Stomatal conductance invalidated not very far from boiling

Limits to the validity for the stomatal conductance (g_s) model of leaf gas exchanges, including CO₂ gain for photosynthesis and water loss by transpiration, become evident when applying the laws of physics to the case of evaporation at the boiling point (BP). Very far from the BP, water vapor is a trace gas whose direct influence is negligible, both on air composition and dynamics; the g_s paradigm is valid under such conditions. At or very near the BP, however, Dalton's law says that water vapor crowds out dry air species and thereby starves photosynthesis for CO₂, and Newton's laws define gas transport as having a non-diffusive nature that is visible in the form of a steam jet. Proximity to the BP thus reduces the water-use efficiency both by impeding the ingress of CO₂ and enhancing the egress of water vapor, versus the classical diffusion-only assumption of plant physiology. A derivation from first principles shows that the fraction of vapor transport that is non-diffusive is determined by water vapor's mass fraction, or specific humidity (q). Thus q is a useful measure of proximity to the BP, and ranges from <1%  (very far from the BP) in temperate environments where g_s is valid, to ~100% very near the BP where g_s is meaningless. Importantly in the context of global warming, with increasing frequency and intensity of heat waves, g_s fails to accurately describe plant functioning for conditions that are not very far from the BP. These include very high temperatures and/or low ambient pressure (sub-stomatal q ~ 5-10%), situations that require amendment of the g_s paradigm to account for the effects of non-diffusive transport.

This work was supported by the projects PID2020-117825GB-C21 (INTEGRATYON3), B-RNM-60-UGR20 (OLEAGEIs) and P18-RT-3629 (ICAERSA) including European Union ERDF funds.