Stomatal conductance
A measure of how readily gas passes through a leaf's stomata, reflecting how far those pores are open. It falls under elevated CO2, which is why enrichment lowers water use.
Conductance is the inverse of resistance. High stomatal conductance means gas moves freely between leaf and atmosphere, so CO2 enters readily and water vapor leaves readily. It is measured with a porometer or a gas exchange system, and reported as the moles of gas crossing each square meter of leaf per second.
Chandra and colleagues measured the response directly in cannabis in 2008. Raising CO2 to 750 ppm suppressed stomatal conductance by roughly 42 percent and transpiration by about 29 percent, while water use efficiency rose 111 percent.
For a grower this is not an abstraction. Less water leaving the leaf means slower substrate drybacks, reduced transpiration-driven nutrient delivery, and a lower dehumidification load, all of which shift the setpoints an enriched room should run.