Garden with Insight v1.0 Help: stomatal conductance

Stomatal conductance is the speed at which water vapor can evaporate from pores called stomata in the plant's leaves. It depends on the difference in the vapor pressure between the spaces inside the leaf (near the stomata) and the vapor pressure in the air surrounding the leaves.

If the speed of conductance is too great, the plant transpires a lot of water and the soil dries out placing the plant in water stress. To avoid this condition, plants try to some extent to control the speed of evaporation by closing the stomata when the sun is bright (which is when evaporation is greatest). However, the speed of photosynthesis depends on being able to release the CO2 produced into the atmosphere, so closing the stomata too much or for too long reduces photosynthate. So plants keep opening and closing their stomata to keep a middle line between the two constraints.