Light-dependent variations in the composition of the internal atmosphere of Phragmites australis (Cav.) Trin. ex steudel

The composition of the lacunal air in Phragmites australis (Cav.) Trin. ex Steudel during a diel cycle and during prolonged exposures to light and dark, respectively, was studied in the laboratory. The experimental system was constructed so that samples of the lacunal air could be withdrawn with syringes at fixed points in the stems and in the rhizomes of an intact shootsystem. The composition of the samples (CO₂, O₂, N₂) was analyzed by gas chromatography.

A steep gradient in O₂ concentration was observed within the lacunae, with highest concentrations in the aerial stems (21%) and lowest concentrations in the deepest-growing rhizomes (3.6%). Inverse gradients of CO₂ and N₂ occurred in the opposite direction. The composition of the lacunal air exhibited a clear diel variation, with highest O₂ and lowest CO₂ and N₂ concentrations during the daytime.

It is concluded that the transport of gases within the lacunae of P. australis is mediated by diffusion following the concentration gradients of the gases, and convective flow of air resulting from solubilization of respiratory CO₂ in the interstitial waters and/or in the transpiration stream. During the daytime, the ventilation is accelerated, probably as a consequence of thermo-osmotic pressurization resulting from temperature differences between the aerial plant tissues and the ambient air.