Young Phragmites plants were grown in two cocktails of monocarboxylic acids (C(1)-C(5)) at pH 6, where the concentration of each acid was innocuous and the total undissociated (potentially toxic) concentrations were 0.35 mmol/L and 0.42 mmol/L. Rice plants were subjected to 1.5 mmol/L acetic acid at pH 4.5 (undissociated concentration = 1.05 mmol/L). In Phragmites, each cocktail curtailed root growth especially and induced premature shoot senescence. In both species, after 3-5 d of treatment, radial oxygen loss (ROL) from apical regions of adventitious roots, and from Phragmites laterals, was reduced to very low values and associated with cell wall lignification and suberization in the surface cell layers. At later stages of treatment, rice responded to acetic acid in similar ways to Phragmites, with the development of intercellular and callus type occlusions in the gas space system, vascular blockages, and the failure of laterals to emerge. The results are relevant to the supply of oxygen from Phragmites roots to sediments for the phytopurification of waste waters, to the efflux of methane and carbon dioxide from wetlands, and to rice cultivation.