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This article in CS

  1. Vol. 19 No. 6, p. 905-908
    Received: Apr 25, 1979

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An Ineffective Nitrogen Fixation Trait in Alfalfa1

  1. D. R. Viands2,
  2. C. P. Vance,
  3. G. H. Heichel and
  4. D. K. Barnes3



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.

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