VARIATIONS IN THE AMINO-ACID-COMPOSITION OF XYLEM SAP OF BETULA-PENDULA ROTH. TREES DUE TO REMOBILIZATION OF STORED N IN THE SPRING

Seasonal patterns of N translocation in the xylem sap of Betula pendul a mere studied, to determine whether specific amino acids mere recover ed in spring as a consequence of N remobilization, Seedlings were grow n in sand culture and provided with (NH4NO3)-N-15-N-15 (at 2.2 atom pe rcent excess) for one growing season. The following winter dormant tre es were transplanted into fresh sand and given N at natural abundance thereafter, Destructive harvests mere taken during bud burst and leaf growth to determine the pattern of N-15 remobilization and N uptake, a long with isolation of xylem sap for analysis of their amino acid prof iles and N-15 enrichment by GC-MS. N-15 remobilization occurred immedi ately following bud burst, while N derived from root uptake did not ap pear in the leaves until 12 d after bud burst. During N remobilization there was a 10-fold increase in the concentration of N in the xylem s ap, due predominantly to increases in citrulline and glutamine, The N- 15 enrichment of these two amino acids demonstrated the increase in th eir concentration in the xylem sap following bud burst was due to N re mobilization, These results are discussed in relation to measuring N r emobilization and storage capacity of trees in the field.