Do pathways of water movement and leaf anatomical dimensions allow development of gradients in H218O between veins and the sites of evaporation within leaves?

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Abstract
The oxygen isotope enrichment of bulk leaf water (ΔL) is often observed to be poorly predicted by the Craig–Gordon‐type models developed for evaporative enrichment from a body of water (Δe). The discrepancy between ΔL and Δe may be explained by gradients in enrichment within the leaf as a result of convection of unenriched water to the sites of evaporation opposing the diffusion of enrichment away from the sites; a Péclet effect. However, this effect is difficult to quantify because the velocities of water movement within the leaf are unknown. This paper attempts to model the complex anatomy of a leaf, and hence such velocities, to assess if the gradients in H218O required for a significant Péclet effect between the vein and the evaporation sites are possible within a leaf. Published dimensions of cells in wheat leaves are used to calculate the cross‐sectional areas perpendicular to the flow velocities of water through assumed pathways. By combining the ratio of actual to ‘slab’ velocities with anatomical lengths, equivalent lengths (L) emerge. In this way, it is concluded that if water moves only through the cell walls, or from cell to cell via either aquaporins or plasmodesmata, and evaporates from mesophyll cells, or the substomatal cells, or from the peristomatal region (a total of 15 combinations of assumptions), then the 15 central estimates of the values of L are between 9 and 200 mm. Each of these central estimates is subject to uncertainty, but overall their magnitude is important and estimates of L are comparable with those made from fitting to isotopic data (8 mm for wheat). It is concluded that significant gradients in enrichment between the vein and the evaporation sites are likely.