Talk:Stomatal conductance

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Doubts about the Definition of Stomatal Conductance
I believe there are some confusions in the introductory paragraph.


 * By definition, stomatal conductance, measured in mmol m⁻² s⁻¹, is the measure of the rate of passage of carbon dioxide (CO2) exiting, or water vapor entering through the stomata of a leaf. Stomata are small pores on the top and bottom of a leaf that are responsible for taking in and expelling CO2 and moisture from and to the outside air. The rate of stomatal conductance, or its inverse, stomatal resistance ...

It is correct that the magnitude of stomatal conductance can be expressed (amongst other units) in mmol m⁻² s⁻¹, which appears to be a rate. However, it is not correct to say the stomatal conductance is a rate.

Let's consider water vapour, in the gaseous intercellular spaces within the leaf, diffusing through stomatal pores to the air in the boundary layer surrounding the leaf. The concentration of water vapour inside the leaf is high (the air is usually reckoned to be saturated with water vapour). The concentration of water vapour in the boundary layer of the leaf is generally a lot lower. This concentration difference creates the transport of water vapour through the stomata by the process of gaseous diffusion. The equation relating the flux density of water vapour through the stomata to the conductance (to gaseous diffusion of water vapour)of the stomata, and the concentration difference (of water vapour) is simply:


 * Flux density of water vapour = Conductance * Concentration Difference

The units in this equation must be consistent. If the flux density is expressed in mol units as mol m⁻² s⁻¹, and if the Concentrations are expressed in the dimensionless units of (mol of water vapour) per (mol of air) - a choice nowadays favoured by plant scientists - then the units of Conductance will be the same as those of the Flux Density, that is: mol m⁻² s⁻¹.

Grammathon 00:23, 18 September 2011 (UTC)

I quite agree with Grammathon, his arguments are correct. I can add that the "the measure of the rate of passage of carbon dioxide (CO2) exiting [...] through the stomata of a leaf" is called the net CO2 assimilation rate and the "the measure of the rate of passage of water vaporous entering [should be leaving] through the stomata of a leaf" is called the transpiration rate. The definition of stomatal conductance could therefore be changed to : By definition, stomatal conductance, measured in mmol m⁻² s⁻¹, conditions the molar flux of CO2 into or water vapour out of the leaf, for a given concentration difference for CO2 or water vapour between the atmosphere and the sub-stomatal cavity. The so conditioned molar fluxes are for CO2 the net CO2 assimilation rate and for water vapour the transpiration rate. --Olbran (talk) 21:26, 15 January 2021 (UTC)