The slope of assimilation rate against stomatal conductance should not be used as a measure of water use efficiency or stomatal control over assimilation

Quantifying 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.