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Host Plant Nodule Enzymes Associated with Selection for Increased N₂ Fixation in Alfalfa¹

² Former graduate research assistant, Univ. of Minnesota (now senior research scientist, Bolton Biological Lab. Rhodendron, OR), research plant physiologist and research geneticist, USDA-ARS, Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, St. Paul, MN 55108, respectively. Reprint requests should be addressed to C.P. Vance.

In legumes symbiotically fixed N₂ is assimilated into C skeletons to form amino acids as a prerequisite for enhancing plant growth. No attempts have been made to determine which nodule enzymes of ammonia and C assimilation are related to N₂ fixation in alfalfa (Medicago sativa L.). The objective of this study was to assess if nodule enzymes of ammonia and C assimilation were associated with acetylene reduction acitivity in alfalfa genotypes previously selected for various traits associated with N₂ fixation. Nodules from eight subpopulations derived by bi-directional selection within each of two alfalfa germplasm sources, an intercross between the two germplasm sources and three alfalfa cultivars were used to measure nodule soluble protein, and in vitro specific activities of the plant enzymes glutamine synthetase (GS), glutamate synthase (GOGAT) glutamate dehydrogenase (GDH), and phosphoenolpyruvate carboxylase (PEPC). Acetylene reduction rate was measured as an indicator of nitrogenase activity. Significant genotypic differences were observed among subpopulations for all variables tested. The GOGAT and PEPC activities were correlated with acetylene reduction activity in one germplasm source but not the other. Phenotypic correlation coefficients among variables showed that GOGAT and PEPC activities were frequently coupled. Germplasm differences in nodule enzyme activity suggest that nodule carbon and N assimilation may be more closely associated with N2 fixation potential in one germplasm than the other. The GOGAT and PEPC activity may be useful traits to include in breeding programs to enhance N₂ fixation in alfalfa.

Key Words: Medicago sativa L. • Ammonia assimilation • Nodule physiology • Nitrogen fixation • Genetic selection • Glutamate synthase • Phosphoenolpyruvate carboxylase

Received for publication November 10, 1983.

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