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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.

Abstract. Mesophyll conductance (gm) has been shown to vary between genotypes of a number of species and with growth environments, including nitrogen availability, but understanding of gm variability in legumes is limited. We might expect gm in legumes to respond differently to limited nitrogen availability, due to their ability to fix atmospheric N2. Using online stable carbon isotope discrimination method, we quantified genetic variability in gm under ideal conditions, investigated gm response to N source (N2-fixation or inorganic N) and determined the effects of N source and water availability on the rapid response of gm to photosynthetic photon flux density (PPFD) and radiation wavelength in three genotypes of chickpea (Cicer arietinum). Genotypes varied 2-fold in gm under non-limiting environments. N-fed plants had higher gm than N2-fixing plants in one genotype, while gm in the other two genotypes was unaffected. gm response to PPFD was altered by N source in one of three genotypes, in which the gm response to PPFD was statistically significant in N-fed plants but not in N2-fixing plants. There was no clear effect of moderate water stress on the gm response to PPFD and radiation wavelength. Genotypes of a single legume species differ in the sensitivity of gm to both long- and short-term environmental conditions, precluding utility in crop breeding programmes.

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