Transport of gases into leaves

Abstract

Abstract. Transport of gases between the intercellular spaces of plant leaves and the surrounding air is analysed in terms of multicomponent collision processes through an isothermal, porous septum. Interaction of diffusing species with each other and with the pore walls is described using a modified Stefan–Maxwell equation and an equation relating the pressure gradient to the sum of the diffusive fluxes, weighted by their appropriate Knudsen diffusivities. Viscous How arising from an excess pressure within the leaf is also considered.Equations are derived which describe the flux densities of water vapour and CO2 through the stomata. The analysis is general and is applicable to trace gases other than CO2. A simple conductance is defined for water vapour to relate the flux and mol fraction difference across the stomata, viz. Nw=−gw, δxw/xa. A simple conductance cannot be defined for CO2 because the flux of water vapour has a significant influence on the CO2 gradient. The equation derived for the intercellular mol fraction of CO2 is in terms of the fluxes of CO2and water vapour and represents a ‘large‐pore’ (d > μm) approximation which requires no information about stomalal geometry. Analogous equations are developed for transfer of gases through the leaf boundary layer. Sample calculations are presented to illustrate the effect of neglecting the interaction of water vapour and CO2 on the calculated intercellular and surface concentrations of CO2. Equations for computing water vapour and CO2 flux densities from leaf chamber measurements are also presented.