Mobilization of protein nitrogen in the spring was studied in bark of stems of unringed and double stem-ringed apple rootstocks M.7 given different nitrogen treatments.A ready protein hydrolysis occurred; the proteins contributed the greater part of the storage nitrogen exported to tlie growing parts. Protein hydrolysis was little affected by the suppjy of newly absorbed nitrogen. Movement of nitrogen out of the bark between the rings could not be demonstrated. Protein breakdown in the isolated bark sections was slightly reduced.Arginine was the predominant amino acid in the proteins of the trees with a high level of storage nitrogen but was not conspicuous in the low-nitrogen trees. The protein composition changed little during hydrolysis. Only the share of arginine in the Hgh-nitrogen trees dropped appreciably. It is suggested that the high-nitrogen trees possess a special storage protein characterized by a high arginine content.Analysis of the nitrogen fraction of isolated bark sections stowed that the composition of the soluble nitrogen was characterized by a high level of asparagine and especially of arginine» and was qtiite different from the composition of the proteins. The data suggest that the asparagine in particular originated largely from transformation of the various amino acids set free during protein hydrolysis.

Apple rootstocks M.7 were given either nitrate or ammonium at the end of September. Until the following June, total and protein nitrogen and the composition of the soluble nitrogen fraction were followed in the roots, stem‐bark, and new growth. Nitrogen from both sources was readily absorbed in roughly the same amounts. Absorption occurred in the autumn and especially in the following spring. Incorporation of the absorbed nitrogen took place exclusively in the young roots. Arginine and, to a lower degree, asparagine were by far the most abundant of the soluble amino compounds and reached levels far above those in the unfertilized trees. The asparagine/arginine ratio was rather high during the first few weeks after fertilization but then dropped steadily to a low winter level. During May and June there was a gradual decrease to very low levels, especially in the new growth. The data suggest that the absorbed nitrogen is incorporated into asparagine and that arginine is formed from asparagine if the supply of nitrogen exceeds the immediate needs. Throughout the experimental period, ammonium fertilization led to higher values of the asparagine/arginine ratio than did nitrate nutrition. This is explained in terms of an enhanced production of asparagine and a retarded conversion into arginine. It is argued that asparagine is the main translocation compound for nitrogen. The possibility is discussed that, in addition, arginine moves upwards by a process of exchange along the negatively charged walls of the xylem vessels.

No abstract

The process of mobilization of nitrogenous compounds in trees during spring development was studied in short isolated shoot sections (usually bearing one bud each) of Golden Delicious apple trees. During leafing‐out of the bud, changes in the amounts of total, protein and soluble nitrogen and of soluble amino acids and amides in bark and wood were followed. The nitrogen required by the growing parts came mainly from protein breakdown in the tissues below the bud; in the tissues above the bud, total nitrogen decreased little, whereas the drop in protein nitrogen was considerable. In de‐budded sections and in internode sections where total nitrogen remained almost unchanged, protein hydrolysis occurred as well. It is concluded that the protein breakdown is not strongly dependent on the demand of the bud for nitrogen. Inversion of the sections did not result in any change in the pattern of nitrogen mobilization: a marked drop occurred in the nitrogen content of the physiologically basal part of the section and only a slight decrease in the apical part. The translocation of stored nitrogenous compounds to the growing parts seems to occur in the phloem, at least over short distances. Asparagine and arginine were found to be the major components of the soluble amino‐nitrogen fraction throughout. The relative importance of asparagine was reduced in tissue where a substantial loss of nitrogen occurred during leafing‐out of the bud. This is explained in terms of a preferential export of asparagine to the bud.

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.