Genotype differences in ¹³C discrimination between atmosphere and leaf matter match differences in transpiration efficiency at leaf and whole‐plant levels in hybrid Populus deltoides ×nigra

¹³C discrimination between atmosphere and bulk leaf matter (Δ¹³Cₗb) is frequently used as a proxy for transpiration efficiency (TE). Nevertheless, its relevance is challenged due to: (1) potential deviations from the theoretical discrimination model, and (2) complex time integration and upscaling from leaf to whole plant. Six hybrid genotypes of Populus deltoides×nigra genotypes were grown in climate chambers and tested for whole‐plant TE (i.e. accumulated biomass/water transpired). Net CO₂ assimilation rates (A) and stomatal conductance (gₛ) were recorded in parallel to: (1) ¹³C in leaf bulk material (δ¹³Cₗb) and in soluble sugars (δ¹³Cₛₛ) and (2) ¹⁸O in leaf water and bulk leaf material. Genotypic means of δ¹³Cₗb and δ¹³Cₛₛ were tightly correlated. Discrimination between atmosphere and soluble sugars was correlated with daily intrinsic TE at leaf level (daily mean A/gₛ), and with whole‐plant TE. Finally, gₛ was positively correlated to ¹⁸O enrichment of bulk matter or water of leaves at individual level, but not at genotype level. We conclude that Δ¹³Cₗb captures efficiently the genetic variability of whole‐plant TE in poplar. Nevertheless, scaling from leaf level to whole‐plant TE requires to take into account water losses and respiration independent of photosynthesis, which remain poorly documented.