Pressurised ventilation in emergent macrophytes: the mechanism and mathematical modelling of humidity-induced convection

Abstract

Humidity-induced diffusion (HID) and convection (HIC: pressure flow) are described for a simple physical model and are mathematically modelled. The physical model comprises a cylindrical chamber, partially filled with water, and capped by a micro-porous partition (membrane); an outflow pipe with a tap vents gases from the header space to the outside. The mathematical model comprises a series of equations which embrace the effects of header space depth and boundary layer thickness, the diffusive- and any Poiseuille-flow resistances of the partition, and the venting path resistance. For membrane pore diameters within (i.e. ≤ 0.1 μm) or outside the Knudsen regime, close correlations are found between experimental values of static and dynamic pressure and convective flows, and those obtained from the mathematical model. The findings suggest that for plants, compromises are necessary between factors such as small pore widths, which can help generate high dynamic pressures, and the need for wider pore widths to support greater flows. It is shown that the fastest flows are generated at pore diameters of ca. 0.2 μm, and it is suggested that the high rates of flow found in Phragmites are achieved because of an optimum leaf sheath stomatal pore width coupled to very low venting resistance through the plant. The benefits of a sustained humidifying source close to the base of the pores is also highlighted, and attention is drawn to thermally enhanced HIC and the continuance of HIC when temperatures within the plant might be lower than outside. The results have important implications for understanding convective gas flow generation in plants and its potential for enhancing ‘greenhouse gas’ emissions from wetlands.