The effect of pH and plaque on the uptake of Cu and Mn in Phragmites australis (Cav.) Trin ex. Steudel

The physico-chemical properties of iron oxyhydroxide plaques formed on the roots of Phragmites australis under field and laboratory conditions were determined using electron microscopy and energy dispersive spectrometry (EDS) analysis. Plaques were present as an amorphous coating on roots with an uneven distribution. They were shown to be composed of iron in both the field and laboratory, and phosphorus was found to be adsorbed onto the surface of plaques formed in the laboratory. The uptake of copper and manganese in the presence and absence of plaque was investigated under two different pH regimes. Concentrations of Cu were lower in the shoots of P. australis in the presence of plaque (565 mg kg^-1) than when it was absent (1400 mg kg^-1), under growth conditions of higher pH (6·0). The adsorption of Cu and Mn onto the plaque surface was not the mechanism by which plaque reduced the uptake of other metals. Alternatively, the plaque may simply act as a physical barrier. Under conditions of lower pH (3·5) the activity of hydrogen ions at the root surface interfered with the movement of metals into the root and masked any potential effect of iron plaque. (C) 2000 Annals of Botany Company.