The boundary layer and the apparent responses of stomatal conductance to wind speed and to the mole fractions of CO₂ and water vapour in the air

The experiments and simulations reported in this paper show that, for stomata sensitive to both CO 2 and water vapour concentrations, responses of stomatal conductance ( g w s ) to boundary layer thickness have two components, one resulting from changes in intercellular CO 2 concentration (χ c i ) and another from changes in leaf surface water vapour saturation deficit ( D w s ). The experiments and simulations also show that the boundary layer conductance ( g w b ) can significantly alter the apparent response of g w s to ambient air CO 2 mole fraction (χ c a ) and water vapour mole fraction (χ w a ). Because of the feedback loop involved the responses of g w s for χ c a and χ w a each include responses to both χ c i and D w s . The boundary layer alters the state of the variables sensed by the guard cells—i.e. χ c i and D w s —and so it is a source of feedback. Thus, when scaling up from responses of stomata to the response of g w s for a whole leaf, the effect of the boundary layer must be considered. The results indicate that, for given responses of g w s to χ c i and D w s , the apparent responses of g w s to D w a and χ c a depend on the size of the leaf and wind speed, showing that this effect of the boundary layer should be considered when comparing data measured under different conditions, or with different methods.