Dead stems of Phragmites australis move air to shoot and root meristems by use of differential air pressure.
"Through flow can also occur in dormant plants with persistent, standing litter. This has been reported for Phragmites australis. Differences in wind speed at the top and near the bottom in the canopy create a differential internal pressure between tall and short dead shoots. The lower air pressure in taller shoots draws air into the shorter dead shoots, down into the rhizomes, and up the taller dead shoots (Fig. 4.8). In the temperate zone in the early spring, this may be an important mechanism for Phragmites to get oxygen to shoot and root meristems."
From Fig. 4.8: "A. Differential air pressure caused by wind blowing across dead culms sucks air into the lower culms through the rhizomes and into the taller culms. B. Pressurization of new culms due to a build up of vapour pressure or higher temperatures causes mass flow of gasses [sic] down the culms into the rhizome and up into more porous older culms. The movement of oxygen from the rhizomes into the roots and out of the roots into the soil is due to diffusion. (Redrawn from Colmer 2003)" (van der Valk 2006: 64-65)
"Internal transport of gases is crucial for vascular plants inhabiting aquatic, wetland or flood-prone environments. Diffusivity of gases in water is approximately 10 000 times slower than in air; thus direct exchange of gases between submerged tissues and the environment is strongly impeded. Aerenchyma provides a low-resistance internal pathway for gas transport between shoot and root extremities. By this pathway, O2 is supplied to the roots and rhizosphere, while CO2, ethylene, and methane move from the soil to the shoots and atmosphere. Diffusion is the mechanism by which gases move within roots of all plant species, but significant pressurized through-flow occurs in stems and rhizomes of several emergent and floating-leaved wetland plants. Through-flows can raise O2 concentrations in the rhizomes close to ambient levels. In general, rates of flow are determined by plant characteristics such as capacity to generate positive pressures in shoot tissues, and resistance to flow in the aerenchyma, as well as environmental conditions affecting leaf-to-air gradients in humidity and temperature. O2 diffusion in roots is influenced by anatomical, morphological and physiological characteristics, and environmental conditions. Roots of many (but not all) wetland species contain large volumes of aerenchyma (e.g. root porosity can reach 55%), while a barrier impermeable to radial O2 loss (ROL) often occurs in basal zones. These traits act synergistically to enhance the amount of O2 diffusing to the root apex and enable the development of an aerobic rhizosphere around the root tip, which enhances root penetration into anaerobic substrates. The barrier to ROL in roots of some species is induced by growth in stagnant conditions, whereas it is constitutive in others. An inducible change in the resistance to O2 across the hypodermislexodermis is hypothesized to be of adaptive significance to plants inhabiting transiently waterlogged soils. Knowledge on the anatomical basis of the barrier to ROL in various species is scant. Nevertheless, it has been suggested that the barrier may also impede influx of: (i) soilderived gases, such as CO2, methane, and ethylene; (ii) potentially toxic substances (e.g. reduced metal ions) often present in waterlogged soils; and (iii) nutrients and water. Lateral roots, that remain permeable to O2, may be the main surface for exchange of substances between the roots and rhizosphere in wetland species. Further work is required to determine whether diversity in structure and function in roots of wetland species can be related to various niche habitats. (Colmer 2003:17)
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