THE BOUNDARY-LAYER AND THE APPARENT RESPONSES OF STOMATAL CONDUCTANCETO WIND-SPEED AND TO THE MOLE FRACTIONS OF CO2 AND WATER-VAPOR IN THEAIR

The experiments and simulations reported in this paper show that, for stomata sensitive to both CO2 and water vapour concentrations, respons es of stomatal conductance (g(s)w) to boundary layer thickness have tw o components, one resulting from changes in intercellular CO2 concentr ation (chi(i)c) and another from changes in leaf surface water vapour saturation deficit (D(s)w). The experiments and simulations also show that the boundary layer conductance (g(b)w) can significantly alter th e apparent response of g(s)w, to ambient air CO2 mole fraction (chi(a) c) and water vapour mole fraction (chi(a)w). Because of the feedback l oop involved the responses of g(s)w for chi(a)c and chi(a)w each inclu de responses to both chi(i)c and D(s)w. The boundary layer alters the state of the variables sensed by the guard cells-i.e. chi(i)c and D(s) w-and so it is a source of feedback. Thus, when scaling up from respon ses of stomata to the response of g(s)w for a whole leaf, the effect o f the boundary layer must be considered. The results indicate that, fo r given responses of g(s)w to chi(i)c and D(s)w, the apparent response s of g(s)w to D(a)w and chi(a)c depend on the size of the leaf and win d speed, showing that this effect of the boundary layer should be cons idered when comparing data measured under different conditions, or wit h different methods.