NITROGEN POOLS IN TAPROOTS OF MEDICAGO-SATIVA L AFTER DEFOLIATION

Alfalfa (Medicago sativa L.) accumulates organic reserves in taproots that are thought to be used as substrates 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 NH2-N 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 concentration 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.