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 co ntents of leaves, xylem sap and phloem exudates of twigs from a conife rous (Picea abies (L.) and a deciduous (Fagus sylvatica L.) tree speci es were analysed at a field site (Hoglwald, Germany) exposed to high l oads of N. In April, total soluble non-protein N (TSNN) in the xylem s ap of twigs from spruce and beech reached a maximum of about 7.0 and 1 5.0 mu mol ml(-1), respectively, probably owing to remobilization of s tored N. After bud break, TSNN in the xylem sap of both tree species d ecreased rapidly, to a minimum of c. 2.0 mu mol ml(-1) in July. Subseq uently, TSNN increased two-fold in the xylem sap of beech until Septem ber, but remained constant in the xylem sap of spruce at the low level observed in July. In both tree species, amides were the predominant T SNN compounds transported in the xylem. In xylem sap of beech, Gin, As n 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, toge ther comparing more than 50% of TSNN. In addition, a number of other p roteinogenic and non-proteinogenic amino acids, and also nitrate and a mmonium, were detected in small amounts in the xylem saps of both tree species. Remobilization of stored N during growth of the new flush re sulted in a 4.5-fold decrease of TSNN in previous year's needles from 9.1 mu mol g(-1) f. wt in April to 2.0 mu mol 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 i n July. High TSNN contents remained constant until September. Within c urrent year's needles TSNN remained relatively constant during the ent ire vegetation period. In leaves of beech, TSNN amounted to c. 5.6 mu mol g(-1) f. wt from April to July and increased to 9.5 mu mol 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 conte nt in the phloem also increased, suggesting a transport of Arg from th e 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, T SNN amounted to c. 34.0 mu mol g(-1) f. wt in April and decreased 10-f old until May, during growth of the new flush. Subsequently, until Sep tember a threefold increase in TSNN was observed. In phloem exudates o f twigs from spruce, TSNN decreased from 13.0 mu mol g(-1) f. wt in Ap ril to 4.8 mu mol g(-1) f. wt in May. By September, TSNN contents were doubled. In April, Arg was the prevailing amino acid in phloem exudat es of beech. Concomitant with the decline in total TSNN the Arg conten t decreased. In September a significant increase in the Arg content wa s observed. In phloem exudates of spruce, Gln and Arg were the predomi nant N compounds in April and May. In July and September a decrease of the Gln content to below 5% of TSNN and an increase of the Arg conten t to c. 60% of TSNN was observed. In addition, phloem exudates of both tree species contained considerable amounts of non-proteinogenic amin o compounds. Besides organic N compounds, nitrate and ammonium were de tected in phloem exudates of both tree species. Apparently, at high lo ads of atmospheric N, N metabolism in the crown is dominated by organi c 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.