Abstract
Wetland plants actively provide oxygen for aerobic processes in submerged tissues and the rhizosphere. The novel concomitant assessment of diurnal dynamics of oxygen and carbon dioxide concentrations under field conditions tests the whole-system interactions in plant-internal gas exchange and regulation. Oxygen concentrations ([O2]) were monitored in-situ in central culm and rhizome pith cavities of common reed (Phragmites australis) using optical oxygen sensors. The corresponding carbon dioxide concentrations ([CO2]) were assessed via gas samples from the culms. Highly dynamic diurnal courses of [O2] were recorded, which started at 6.5-13 % in the morning, increased rapidly up to 22 % during midday and declined exponentially during the night. Internal [CO2] were high in the morning (1.55-17.5 %) and decreased (0.04-0.94 %) during the rapid increase of [O2] in the culms. The observed negative correlations between [O2] and [CO2] particularly describe the below ground relationship between plant-mediated oxygen supply and oxygen use by respiration and biogeochemical processes in the rhizosphere. Furthermore, the nocturnal declining slopes of [O2] in culms and rhizomes indicated a down-regulation of the demand for oxygen in the complete below ground plant-associated system. These findings emphasize the need for measurements of plant-internal gas exchange processes under field conditions because it considers the complex interactions in the oxic-anoxic interface.
Highlights
Under strongly anoxic conditions in permanently watersaturated soils of wetland ecosystems (Ponnamperuma1972), oxygen is a very rare resource with limited occurrence and, if present, it is immediately utilized by biological and chemical processes
Courses of rhizome-[O2] in the reed plants were similar to those observed in culms and were correlated strongly to relative air humidity (RH) (r < À0.94) and air temperature (r ! 0.93), and moderate with surface water temperature (r ! 0.50) and photon flux density (PFD) (r ! 0.54)
The relationship between internal oxygen and carbon dioxide concentrations measured under field conditions reflects the dependence of respiration of the submerged organs and biogeochemical processes in the rhizosphere on plant-internal oxygen supply
Summary
Under strongly anoxic conditions in permanently watersaturated soils of wetland ecosystems (Ponnamperuma1972), oxygen is a very rare resource with limited occurrence and, if present, it is immediately utilized by biological and chemical processes. Faußer et al — Diurnal dynamics of oxygen and carbon dioxide concentrations in shoots and rhizomes of a perennial conditions frequently develop an enlarged gas-space continuum which runs from shoots through the extensive rhizome system to the roots (Cardoso et al 2014; Jackson and Armstrong 1999; Koncalova 1990). This aerenchyma system enables emergent wetland plants to supply submerged organs with oxygen. In return respiratory gases (e.g. CO2) from below ground aerobic, anaerobic and fermentation processes (e.g. aerobic respiration, denitrification, iron reduction) as well as methane from the anaerobic sediments can ventilate to the atmosphere (Armstrong 1980; Armstrong et al 2000; Kayranli et al 2010; Le Mer and Roger 2001; Laanbroek 2009; Maltais-Landry et al 2009)
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