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

Abstract

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 H2O 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. Show more