species to different vapour pressure deficits (VPD) in controlled climate chambers to produce steady-state leaf water enrichment (in (18) O). We measured leaf gas exchange, stable oxygen isotopic composition (δ(18) O) of input and plant waters as well as leaf anatomical characteristics. Variation in bulk needle water δ(18) O was strongly related to VPD. Conifer needles had large amounts of water within the vascular strand that was potentially unenriched (up to 40%). Both standard Craig-Gordon and Péclet models failed to accurately predict conifer leaf water δ(18) O without taking into consideration the unenriched water in the vascular strand and variable L. Although L was linearly related to mesophyll thickness, large within-species variation prevented the development of generalizations that could allow a broader use of the Péclet effect in predictive models. Our results point to the importance of within needle water pools and isolating mechanisms that need further investigation in order to integrate Péclet corrections with 'two compartment' leaf water concepts.

Leaf vein fraction influences the Péclet effect and 18O enrichment in leaf water

“…In pine needles, attributing an unenriched water fraction to the vascular strand reduced the ∆ b offset from ∆ e so that the remaining mesophyll tissue was closer to the predicted Craig–Gordon composition (Roden et al . ). However, in the case of Larix decidua (Fig.…”

“…In pine needles, the bottleneck was suggested to be the suberized endodermis surrounding the vascular tissue (Roden et al . ). Secondly, there is a large transfer surface area outside the bundle sheath associated with many parallel pathways (Sack ; Sack & Holbrook ), reducing the velocity and therefore the Péclet effect.…”

“…The major vein water is usually taken to be unenriched so that the leaf can be similarly described in a two‐pool manner, as used by Roden et al . (). In this case, Eqn 2 becomes…”

“…This is perhaps why some researchers have removed the midvein (which represents the majority of the xylem water) before extracting and analysing the bulk leaf water, or, as was recently applied to pine needles, quantitatively from anatomical measurements of the vascular volume fraction (Roden et al . ). Even with removal of the midvein, estimates of L from single point measurements range from 3.2 mm ( Eucalyptus incrassata ; Kahmen et al .…”

Stable isotopes in leaf water of terrestrial plants

“…A convenient way to probe observed leaf water isotopic composition for evidence of Péclet effects is to plot the proportional difference between Δ e and Δ L as a function of transpiration rate. Such plots have yielded variable results, with some authors finding a positive relationship, as predicted by the Péclet model (Barbour et al ; Loucos et al ; Ripullone et al ), and others, either no detectable relationship or a negative relationship (Cernusak et al ; Roden & Ehleringer ; Roden et al ; Song et al ; Song et al ). For our combined dataset given in the Supporting Information and shown in Fig.…”