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This article in CS

  1. Vol. 36 No. 5, p. 1217-1223
    Received: June 7, 1995

    * Corresponding author(s): bjoern@dept.agry.purdue.edu
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Supplemental Nitrogen Effects on Alfalfa Regrowth and Nitrogen Mobilization from Roots

  1. L. D. Barber,
  2. B. C. Joern ,
  3. J. J. Volenec and
  4. S. M. Cunningham
  1. Dep. of Agronomy, Lilly Hall of Life Sciences, Purdue Univ., West Lafayette, IN 47907-1150



Recent studies suggest that alfalfa (Medicago sativa L.) root carbohydrate concentrations are not correlated closely with shoot regrowth, and that root N reserves may influence shoot regrowth after harvest. The objectives of this greenhouse study were to (i) determine the effects of supplemental N on alfalfa regrowth and N uptake, (ii) quantify N remobilization from roots to regrowing shoots, and (iii) determine if certain root N pools, including vegetative storage proteins (VSPs), were utilized preferentially as N sources during early shoot regrowth. Four cultivars with contrasting fall dormancy reaction were defoliated and supplied with 85 mg N (kg soil)−1 as 15N-enriched (NH4)2SO4 or KNO3. Samples were taken immediately and once weekly for 28 d. After 28 d, plants were again defoliated and shoots, crowns, and roots of one-half of the plants sampled immediately. Intact crowns and roots of remaining plants were transplanted into sand, and shoots, crowns, and roots sampled 10 d after transplanting. Taproot N from these samples was partitioned into four specific pools. Nitrate-N uptake was greater than NH4-N uptake at 7 and 28 d after N application. Ten days after transplanting, 25% of crown and root total N and 35% of crown and root 15N was remobilized to regrowing shoots. Fifty-five percent of total root N was soluble in phosphate buffer. The buffer-soluble N pool was fractionated into a low molecular weight N pool (≈30% of total root N), a non-VSP protein N pool (≈20% of total root N), and a VSP-enriched protein N pool (≈5% of total root N). The low molecular weight N and VSP-enriched protein N pools in roots were utilized preferentially as N sources for regrowing shoots 10 d after transplanting. Accumulation of these root N pools may be important for rapid early shoot regrowth after defoliation.

Journal Series no. 14741.

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