A three-dimensional stomatal CO2 exchange model including gaseous phase and leaf mesophyll separated by irregular interface

A three-dimensional model was constructed for studying the diffusion of CO2 molecules from air into stomatal air spaces and further into leaf mesophyl l cells. The model covers two different phases, mesophyll and air, with app ropriate boundary conditions. Sinks for CO2 molecules were obtained from a well-known biochemical photosynthesis model parametrized for Scots pine. Th e effects of the varying stomatal geometry on the CO2 flux and intercellula r-air-space (IAS) and mesophyll concentrations were examined. The net CO2 f lux is sensitive to the size of the IAS-mesophyll interface when the mesoph yll transport coefficient is low, equalling the diffusivity of CO2 in water . The simulations have revealed an optimum between the volume of the assimi lating mesophyll and the air-mesophyll interface. The solubility of CO2 mol ecules into water in the cell surfaces, which depends on temperature and pH , had a clear effect on the flux. Increasing pH from the value indicated by the physical solubility (pH similar to 6) leads to an increase in the temp erature maximum for CO2 flux into higher temperatures and to unrealisticall y high CO2 concentrations in the mesophyll. (C) 1999 Academic Press.