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Phragmites Australis– A Preliminary Study Of Soil‐oxidizing Sites And Internal Gas Transport Pathways
Published 1988 · Biology
SUMMARY The possible pathways of oxygen transport within Phragmites australis and out into the soil were investigated by means of pressurized air flow, methylene-blue dye and by anatomical studies. In most cases, gas was easily forced through the plants, thus demonstrating gas-space connexion between aerial shoot, rhizomes and roots and a relatively low resistance to flow. Gas-space connexions between the pith cavity and cortical aerenchyma in the rhizomes were also detected and were investigated anatomically they are present only in nodal regions just above the insertion of the pith diaphragm and take the form of radial channels through the stelar cylinder. Similar pathways in the culm connect the pith cavity with nodal lines of stomata and it is suggested that these regions may often form the major oxygen entry point to the rhizome-root system. A failure to effect pressurized gas-flow through some rhizomes led to the detection of secondary barriers to longitudinal gas-flow. Seemingly a response to wounding, these are an impenetrable barrier to gases, to flooding, and probably also to microorganisms, and take the form of tylosoidal growths from and within regions of stellate-and/or circumvascular-parenchymas such as those of the nodal diaphragms. Stellate parenchymas of various sorts were found to be a common feature of the gas-path in Phragmites and evidently they form an important component of the gas-space system occurring within the nodal radial channels, the nodal diaghragms and, rather unusually, the root-shoot junctions where they must confer a porosity which is higher than for most plants. As measured by methylene-blue oxidation, oxygen release from underground parts was most rapid from young adventitious and secondary roots and particularly basal tufts of fine laterals. Zones of oxidation also occurred around the sprouting tips of horizontal and vertical rhizomes. Dormant tips showed very little oxidizing power, whilst the rhizomes themselves and the older parts of adventitious roots showed none. This was associated with cuticularization of the rhizome and hypodermal suberization and lignification; oxygen-impermeable regions of the roots were also hyperdermally suberized and lignified. The findings are discussed in relation to the use of Phragmites in sewage-treatment beds.