Can hydraulic design explain patterns of leaf water isotopic enrichment in C3 plants?

H218O enrichment develops when leaves transpire, but an accurate generalized mechanistic model has proven elusive. We hypothesized that leaf hydraulic architecture may affect the degree to which gradients in H218O develop within leaves, influencing bulk leaf stable oxygen isotope enrichment (ΔL) and the degree to which the Péclet effect is relevant in leaves. Leaf hydraulic design predicted the relevance of a Péclet effect to ΔL in 19 of the 21 species tested. Leaves with well‐developed hydraulic connections between the vascular tissue and the epidermal cells through bundle sheath extensions and clear distinctions between palisade and spongy mesophyll layers (while the mesophyll is hydraulically disconnected) may have velocities of the transpiration stream such that gradients in H218O develop and are expressed in the mesophyll. In contrast, in leaves where the vascular tissue is hydraulically disconnected from the epidermal layers, or where all mesophyll cells are well connected to the transpiration stream, velocities within the liquid transport pathways may be low enough that gradients in H218O are very small. Prior knowledge of leaf hydraulic design allows informed selection of the appropriate ΔL modelling framework. We show that leaf hydraulic design contributes to the development of gradients in oxygen isotope composition within leaf water, and that a priori knowledge of leaf hydraulic design can guide selection of appropriate leaf water isotope models.