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An Ineffective Nitrogen Fixation Trait in Alfalfa¹

An alfalfa (Medicago sativa L.) plant designated MnPL- 480 formed large, white nodules incapable of fixing atmospheric N when grown in association with five strains of Rhizobium meliloti Dang. in nil-nitrate greenhouse sand culture. Under the same conditions, control plants capable of fixing atmospheric N were vigorous, dark green, and had pink corraloid nodules. To verify the ineffectiveness of the rhizobium-MnPL-480 association, cuttings from control plants and from MnPL-480 were compared for both physiological and histological characteristics. MnPL-480 was self-pollinated and cross-pollinated with control plants having functional nodules to determine if the ineffective association was a heritable trait.

Acetylene reduction rates and nodule leghemoglobin concentrations were zero in MnPL-480. Forage and root dry weights of control plants were both about fivefold greater than those of MnPL-480. Nitrogen concentrations in forage and shoots of control plants were 1.5 fold greater than those of MnPL-480. However, nodule fresh weight and .root nonstructural carbohydrates were greater in MnPL-480 than in controls. Histological examinations of MnPL-480 nodules showed only a few cells that contained bacteroids. Bacteroids that were found in MnPL-480 nodules cells appeared to undergo rapid senescence. Nodules collected from MnPL-480 showed larger numbers of starch granules than nodules from control plants. MnPL-480 formed white, ineffective nodules with five Rhizobium strains tested in Leonard jar culture.

Progenies from self-pollination of MnPL-480 produced only ineffective nodules. F1 progenies from crosses of control plants and MnPL-480 produced functional nodules, thereby indicating that the ineffective association was a heritable (probably recessive), host-conditioned trait.

The MnPL-480 ineffective trait should be useful in studying the morphogenesis of alfalfa nodules, the genetic control of N fixation by the host, and the N economy of the alfalfa plant.

Key Words: Rhizobium meliloti Dang. • N Fixation • Ineffective nodules • Nodule histology • Bacteroids

² Formerly graduate student, Dep. of Agronomy and Plant Genetics, Unix. of Minnesota. Now assistant professor, Dep. of Plant Breeding, Cornell Univ., Ithaca, NY 14850.

³ Plant physiologists and research geneticist, respectively, USDA-SEA-AR, in the Dep. of Agronomy and Plant Genetics, Univ. of Minnesota.

Received for publication April 25, 1979.

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