Effect of Time of Nitrogen Application on Amino‐Nitrogen Composition of Roots and Xylem Sap of Apple

Abstract Apple rootstocks M.7 were given a nitrogen application either in the spring or in the preceding autumn. At the time of the spring application some rootstocks were ringed. During the 50‐day experimental period from bud‐break, shoot growth and the amount of nitrogen incorporated into the new shoots were slightly reduced in the spring‐treated trees and strongly reduced in the ringed trees of both treatments. Roots of unringed autumn‐fertilized trees showed higher levels of total and amino nitrogen than those of similar trees in the spring treatment; to a lesser degree, the reverse held for xylem sap from the stem. Ringing increased the amino‐nitrogen level in the roots, which suggests a reduced translocation rate. The nitrogen treatments led to marked differences in the percentage composition of the amino‐nitrogen fraction of roots and xylem sap. The distribution of amino acids and amides in the roots and that in xylem sap of the same trees was divergent, but arginine and asparagine often were the most important constituents. Aspartic acid was rather abundant in xylem sap. Ringing did not affect the composition of the amino‐nitrogen fraction in the roots quantitatively but increased the proportion of arginine in the sap. The possible relationship between the composition of xylem sap and soluble nitrogen in the roots is discussed. It is argued that especially in spring‐fertilized trees appreciable amounts of nitrogen must be translocated via the phloem in addition to the transport in the xylem.