J. Plant Physiol. Vol. 141. pp. 129-135 (1993) Introduction Nitrogen Pools in Taproots of Medicago sativa L. After Defoliation KERRY L. HENDERSHOT! and JEFFREY J. VOLENEc 2 * I Department of Agronomy, Horticulture, and Entomology, Texas Tech University, Lubbock, TX 79409, USA 2 Department of Agronomy, Purdue University, West Lafayette, IN 47907, USA * To whom all correspondence should be addressed Received March 16, 1992 . Accepted June 9,1992 Summary Alfalfa (Medicago sativa L.) accumulates organic reserves in taproots that are thought to be used as sub- strates for newly developing shoots after defoliation. Two experiments were conducted to determine if specific N pools in taproot tissues undergo depletion and reaccumulation following defoliation. In Exp. 1, bark tissues of taproots of <Hi-Phy> alfalfa had higher concentrations of total N, soluble NHrN and buffer-soluble protein than did wood tissues. Concentrations of these N pools declined in both tissues after defoliation and then reaccumulated after 21 d of regrowth. In Exp. 2, two genotypes differed in con- centration of N-containing pools, although trends following defoliation of both genotypes were similar to those observed in Exp. 1. ASP + ASN were the most prevalent of the amino acids found in bark and wood tissues, together comprising approximately 50 % of the total amino acid pool. Concentration of the ASP + ASN pool declined markedly in roots following defoliation, while concentrations of other amino acids (LEU, ILE, TYR, and PHE) increased. Characterization of buffer-soluble proteins using SDS-PAGE indicated that specific proteins with molecular masses of 15 and 19 kDa were depleted, especially in bark tissues, as soluble protein concentrations declined. The depletion of specific amino acids and certain buffer-soluble proteins from taproots during regrowth of defoliated alfalfa suggests that these N-pools may be utilized as a source of N during foliar regrowth after defoliation. Key words: Medicago sativa L., defoliation, regrowth, root reserves, storage proteins. Abbreviations: TNC = total nonstructural carbohydrates; LSD = least significant difference; NHrN = amino-N; HSt = high starch; LSt = low starch; % TP = percent of total amino acid pool; PPF = photo- synthetic photon flux; SDS-PAGE = sodium dodecyl sulfate polyacrylamide gel electrophoresis. Organic compounds in alfalfa taproots are thought to be utilized in early shoot regrowth after herbage removal. While many previous studies have shown a correlation be- tween use of TNC (primarily starch and sugars) and shoot regrowth of alfalfa (Grandfield, 1935; Hodgkinson, 1969), the contribution of nitrogenous compounds to regrowth after defoliation has not been examined in detail. Millard (1988) suggested that N storage and remobilization occurs in most herbaceous plants and therefore might be an important determinant of plant growth and development. 15N-fertiliza- tion studies of subterranean clover ( Trifolium subterraneum L.) revealed that 100 to 143 % of N present in crowns and roots turned over during four consecutive harvests (Phillips et al., 1983). While the mechanism for this phenomenon was not identified, these authors suggested that roof protein de- gradation may occur to provide the N necessary for leaf re- growth following defoliation. Recently, Culvenor and Simpson (1991) reported that much of the N found in new leaves of defoliated subterranean clover was mobilized from vegetative tissues. Nodulated roots supplied 75 % of the mo- © 1993 by Gustav Fischer Verlag, Stuttgan