Phragmites australis: a critical appraisal of the ventilating pressure concept and an analysis of resistance to pressurized gas flow and gaseous diffusion in horizontal rhizomes.

Abstract

SUMMARY The concept of plant ‘ventilating pressures’ is considered by comparing the pressures required to cause various rates of gas‐How through rhizomes of Phragmites australis venting either beneath water or into air. The resistances to pressurized flow in nodal and internodal regions are quantified, and the pressure: flow data analysed mathematically to deduce effective pore‐space diameter in the nodal diaphragms, and the diffusive resistances of the diaphragms and intemodes. The value traditionally termed ‘ventilating‐pressure’, the minimum pressure required to cause a venting of gas into water, is shown to be a function of surface tension effects at the venting surfaces and not a measure of gas‐flow resistance within the plant. With venting to air rather than water, gas flows occurred at all applied pressures greater than atmospheric and pressure‐flow resistance within the plant was determined from the gas flow: pressure relationship. Pressure‐flow resistance was much greater in the nodal pith diaphragms (6–210 MPa s m−3) than in internodal pith cavities (0.001 5.040 MPa s m−3; path length, 100 mm) and was chiefly a function of pore‐size, pore numbers and path length within the small stellate parenchyma layers of the diaphragms. The domed nature of diaphragms reduces resistance by increasing the pore numbers. The effective pore diameters within the small stellate parenchyma of the diaphragms were estimated from the pressure: flow data using a modification of the Poiseulle equation. The values obtained ranged from c. 3.7 μm, and corresponded closely with diameters found previously by scanning electron microscopy. It is concluded that both large and small stellate parenchyma must contribute significantly to the diffusive resistance of tile diaphragms in accordance with path lengths and porosity; and that, in contrast with pressure‐flow resistance, the internodes because of their length, should have the greater impedance. Estimated values for the pith diaphragm diffusive resistance were 1–13 Ms m−3 (path length, 1.7–3 mm), whilst pith cavity resistances, (path length 100 mm), were 25–1100 Ms m−3. In the latter, the variation was attributable chiefly to the widely varying rhizome diameters: porosities of wide rhizomes were greater than those of narrower ones because of large pith cavities. The fractional porosity due to cortical aerenchyma channels (10–20%) was not related to rhizome thickness. It is concluded that gas‐flow resistances in Phragmites are sufficiently small to allow for substantial long‐distance transport by convection or diffusion.

Internal pressurization and convective gas flow in some emergent freshwater macrophytes

“…Root aeration is enhanced by increased oxygen partial pressure at the rhizome-root junction and the consequently higher diffusive flux into the gas spaces of the root. Oxygen leakage from roots and the ability of plants to main-tain an oxidized rhizosphere are enhanced by convective through-flow (Armstrong and Armstrong 1990). The release of potentially asphyxiating gases produced in the sediment (COZ, CH4) is also enhanced by convective gas flow in emergent plants (Dacey and Klug 1979;Sebacher et al 1985).…”

“…The unit used here for resistance (Pa s mmm2) differs from those (Pa s rnrnp3 or Pa s mme4) usually used to measure resistance (e.g. see Armstrong et al 1988). This unit is used so we can directly compare the relative tissue resistances for different species with different cross-sectional areas.…”

“…The pressure, which is generated by gradients in temperature and water-vapor pressure between the internal gas spaces and the surrounding atmosphere, is vented through rhizomes and old shoots to the atmosphere (Dacey 198 1;Armstrong and Armstrong 199 1). These convective flows increase the oxygen concentration of gases venting through the rhizome, thus enhancing the diffusive flux of oxygen from rhizomes to roots and into the rhizosphere (Armstrong and Armstrong 1990). Internal pressurization and convective gas flow are important adaptations to growth in an anoxic substrate and may offer a competitive advantage over species relying exclusively on diffusive gas transport.…”

Phragmites die–back: bud and root death, blockages within the aeration and vascular systems and the possible role of phytotoxins

“…A separate pressure transducer on the stubble side of the fiow cell helped to record the applied, and any stubble, pressures. Conducti\'ities into the beds were calculated by appropriate substitution in the Poiseuille equation (Armstrong, Armstrong & Beckett, 1988).…”

Ventilation and respiration in roots of one-year-old seedlings of grey mangrove Avicennia marina (Forsk.) Vierh.

“…Root aeration of wetland plants has been studied extensively (e.g . Armstrong et al, 1988 ;Dacey, 1981 ;Grosse & Mevi-Schutz, 1987 ;Jackson & Drew, 1984 ;Raskin & Kende, 1983 ;Yamasaki, 1984) . Wetland plants' roots typically contain a large proportion of aerenchyma providing an extensive internal gas space (Kozlowski, 1984) which serves to store and conduct gases throughout the root system (Chapman, 1976 ;Hook & McKevlin, 1988) .…”