H₂¹⁸O labelled vapour reveals evidence of radial Peclet effects in some leaves, but not all.

Previous studies demonstrated the importance of pathways of water movement, hydraulic conductance, and leaf hydraulic design on leaf water isotopic enrichment. The current study addresses the relevance of Péclet effects to leaf water isotopic enrichment by assessing spatial variation and comparing bulk leaf water enrichment to Craig-Gordon modelled values. Vapour isotope labelling was also utilised in an experimental first to quantify the carry-forward of mesophyll water isotope compositions to more distal parts of corn (Zea mays), oat (Avena sativa), sunflower (Helianthus annuus) and foxglove (Digitalis purpurea) leaves through coupled leaf gas exchange and isoflux measurements. In this study, we found that the Péclet effect was relevant to patterns of leaf water isotopic enrichment for three out of four species tested. The change in H₂¹⁸O for both monocots highlighted a vascular Péclet effect, while the change in foxglove showed a mesophyll Péclet effect, both of which were found to be relevant for the isotopic enrichment of transpired vapour and leaf water. In contrast, sunflower did not present any evidence of either a vascular or a mesophyll Péclet effect, suggesting a difference in leaf hydraulic design. These findings help us to understand how differences in leaf hydraulic design and anatomy influence the development of gradients in leaf water isotopes. This improved understanding will assist in the most appropriate choice of theoretical models to accurately predict leaf water isotope composition for applications as varied as predicting past climates from tree ring isotopes or selecting water-use efficient genotypes in crop plants