A three-dimensional model of CO2 transport in airspaces and mesophyll cells of a silver birch leaf

A realistic numerical three-dimensional (3D) model was constructed to study CO2 transport inside a birch leaf. The model included chloroplasts, palisade and spongy mesophyll cells, airspaces, stomatal opening and the leaf boundary layer. Diffusion equations for CO2 were solved for liquid(mesophyll) and gaseous(air) phases. Simulations were made in typical ambient field conditions varying stomatal opening, photosynthetic capacity and temperature. Doubled ambient CO2 concentration was also considered. Changes in variables caused non-linear effects in the total flux, especially when compared with the results of double CO2 concentration. The reduction in stomatal opening size had a smaller effect on the total flux in doubled concentrations than ambient CO2. The reduced photosynthetic capacity had a similar effect on the flux in both cases. The palisade and spongy mesophyll cells had unequal roles mainly due to the light absorption profile. Results from the 3D simulation were also compared to the classical one-dimensional resistance approach. Liquid and gas phase resistances were estimated and found strongly variable according to changes in temperature and degree of stomatal opening. For the birch leaves modelled, intercellular airspace resistance was small (2% of the total resistance in saturating irradiance conditions at 25 degreesC at stomatal opening diameter of 4 mum) whereas the liquid phase resistance was significant (23% for mesophyll and chloroplasts in the same 'base case'). The absorption of CO2 into water at cell surfaces caused additional (strongly temperature dependent) resistance which accounted for 36% of the total resistance in the base case.