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

Carbon isotope discrimination (Δ) has been used widely to infer intrinsic water-use efficiency (iWUE) of C3 plants, a key parameter linking carbon and water fluxes. Despite the essential role of mesophyll conductance (gm ) 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 gm (iWUEmes ) and exploits the gm -stomatal conductance (gsc ) relationship across drought-stress levels and plant functional groups (deciduous or semideciduous woody, evergreen woody and herbaceous species) in a global database. iWUEmes was further validated with an independent dataset of online-Δ and CO2 and H2 O gas exchange measurements with seven species. Drought stress reduced gsc and gm by nearly one-half across all plant functional groups, but had no significant effect on the gsc : gm ratio, with a well supported value of 0.79 ± 0.07 (95% CI, n = 198). gm was negatively correlated to iWUE. Incorporating the gsc : gm ratio greatly improved estimates of iWUE, compared with calculations that assumed infinite gm . The inclusion of the gsc : gm ratio, fixed at 0.79 when gm was unknown, proved desirable to eliminate significant errors in estimating iWUE from Δ across various C3 vegetation types.