Two Cycles of Divergent Selection for Root Architecture in AlfalfaJ. F. S. Lamb, D.K. Barnes, and K.I. HenjumFertilizer N is the single most expensive input in most crop productions systems and has been implicated in declining ground water quality due to nitrate contamination. Improvements in N cycling and efficient use of symbiotically -fixed N can have a marked effects on the economic and environmental impact of agricultural systems. A major limitation to better management of symbiotically-fixed N from alfalfa is the lack of varieties with specific characteristics that influence the N cycle. Johnson (1992) showed that modern alfalfa varieties (released after 1980) had little variability for root morphological traits. Most alfalfa varieties were tap rooted with a few secondary roots and a small amount of fibrous roots. Older varieties (released before 1970) and Plant Introductions showed a large amount of variability for the number of secondary roots, the amount of fibrous root mass, and taproot diameter. The objective of this study was to evaluate experimental alfalfa germplasms that have undergone two cycles of divergent selection for root morphological traits for the number of secondary roots, amount of fibrous root mass and taproot diameter. Four experimental alfalfa germplasm sources were evaluated after two cycles of divergent selection for root architecture. MN NDRN (dormancy=8), and MN FLEM (dormancy=5) were selected for few (T, taproot) vs. many (B, branched) secondary roots. MN NCPL and MN MWNC (dormancy=3) underwent two cycles of selection for no or few (LF, low fibrous) vs. many (HF, high fibrous) fibrous roots and one cycle of selection for few (T) vs. many (B) secondary roots. Selected and unselected populations were established in two experiments at both Becker, and Rosemount, MN in May 1994. The experimental design was eight replications of a randomized complete block with a split plot arrangement of the treatments, with fertilizer rates (0 kg N ha-1 and 200 kg N ha-l) as whole plots and alfalfa populations as subplots. One experiment at each location was dug in October 1994 and evaluated for taproot diameter (measured in mm), number of secondary roots (scored, 1= few, 5= many), and amount of fibrous roots (scored, 1= few, 5=many). The remaining experiment at each location will be evaluated in 1995. MN NDRNBC2 and MN FLEMBC2 had more secondary roots while MN NDRNTC2 and MN FLEMTC2 had fewer secondary roots than their unselected parent populations. The branched selections had more fibrous roots than the unselected population in the MN NDRN source, but there was no difference in fibrous root score among the MN FLEM populations. MN MWNCHFC2B and MN NCPLHFC2B had more fibrous and secondary roots when compared to there unselected parent populations. MN MWNCLFC2T was not significantly different from the unselected parent population for fibrous or secondary root score. MN NCPLLFC2T had the same amount of fibrous root but fewer secondary roots when compared to its unselected parent population. Preliminary results indicate that two cycles of divergent selection for root morphological traits in all four germplasm sources produced populations that were significantly different in root architecture. References:
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