ANALYSIS OF STOMATAL CO2 UPTAKE BY A 3-DIMENSIONAL CYLINDRICALLY SYMMETRICAL MODEL

A numerical model is introduced that solves the steady-state diffusion equation for a single stoma and the mesophyll surrounding. This syste m has cylindrical symmetry, and diffusive transport of carbon dioxide in the gas phase is coupled with transfer in mesophyll along with a ph otosynthetic sink rate and respiratory production rates. The mesophyll is treated as a continuously distributed liquid phase, and the photos ynthetic rate is determined by the carbon dioxide concentration, the p hotosynthetic photon flux density and the chlorophyll concentration. P hotorespiration is proportional to the photon flux density, and dark r espiration is assumed to be constant. The model offers a rigorous way to investigate the roles of physics and geometrical structure in stoma tal gas exchange. Lateral (radial) diffusion and differences between h ypostomatous and needle-like leaves are analysed with special attentio n. To yield realistic stomatal behaviour, the model requires that the diffusion coefficient describing mesophyllic transport must be somewha t larger than carbon dioxide diffusivity in pure liquid water. The mes ophyllic carbon dioxide concentration slopes sharply towards a constan t value as a function of distance from the surface of a sub-stomatal c avity. The optimal placement of chlorophyll is close to the surface co ntaining stomata.