An Ineffective Nitrogen Fixation Trait in Alfalfa1
- D. R. Viands2,
- C. P. Vance,
- G. H. Heichel and
- 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.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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