Accounting for mesophyll conductance substantially improves <sup>13</sup> C‐based estimates of intrinsic water‐use efficiency
Wei Ting, Guillaume Tcherkez, Xuming Wang, Rudi Schäufele, H. Schnyder, Yusheng Yang, Xiao Ying Gong
Abstract
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.