Increased Herbage Yield in Alfalfa Associated with Selection for Fibrous and Lateral Roots

greenhouse pots was higher (r 5 0.60) under water stress than under non-stress conditions (r 5 0.49) (McIntosh A positive association between root morphology and herbage yield and Miller, 1981). Peterson (1982) reported that lateral in alfalfa (Medicago sativa L.) has been reported previously. To root number and total forage yield were moderately further investigate this association, we created populations that differ correlated in space-planted field studies at two locations in root morphology within four unrelated experimental germplasm (r 5 0.48 and 0.51), while a weaker relationship was sources. Two germplasm sources were divergently selected for lateral found between fibrous root mass and total forage yield root number, and two sources underwent divergent selection for fi(r 5 0.25 and 0.34). Pederson et al. (1984b) reported brous root mass followed by divergent selection for lateral root number. Selected and unselected populations from all germplasm sources that progenies from high root weight selections had were evaluated for herbage yield, root morphology, fall dormancy greater root diameters, root branching, and shoot response, and disease resistance. Herbage yield was evaluated using weights than progenies from low root weight selections eight replicates of a randomized complete block design with a splitin greenhouse trials. While conducting selection for inplot arrangement of fertilizer rates (0 and 200 kg N ha21) as whole creased alfalfa root yield in the greenhouse, Saindon plots and alfalfa populations as subplots. Experiments were estabet al. (1991) reported moderately high (r 5 0.71–0.76) lished twice at each of two locations in May 1994. Two herbage yield correlations between root branching and top yield. Conharvests were recorded from one experiment at each location, and trary to all other reports, Busch and Davis (1969) transplots were dug and evaluated for root traits in fall 1994. Herbage planted alfalfa into competitive stands in the field and yields were taken from the other experiment at each location twice detected no correlation between herbage yield and the in 1994 and four times in 1995, and again plots were dug and evaluated number of lateral roots. for root traits in fall 1995. All populations were evaluated for fall Several studies have reported that the number of latdormancy response in 1994 and disease resistance in 1995 according eral roots or branched roots (Burton, 1937; Macintosh to standard protocols. Populations selected for more fibrous or lateral and Miller, 1981; Peterson, 1982; Pederson et al., 1984a; roots had greater herbage yield than populations selected for no or few Johnson et al., 1996; Lamb et al., 1999) and fibrous root fibrous or lateral roots in all four germplasm sources. No differences in mass (Viands et al., 1981; Peterson, 1982; Johnson et root size or weight, dormancy, or disease resistance were found beal., 1996; Lamb et al., 1999) are heritable traits in alfalfa. tween fibrous or branch-rooted vs. taprooted populations. Selection We created unique experimental alfalfa populations for fibrous and lateral roots within these alfalfa germplasms increased that differ in fibrous root mass and/or lateral root numherbage yield in the tested environments. ber within four unrelated alfalfa germplasm sources (Lamb et al., 1999). The largest, healthiest plants displaying the root morphology trait of interest were seA goal of most plant breeding programs is to lected in creating these divergent root morphology popincrease yield. In alfalfa, forage yield increases ulations. Selecting plants in this manner may have have been mostly credited to improved disease and pest influenced herbage yield by inadvertently selecting for resistance. Changing root growth patterns in alfalfa changes in disease resistance response or root size and could possibly increase forage yield by affecting water weight. Johnson et al. (1998) reported moderate negaand nutrient-uptake capacities. Several researchers have tive correlations between fall dormancy and fibrous root investigated the association between herbage yield and mass (r 5 20.59) and lateral root number (r 5 20.42). root morphology in alfalfa. Burton (1937) reported a Changes in dormancy response may have occurred while low correlation (r 5 0.30) between branched-root type selecting for differences in root morphology and could and yield for individual plants grown in pots in a greenhave influenced herbage yield among these populations. house and in a space-planted field. McIntosh and Miller We investigated the response of total herbage yield re(1980) evaluated three alfalfa cultivars in space-planted sulting from selection for root morphology traits in alfield conditions and found that correlations between falfa. Our objectives were (i) to evaluate total herbage root branching habit and shoot weight varied from 0.27 yield in these experimental populations, (ii) to identify to 0.52. They also found that the association between any association between total herbage yield and root root branching and plant weight when evaluated in morphology traits, and (iii) to assess any changes in dormancy response or disease resistance that may have J.F.S. Lamb, USDA-ARS Plant Sci. Res. Unit and Dep. Agronomy influenced total herbage yield. and Plant Genetics, 411 Borlaug Hall, Univ. of Minnesota, St. Paul, MN 55108-6026; D.A. Samac, USDA-ARS Plant Sci. Res. Unit and MATERIALS AND METHODS Dep. of Plant Pathology, 495 Borlaug Hall, Univ. of Minnesota, St. Paul, MN, 55108-6030; D.K. Barnes, USDA-ARS retired; K.I. Henjum Development of Plant Materials Dep. Agronomy and Plant Genetics, 411 Borlaug Hall, Univ. of MinFour experimental germplasm sources were created for this nesota, St. Paul, MN 55108-6026. Joint contribution of the USDAstudy. MWNC (UMN 2892) was a composite of dormant exARS, Plant Sci. Res. Unit, and Minn. Agric. Exp. Stn. Paper no. 991131023 of the MAES scientific journal series. Received 25 May Abbreviations: BRH, branch-rooted population; HF, population high 1999. *Corresponding author ([email protected]). in fibrous root mass; LF, population low in fibrous root mass; TAP, tap-rooted population. Published in Crop Sci. 40:693–699 (2000).