Uptake and Distribution of Nitrogen in Young Apple Trees after Application of Nitrate or Ammonium, with Special Reference to Asparagine and Arginine

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.