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The experiments and simulations reported in this paper show that, for stomata sensitive to both CO2 and water vapour concentrations, responses of stomatal conductance (gsw) to boundary layer thickness have two components, one resulting from changes in intercellular CO2 concentration (χic) and another from changes in leaf surface water vapour saturation deficit (Dsw). The experiments and simulations also show that the boundary layer conductance (gbw) can significantly alter the apparent response of gsw) to ambient air CO2 mole fraction (χac) and water vapour mole fraction (χ aw). Because of the feedback loop involved the responses of gsw) for χac) and χaw) each include responses to both χic) and Dsw), The boundary layer alters the state of the variables sensed by the guard cells-i.e. χic) and Dsw)-and so it is a source of feedback, Thus, when scaling up from responses of stomata to the response of gsw) for a whole leaf, the effect of the boundary layer must be considered. The results indicate that, for given responses of gsw) to χic)and Dsw), the apparent responses of gsw) to Daw and χac) 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. [ABSTRACT FROM AUTHOR]

The objective of the work reported was to answer the following questions. (1) Do stomata respond to both humidity and temperature? (2) Do these responses interact in such a way that relative humidity at the leaf surface is a more appropriate variable than water vapour saturation deficit at tile leaf surface and yields a simpler description of the compound response? To answer these questions, we measured the response of leaf conductance to humidity under constant leaf temperature, and the response to increasing leaf temperature tinder constant relative humidity and under constant water vapour saturation deficit. We found that, in Hedera helix subsp. canariensis (Willd ) Coutinho, there was a reversible response to humidity under constant temperature, and that there was also a response to temperature under constant relative humidity. The relationship between leaf conductance and relative humidity was different when measured at the same temperature rather than at different temperatures. An inversely proportional response was consistently obtained when stomatal conductance was expressed in relation to water vapour saturation deficit. The interaction between the effects of leaf temperature and water vapour saturation deficit was not compatible with a mechanism of response to humidity and temperature based solely on relative humidity From these data, we conclude that water vapour saturation deficit is a more appropriate variable lot describing stomatal responses to humidity. [ABSTRACT FROM AUTHOR]