Variation Among Alfalfa Genotypes for Reduced Nitrogen Concentration1

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Abstract
Kjeldahl N concentration was measured in ‘African’ and ‘Hairy Peruvian’ germplasm sources of alfalfa (Medicago sativa L.) grown first with N2 and then with 8 mM NH4NO3 to assess variation in shoot protein content for possible improvement of alfalfa quality. Shoot N concentration was determined on 120 seedlings from each germplasm source grown without combined N and inoculated with Rhizobium meliloti Dang. strain 102F28. Each plant shoot was harvested when the first flower opened during three successive growth cycles. After the second harvest following growth on N2, plants again were grown to flowering with 8 mM NH4NO3, and no significant contribution to N content was detected by the C2H2‐reduction assay. Although herbage dry weight and percent N generally were correlated negatively, it was possible to select two groups of plants from each germplasm source: I) high percent N in shoots on both sources with dry matter production near the average of the unselected population, and II) low percent N in shoots on both N sources with dry matter production near the average of the unselected population. Herbage of plants selected for high percent N averaged 13 to 16 % greater crude protein concentration on the two N sources than the original population and 30 to 35 % greater crude protein concentration than seedlings selected for low percent N. Ramets established from three seedlings within each selected population were tested separately for percent N content on each N source in two trials. All ramets within a N treatment produced similar herbage yield, and genotypes selected for high percent N had significantly higher shoot N concentration on both N sources than genotypes selected for low percent N. The results show that when N nutrition is controlled carefully by separating combined N and Rhizobium effects, a small percentage of alfalfa genotypes with superior crude protein content and normal dry matter production can be identified.
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