Occurrence, structure, and nitrogen-fixation of root nodules of actinorhizal Arizona alder

Actinorhizal plants are nodulated by the symbiotic, nitrogen-fixing actinomycete Frankia. The genus Alnus in the family Betulaceae is one of the 24 genera in 8 families of angiospermous plants that are actinorhizal. Arizona alder (Alnus oblongifolia Torr.) occurs in isolated populations associated with the watersheds of Madrean Sky Islands in the Southwestern United States between 1,370 and 2,285 m in elevation. We have found root nodules on alder trees from Oak Creek Canyon in central Coconino County, Arizona, and from the Santa Catalina Mountains in southeastern and central Pima County, Arizona. We describe the occurrence of nodules at two locations at or near opposite latitudinal limits of Arizona alder’s main range. Capacity of Arizona alder nodules to fix atmospheric nitrogen is confirmed by the acetylene reduction assay and the occurrence of vesicles in infected cortical cells of nodule lobes. Nodule location on roots, nodule morphology, and cellular anatomy confirm symbiotic structure similar to that of other alder species.