Soluble N Compounds in Trees Exposed to High Loads of N: A Comparison of Spruce (Picea abies) and Beech (Fagus sylvatica) Grown Under Field Conditions

During the vegetation period 1994, the nitrogen (N) composition and contents of leaves, xylem sap and phloem exudates of twigs from a coniferous (Picea abies (L.) and a deciduous (Fagus sylvatica L.) tree species were analysed at a field site (Hoglwald, Germany) exposed to high loads of N. In April, total soluble non-protein N (TSNN) in the xylem sap of twigs from spruce and beech reached a maximum of about 7.0 and 15.0 μmol ml-1, respectively, probably owing to remobilization of stored N. After bud break, TSNN in the xylem sap of both tree species decreased rapidly, to a minimum of c. 2.0 μmol ml-1in July. Subsequently, TSNN increased two-fold in the xylem sap of beech until September, but remained constant in the xylem sap of spruce at the low level observed in July. In both tree species, amides were the predominant TSNN compounds transported in the xylem. In xylem sap of beech, Gln, Asn and Arg were most abundant, together comparing more than 60% of TSNN. Gln and Asp, but not Arg, prevailed in the xylem sap of spruce, together comparing more than 50% of TSNN. In addition, a number of other proteinogenic and non-proteinogenic amino acids, and also nitrate and ammonium, were detected in small amounts in the xylem saps of both tree species. Remobilization of stored N during growth of the new flush resulted in a 4.5-fold decrease of TSNN in previous year's needles from 9.1 μmol g-1f. wt in April to 2.0 μmol g-1f. wt in May. This decrease was mainly attributed to Arg, the prevailing amino compound in needles of spruce. Subsequently, a 5.5-fold increase was observed in July. High TSNN contents remained constant until September. Within current year's needles TSNN remained relatively constant during the entire vegetation period. In leaves of beech, TSNN amounted to c. 5.6 μmol g-1f.wt from April to July and increased to 9.5 μmol g-1f.wt in September. In April and May the predominant amino compounds in leaves of beech were Asn and Glu, in September they were Arg and Gln. Concomitant with the increase in Arg content of the leaves, its content in the phloem also increased, suggesting a transport of Arg from the senescencing leaves to storage tissues. Nitrate was found neither in needles nor in leaves, whereas ammonium amounted to up to 25% of TSNN in both needles and leaves. In phloem exudates of twigs from beech, TSNN amounted to c. 34.0 μmol g-1f.wt in April and decreased 10-fold until May, during growth of the new flush. Subsequently, until September a threefol