The convective throughflow of gases found recently in Phragmites australis increased with increasing fluxes of photosynthetically active radiation. Despite this, the oxygen in the venting gases remained lower than in air and the convection is thought to depend mainly on a humidity-induced diffusion of atmospheric gases across the leaf sheaths. When convection was prevented, and aeration was dependent entirely upon diffusion, no photosynthetic enhancement of rhizome and root aeration could be detected.
In terms of oxygen transport, rapid rates of convection were much more effective than diffusion and substantially enhanced the entry of oxygen into the rhizome system. This resulted in large increases in the efflux of oxygen into the rhizospheres of basal laterals and apical and sub-apical regions of the adventitious roots. This effect was attributed to a greater diffusion of oxygen from rhizome to root. Substantial diurnal fluctuations in rhizome oxygen regimes and in root oxygen efflux occurred.
It is estimated that because of convection and diffusion, up to 0.31 kg of oxygen d−1 m−1 may enter the rhizome system of vigorous stands of Phragmites during summer months. By comparison, diffusion alone could be expected to supply less than 0.01 kg d−1. Scanning electron micrographs of root-shoot junctions showed that the resistance to gaseous diffusion from rhizome to roots must be especially small in Phragmites.