Pressure gradients along whole culms and leaf sheaths, and other aspects of humidity-induced gas transport in Phragmites australis
Emergent aquatic macrophytes growing in waterlogged anaerobic sediments overlain by deep water require particularly efficient ventilating systems. In Phragmites australis (Cav.) Trin. ex Steud, pressurized gas flows, generated by humidity-induced diffusion of air into leaf sheaths, enhance oxygen transport to below-ground parts and aid in the removal of respiratory CO2 and sediment-generated CO2 and methane. Although modelling and flow measurements have pointed to the probable involvement of all leaf sheaths in the flow process and the development of pressure gradients along the whole lengths of living culm and leaf sheaths, direct measurements of pressure gradients have never been reported. The aim of this study was to search for pressure gradient development in Phragmites culms and leaf sheaths and to determine their magnitudes and distribution. In addition, dynamic (with gas flow) and static pressures (no flow condition) and their relationship to flows, leaf sheath areas, and living-to-dead culm ratios were further investigated. Dynamic pressures (ΔPd) recorded in the pith cavities of intact (non-excised) leafy culms, pneumatically isolated from the below-ground parts and venting through an artificial bore-hole near the base, revealed a curvilinear gradient of pressure ‘asymptoting’ towards the tips of the culms. Similarly, ΔPd in upper and lower parts of leaf sheaths increased with distance from the base of the culm, with values in the upper parts always being greater. Curvilinear gradients of pressure were also found along pneumatically isolated individual leaf sheaths, but radial channels linking the leaf sheath aerenchyma with the pith cavity of the culm appeared to offer little resistance to flow. In keeping with predictions, static pressure differentials (ΔPs) achieved in intact and excised culms and single leaf sheaths on intact culms proved to be relatively independent of leaf sheath area, whereas the potential for developing convective flows (pressure-driven flows) increased with increasing leaf sheath area. As measured by the ventilating coefficient [1–(ΔPd/(ΔPs)] the old dead (efflux) to living (influx) culm ratio of 1:12 compared with 1:25 raised ventilating efficiency from 31% to 71%, giving flows per tall culm into the rhizome system of c. 2.8 cm3 and 6.5 cm3 min−1, respectively. It was concluded that dynamic pressure gradients probably extend along the whole length of the leafy culms and leaf sheaths of Phragmites and that all leaf sheaths and all exposed points along the leaf sheaths can contribute convective gas-flow to the rhizome system.