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Photosynthate Partitioning in Plants of Alfalfa Population Selected for High and Low Nodule Mass¹

Alfalfa (Medicago sativa L.) N₂ fixation can be improved by recurrent selection for associated characters, especially nodule mass. Because of the interdependence of nodule metabolism and shoot CO₂ assimilation, selection for nodule mass might have correlated effects on photosynthate partitioning to dry matter. This possibility was examined by investigating leaf CO₂ assimilation, dry matter distribution among organs, and photosynthate partitioning in alfalfa plants from experimental populations developed for high and low nodule mass. Experimental populations with low (LNM), intermediate (INM), and high (HNM) nodule mass/plant were developed by crossing two subpopulations produced from two different germplasm sources. The subpopulations had been developed for high and low nodule mass by two cycles of divergent recurrent selection. Glasshouse grown plants at two stages of ontogeny were radiolabeled with ¹⁴CO₂ and analyzed for allocation of radiolabel. Selection for HNM yielded plants with larger organs than those of the INM control, and selection for LNM yielded plants with smaller organs. Compared to the LNM population, plants from the HNM population had 40% greater nodule mass, 29% greater leaf mass, 34% greater mass of crown shoots, 35% greater crown mass, 41% greater root mass, and 35% greater phytomass. Despite the direct response of nodule mass and the correlated responses of other organs to selection for nodule mass, the relative distribution of whole-plant dry matter among organs was unaffected by selection. Furthermore, selection for nodule mass did not materially change the partitioning of photosynthate to dry matter among organs of the experimental populations, leaf CO₂ assimilation, or export of photosynthate by labeled leaves. The results indicate that photosynthate partitioning was unaltered by selection of experimental populations differing by 40% in nodule mass. The results also indicate that selection of alfalfa for greater N₂-fixation capability by selection for nodule mass may maintain or increase yields without changes in C allocation.

Key Words: Physiological traits • Source-sink relations • Radiotracer • Correlative control • Organ growth • Transport • Dinitrogen fixation • Carbon dioxide exchange • Medicago sativa L.

² Former research assistant, Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, currently, assistant professor, Dep. of Soil and Crop Sciences, Texas A & M Univ., College Station, TX 77843; plant physiologist; and research geneticist, USDA-ARS, Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, 1991 Buford Circle, St. Paul, MN 55108.

Received for publication May 24, 1986.

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