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 (Höglwald, 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−1 in 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. Gin, Asn and Arg were most abundant, together comparing more than 60% of TSNN. Gin 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−l f. wt in April to 2.0 μmol g−1 f. 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−1 f, wt from April to July and increased to 9.5 μmol g−1 f. 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 Gin. 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−1 f. wt in April and decreased 10-fold until May, during growth of the new flush. Subsequently, until September a threefold increase in TSNN was observed. In phloem exudates of twigs from spruce, TSNN decreased from 13.0 μmol g−1 f. wt in April to 4.8 μmol g−1 f. wt in May, By September, TSNN contents were doubled. In April, Arg was the prevailing amino acid in phloem exudates of beech. Concomitant with the decline in total TSNN the Arg content decreased. In September a significant increase in the Arg content was observed. In phloem exudates of spruce, Gin and Arg were the predominant N compounds in April and May. In July and September a decrease of the Gin content to below 5 % of TSNN and an increase of the Arg content to c. 60 % of TSNN was observed. In addition, phloem exudates of both tree species contained considerable amounts of non-proteinogenic amino compounds. Besides organic N compounds, nitrate and ammonium were detected in phloem exudates of both tree species. Apparently, at high loads of atmospheric N, N metabolism in the crown is dominated by organic N compounds in spruce and beech. Arg can be considered as a central storage compound under these conditions. The possible origins of the N compounds in leaves, xylem and phloem are discussed.
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