Contrasting oxygen dynamics in the freshwater isoetid Lobelia dortmanna and the marine seagrass Zostera marina

Background and Aims Submerged plants possess well-developed aerenchyma facilitating intra-plant gas-phase diffusion of O-2 to below-ground tissues, which are usually buried in anoxic sediments. However, aquatic habitats differ in terms of O-2 fluctuations in the water column and in O-2 consumption of the sediment, and aquatic plants differ in aerenchymal volume and resistance to O-2 diffusion through the plant and across leaf and root surfaces. The hypothesis that the freshwater isoetid Lobelia dortmanna and the marine seagrass Zostera marina should display pronounced contrasts in intra-plant O-2 dynamics because of differences in morphology/anatomy, physiology and growth habitat was tested. Methods In order to determine the O-2 dynamics and relate this to the anatomy and morphology of the two species, O-2 microelectrodes were inserted in the aerenchyma of leaves and roots, the sediment pore-water, and the water column in the field. Manipulation of water column O-2 in the laboratory was also carried out. Key Results It was found that intra-plant transport of O-2 between leaf and root tips takes place more readily in L. dortmanna than in Z. marina due to shorter distances and greater cross-sections of the aerenchyma. The major exchange of O-2 across roots of L. dortmanna can be accounted for by small intra-plant resistances to diffusion, larger root than leaf surfaces, and greater radial diffusive resistance of leaves than roots. In contrast, the major O-2 exchange across leaves than roots of Z. marina can be accounted for by the opposite anatomical-morphological features. The larger aerenchymal volume and the smaller metabolic rates of L. dortmanna compared to Z. marina imply that turnover of O-2 is slower in the aerenchyma of L. dortmanna and O-2 fluctuations are more dampened following changes in irradiance. Also, O-2 accumulated in the aerenchyma can theoretically support dark respiration for a few hours in L. dortmanna but for only a few minutes in Z. marina. Conclusions The build-up of O-2 in the pore-water of L. dortmanna sediments during the day as a result of high release of photosynthetic O-2 from roots and low O-2 consumption of sediments means that sediment, aerenchyma and water are important O-2 sources for respiration during the following night, while Z. marina relies on the water column as the sole source of O-2 because its sediments are anoxic. These differences between L. dortmanna and Z. marina appear to represent a general difference between the isoetid species mainly inhabiting sediments of low reducing capacity of oligotrophic lakes and the elodeid freshwater species and marine seagrasses mainly inhabiting sediments of higher reducing capacity in more nutrient-rich habitats.