Bellasio, Chandra2025-02-032025-02-030166-8595https://hdl.handle.net/1885/733734628Quantifying water use efficiency, and the impact of stomata on CO2 uptake are pivotal in physiology and efforts to improve crop yields. Although tempting, relying on regression slopes from assimilation-stomatal conductance plots to estimate water use efficiency or stomatal control over assimilation is erroneous. Through numerical simulations, I substantiate this assertion. I propose the term ‘instantaneous transpiration efficiency’ for the assimilation-to-transpiration ratio to avoid confusion with ‘intrinsic water use efficiency’ which refers to the assimilation-to-stomatal conductance ratio, and recommend to compute both metrics for each gas exchange data point.Open Access funding provided thanks to the CRUE-CSIC agreement with Springer Nature. I gratefully acknowledge funding by European Union’s Horizon 2020 research and innovation programme through a MSCA individual fellowship (Grant Agreement ID No: 702755), and by a Science Foundation Ireland Individual Pathway Fellowship (21/PATH-S/9322).application/pdfen-AU© 2023 The authorshttp://creativecommons.org/licenses/by/4.0/202310.1007/s11120-023-01054-62024-01-07Creative Commons Attribution licence