Biology of the Frankia-Alnus maritima subsp. maritima symbiosis
Alnus maritima subsp. maritima is a rare shrub that is unique among alders in its restriction to waterlogged soils. An actinorhizal species with great horticultural potential, this plant develops a root-nodule symbiosis with the nitrogen-fixing bacterium Frankia . Oxygen in the root zone is critical to establishment of an effective symbiosis, and both symbiotic partners are challenged by growth in low-oxygen wetland soils. My overall goal was to understand the conditions that foster or limit the success of this subspecies in its native habitat and in horticultural settings. I designed an aeroponic gas-delivery system and used the acetylene-reduction assay to test the influence of oxygen on nodulation and nitrogenase activity. Oxygen-dependent changes in nodule anatomy were confirmed by using microscopic analyses. My results show that nodules of this subspecies are capable of forming and functioning on submerged roots and exhibit evidence of adaptation to growth under hypoxic conditions. The subspecies should not be limited to wetland planting sites, however, because plant survival, nodulation, and nitrogen fixation are not inhibited by higher oxygen concentrations typical of managed settings.