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This article in CS

  1. Vol. 27 No. 6, p. 1205-1209
    Received: Dec 26, 1987

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Dry Matter, Nitrogen Distribution, and Dinitrogen Fixation in Contrasting Alfalfa Symbioses1

  1. K. A. Fishbeck,
  2. G. H. Heichel and
  3. C. P. Vance2



Improving dinitrogen (N2) fixation of alfalfa (Medicago sativa L.) requires combining effective rhizobial strains and host genotypes capable of enhanced N2 fixation. Glasshouse experiments on pot-grown alfalfa determined how changes in the rhizobial and host genomes associated with N2 fixation affect dry matter and N yields and distribution at vegetative, flower bud, and 30% flowering developmental stages. We compared dry matter and N yield and distribution in a host- and a rhizobial-conditioned ineffective (Fix+) and in a related effective (Fix+) symbiotic combination, and investigated the effect of combined N on N2 fixation and on the distribution of symbolically fixed N2 in two unrelated Fix+ alfalfa germplasms. Dry matter and N yield and response to added N (0, 40, 80 kg/ha) of the host-conditioned and the rhizobial-conditioned Fix combinations were similar. Without combined N, both Fix combinations distributed relatively less dry matter and N to aerial organs than did the Fix+ combination. Neither the type of Fix combination nor traits in the Fix+ germplasms supportive of increased N2 fixation altered the relative distribution of dry matter among plant organs; however, N concentration in individual organs varied with supplemental N in the Fix but not the Fix+ combinations. Dry matter and N yields of the two unrelated Fix+ combinations differed only without combined N. Supplemental N was more inhibitory to N2 fixation by Fix+ germplasm selected for enhanced nitrogenase activity compared with unselected Fix+ germplasm. Either source of ineffective nodulation in Fix combinations would be appropriate for developing non-fixing controls for physiological investigations of N2 fixation and for use in programs to evaluate and enhance N2 fixation in alfalfa.

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