Formation of root plaques and their influence on tissue phosphorus content in Lobelia dortmanna

Abstract

Oxygen release from the roots of the freshwater isoetid, Lobelia dortmanna L., influences redox related processes of iron (Fe) and manganese (Mn) in the sediments. Because oxidized species of Fe and Mn have high capacity for phosphorus (P) binding, the availability of sediment P to aquatic plants is influenced by root oxygen release. We investigated 30 sediment cores vegetated with L. dortmanna from oligotrophic Mirror Lake to analyse sediment variables responsible for precipitation of Fe and Mn on root surfaces (plaques), and P content of plant tissue. Close negative correlations between redox potential and concentrations of Fe ( r=−0.924) and Mn plaques ( r=−0.897) were found. Also, correlations between exchangeable Fe and Fe plaques ( r=0.716) and between exchangeable Mn and Mn plaques ( r=0.783) were found indicating that plaques are formed in reduced sediments with high concentrations of reduced Fe and Mn. For plants with Fe plaque concentrations lower than 30 μmol g −1 there were significant positive correlations between the tissue P content and the concentration of the less-plant-available P fractions in the sediment. In contrast, plants with Fe plaque concentrations higher than 30 μmol g −1 showed low tissue-P content (about 20 μmol g −1) independent of the P concentration in the sediment indicating that P uptake by L. dortmanna is reduced by Fe plaques. L. dortmanna with low tissue P content (<30 μmol g −1) showed significantly higher mycorrhizal infection than plants with high tissue P content (>30 μmol g −1) indicating that fungal association may be important in P assimilation by rooted aquatic plants in habitats with low P availability.