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Ji-Jhong Chen, Heidi Kratsch, Jeanette Norton, Youping Sun, and Larry Rupp

. rotundifolia are actinorhizal plants ( Benson and Silvester, 1993 ), and S × utahensis ‘Torrey’ can form a symbiotic association with Frankia to fix N 2 in its root nodules (J. Chen, unpublished data). This biological N 2 -fixing capacity may reduce the

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Ji-Jhong Chen, Jeanette Norton, Heidi Kratsch, Youping Sun, and Larry Rupp

Actinorhizal plants are able to fix atmospheric nitrogen (N 2 ) through symbiosis with Frankia , a genus of actinobacteria, and have great potential for sustainable landscaping ( Kratsch and Graves, 2004 ). Plant growth and development may be

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Heidi A. Kratsch* and William R. Graves

Alnus maritima (Marsh.) Muhl. ex Nutt. is unique among alders in its degree of preference for low-oxygen soils of wetlands. An actinorhizal species with promise for use in sustainable horticulture, A. maritima develops a root-nodule symbiosis with nitrogen-fixing Frankia. Nodules of other actinorhizal species that are obligate wetland natives are adapted to low oxygen, and expression of hemoglobin is common to these taxa. Our objectives were to determine the range of oxygen tension under which Alnus maritima subsp. maritima fixes nitrogen and to investigate a potential role for hemoglobin in adaptation of nodules to low oxygen. Roots of plants, cultured aeroponically, were subjected to eight oxygen tensions from 0 to 32 kPa. After four weeks, plant dry weight, nodule fresh weight, nitrogenase activity, and photosynthetic rate were measured. In addition, nodules were assayed spectrophotometrically for the presence of hemoglobin. A quadratic function best described the influence of oxygen on plant dry weight, nodule fresh weight, nitrogenase activity, and photosynthetic rate with maximal values above 20 kPa. Alnus serrulata (Ait.) Willd. is sympatric with A. maritima subsp. maritima but is not an obligate inhabitant of wetlands. In a separate experiment, we found higher nitrogenase activity in A. maritima subsp. maritima than in A. serrulata (0.74 vs. 0.26 μmol/h per plant) at hypoxic oxygen tensions. Further, optical absorption spectra of nodule extracts confirmed hemoglobin within nodules of A. maritima subsp. maritima. Our data suggest that hemoglobin contributes to oxygen regulation in nodules of A. maritima subsp. maritima.

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Heidi A. Kratsch and William R. Graves

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.

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Taun Beddes and Heidi A. Kratsch

nursery production. However, no studies have investigated effects of CRF on nodule formation in N-fixing plants. It has been demonstrated with various actinorhizal species that nodulation can be limited when plant roots are exposed to high concentrations

Open access

Asmita Paudel, Youping Sun, Larry A. Rupp, and Richard Anderson

). Sego Supreme TM plants are selected for their special characteristics, such as aesthetic value, ease of propagation, demand, disease/pest resistance, and drought tolerance. Cercocarpus is the largest genus of actinorhizal plants in the Rosaceae (rose

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Roland Ebel

. Additionally, ahuejotes provide shade, create a protective barrier against wind and pests, and serve as trellises for vine crops. After the planting, the willow is interwoven with reeds and branches of other plants. The result is the chinamil (a solid fence