Computer-based studies of diffusion through stomata of different architecture

Background and Aims The influence of stomatal architecture on stomatal conductance and on the developing concentration gradient was explored quantitatively by comparing diffusion rates of water vapour and CO2 occurring in a set of three-dimensional stoma models. The influence on diffusion of an internal cuticle, a sunken stoma, a partially closed stoma and of substomatal chambers of two different sizes was considered. Methods The study was performed by using a commercial computer program based on the Finite Element Method which allows for the simulation of diffusion in three dimensions. By using this method, diffusion was generated by prescribed gas concentrations at the boundaries of the substomatal chamber and outside of the leaf. The program calculates the distribution of gas concentrations over the entire model space. Key Results Locating the stomatal pore at the bottom of a stomatal antechamber with a depth of 20 pm decreased the conductance significantly (at roughly about 30 %). The humidity directly above the stomatal pore is significantly higher with the stomatal antechamber present. Lining the walls of the substomatal chamber with an internal cuticle which suppresses evaporation had an even stronger effect by reducing the conductance to 60 % of the original value. The study corroborates therefore the results of former studies that water will evaporate preferentially at sites in the immediate vicinity to the stomatal pore if no internal cuticle is present. The conductance decrease affects only water vapour and not CO2. Increasing the substomatal chamber increases CO2 uptake, whereas transpiration increases if an internal cuticle is present. Conclusions Variation of stomatal structure may, with unchanged pore size and depth, profoundly affect gas exchange and the pathways of liquid water inside the leaf. Equations for calculation of stomatal conductance which are solely based on stomatal density and pore depth and size can significantly overestimate stomatal conductance.