Accounting for mesophyll conductance substantially improves C-based estimates of intrinsic water-use efficiency.

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