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Nitrogen Assimilation in an Improved Alfalfa Population¹

Alfalfa (Medicago sativa L.), which is capable of fixing N₂ in symbiosis with Rhizobium meliloti, normally is grown in soils with varying amounts of available N. We have demonstrated previously that population selected from ‘African’ and ‘Hairy Peruvian’ for increased dry matter and N assimilation under both N₂- and NH₄NO₃-dependent conditions outperformed the original parental materials when grown with the extremes of N nutrition, i.e., totally dependent on N₂ or completely sustained by mineral N. The objective of this study was to compare i) growth and N assimilation of the original Hairy Peruvian (HP) and the improved Hairy Peruvian 32 (HP32) with various strains of R. meliloti and no nitrate and ii) one strain of R. meliloti and different levels of nitrate. Tests showed that, although only one strain of Rhizobium had been present during selection, HP32 grown in the absence of combined N increased N₂ fixation in three other strains of R. meliloti by 22 to 53% relative to the same strains in HP. When HP and HP32 were tested with 0, 1, 2, or 8 mM nitrate in symbiosis with the Rhizobium strain used during the selection process, HP32 assimilated an average of 32% more total N than HP under all N regimes. Estimates of N₂ fixation using the ¹⁵N-dilution method under those conditions showed that HP32 fixed an average of 38% more N₂ than HP under all nitrate treatments. The inhibition of N₂ fixation by nitrate, however, was proportionately similar in HP and HP32. The fact that HP32 increased N₂ fixation by all strains of R. meliloti tested suggests that HP32 has some general physiological trait which might be used to improve the symbiotic performance of the mediocre indigenous Rhizobium strains that nodulate alfalfa in many soils.

Key Words: Nitrogen fixation • Nitrate reduction • Rhizobium • Isotopic N.

² Professor, research associates, and associate professor, respectively; Dep. of Agronomy and Range Science, Univ. of California, Davis, CA 95616. Present address of E.J.B.: Estacion Experimental del Zaidin, C.S.I.C., Granada, Spain.

Received for publication January 17, 1985.