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Field-based Selection Method Creates Alfalfa Populations That Differ in Nitrate Nitrogen Uptake

^a USDA-ARS Plant Science Research Unit and Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, 411 Borlaug Hall, 1991 Upper Buford Cir., Saint Paul, MN 55108
^b USDA-ARS-PSRU and U.S. Dairy Forage Research Center (Minnesota Cluster), 1991 Upper Buford Cir., Rm. 439, Saint Paul, MN 55108
^c 1319 West Oakes Dr., St. Cloud, MN 56303

^* Corresponding author (JoAnn.Lamb{at}ars.usda.gov).

Sustainable agricultural systems could benefit from alfalfa (Medicago sativa L.) germplasm that reduce N losses to the ground water and decrease N fertilizer inputs. With the goal of producing alfalfa populations with altered nitrate N uptake, our objectives were to develop, assess, and refine an inexpensive selection method using Br^– as an analog for nitrate under field conditions. Two selection schemes using divergent herbage Br^– concentration and/or divergent herbage Br^– uptake, both in combination with high herbage N content, produced two Cycle 1 and four Cycle 2 alfalfa populations. We evaluated these populations for both Br^– uptake and nitrate N uptake (estimated by N derived from fertilizer, Ndff [¹⁵N-labeled]) for one regrowth period in two locations in each of 2 yr. Weak associations between Br^– uptake and Ndff were found during the establishment year, but strong correlations were found between Br^– concentration and fraction of Ndff (fNdff) (r = 0.83) and between Br^– uptake and Ndff (r = 0.85 to 0.90) when populations were evaluated during the first production year. Populations selected for high Br^– concentration or uptake had greater Br^– concentration and uptake, fNdff, and Ndff than populations selected for low Br^–. Our method of selecting alfalfa plants for differences in herbage Br^– concentration or uptake in combination with high herbage N content is the first to successfully produce alfalfa populations that differ in nitrate N uptake in a field environment.

Abbreviations: DM, dry matter • fNdff, fraction of N derived from fertilizer • Ndff, N derived from fertilizer

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Received for publication April 16, 2007.