Accounting for mesophyll conductance substantially improves 13 C‐based estimates of intrinsic water‐use efficiency

Summary Carbon isotope discrimination (Δ) has been used widely to infer intrinsic water‐use efficiency (iWUE) of C 3 plants, a key parameter linking carbon and water fluxes. Despite the essential role of mesophyll conductance ( g m ) in photosynthesis and Δ, its effect on Δ‐based predictions of iWUE has generally been neglected. Here, we derive a mathematical expression of iWUE as a function of Δ that includes g m (iWUE mes ) and exploits the g m ‐stomatal conductance ( g sc ) relationship across drought‐stress levels and plant functional groups (deciduous or semideciduous woody, evergreen woody and herbaceous species) in a global database. iWUE mes was further validated with an independent dataset of online‐Δ and CO 2 and H 2 O gas exchange measurements with seven species. Drought stress reduced g sc and g m by nearly one‐half across all plant functional groups, but had no significant effect on the g sc  :  g m ratio, with a well supported value of 0.79 ± 0.07 (95% CI, n  = 198). g m was negatively correlated to iWUE. Incorporating the g sc  :  g m ratio greatly improved estimates of iWUE, compared with calculations that assumed infinite g m . The inclusion of the g sc  :  g m ratio, fixed at 0.79 when g m was unknown, proved desirable to eliminate significant errors in estimating iWUE from Δ across various C 3 vegetation types.