Location and anatomy of nodules on Alnus maritima subjected to flooding

Although many species of Alnus Miller grow in wet soils, none is as closely associated with low-oxygen, waterlogged soils as Alnus maritima (Marsh.) Muhl. ex Nutt. (seaside alder). An actinorhizal species with promise for use in horticultural landscapes, land reclamation, and sustainable systems, A. maritima associates with Frankia Brunchorst, thereby forming root nodules in which gaseous nitrogen is fixed. Our objective was to determine how root-zone moisture conditions influence the occurrence, location, and anatomy of nodules on A. maritima. Plants of Alnus maritima subsp. maritima Schrader and Graves were established in root zones with compatible Frankia and subjected to four moisture regimens (daily watered/drained, partially flooded, totally flooded, and totally flooded with argon bubbled through the flood water) for 8 weeks. Oxygen content of the root zone, number and location of nodules on root systems, and dry weight and nitrogen content of shoots were determined. Root-zone oxygen content ranged from 17.3 kPa for daily watered/drained plants to 0.9 kPa for argon-treated plants. Across all treatments, 87% of the nodules were within the upper one-third (4 cm) of the root zone. Although shoot dry weights of daily watered/drained and partially flooded plants were not different, daily watered/drained plants had more nitrogen in their leaves (2.53 vs. 2.21 mg.g(-1)). Nodulation occurred in all treatments, but nodules on totally flooded roots (with or without argon) were limited to a single lobe; in contrast, multilobed nodules were prevalent on partially flooded and daily watered/drained plants. Frankia infection within submerged nodule lobes was limited to one or two layers of cortical cells. Submerged nodules developed large air spaces between cortical cells, and phenolic-containing cells appeared to inhibit Frankia expansion within the nodule. These data suggest that access to root-zone oxygen is critical to the Frankia-A. maritima subsp. maritima symbiosis, and that plants of this subspecies in the drained soils of managed landscapes may benefit more than plants in native wetland habitats from nodulation and nitrogen fixation.