FORMATION OF ROOT PLAQUES AND THEIR INFLUENCE ON TISSUE PHOSPHORUS-CONTENT IN LOBELIA-DORTMANNA

Oxygen release from the roots of the freshwater isoetid, Lobelia dortm anna L,, influences redox related processes of iron (Fe) and manganese (Mn) in the sediments. Because oxidized species of Fe and Mn have hig h capacity for phosphorus (P) binding, the availability of sediment P to aquatic plants is influenced by root oxygen release. We investigate d 30 sediment cores vegetated with L.. dortmanna from oligotrophic Mir ror Lake to analyse sediment variables responsible for precipitation o f Fe and Mn on root surfaces (plaques), and P content of plant tissue. Close negative correlations between redox potential and concentration s of Fe (r=-0.924) and Mn plaques (r=-0.897) were found. Also, correla tions between exchangeable Fe and Fe plaques (r=0,716) and between exc hangeable Mn and Mn plaques (r=0.783) were found indicating that plaqu es are formed in reduced sediments with high concentrations of reduced Fe and Mn, For plants with Fe plaque concentrations lower than 30 mu mol g(-1) there were significant positive correlations between the tis sue P content and the concentration of the less-plant-available P frac tions in the sediment. In contrast, plants with Fe plaque concentratio ns higher than 30 mu mol g(-1) showed low tissue-P content (about 20 m u mol g(-1)) independent of the P concentration in the sediment indica ting that P uptake by L. dortmanna is reduced by Fe plaques. L. dortma nna with low tissue P content (<30 mu mol g(-1)) showed significantly higher mycorrhizal infection than plants with high tissue P content (> 30 mu mol g(-1)) indicating that fungal association may be important i n P assimilation by rooted aquatic plants in habitats with low P avail ability. (C) 1998 Elsevier Science B,V.