SOLUBLE N-COMPOUNDS IN TREES EXPOSED TO HIGH LOADS OF N - A COMPARISON BETWEEN THE ROOTS OF NORWAY SPRUCE (PICEA-ABIES) AND BEECH (FAGUS-SYLVATICA) TREES GROWN UNDER FIELD CONDITIONS

During the growing session of 1995, the total soluble non-protein nitr ogen (TSNN) composition and contents of mycorrhizal fine roots, xylem sap and phloem exudates of roots from a coniferous (Picea abies L.(Kar st)) and a deciduous (Fagus sylvatica L.) tree species were analysed a t a field site ('Hoglwald', Germany) exposed to high loads of N. In Ap ril, TSNN in fine roots of spruce and beech trees amounted to 16 mu mo l N g(-1) f. wt and 23.3 mu mol N g(-1) f. wt, respectively. It decrea sed to 9.2 mu mol N g(-1) f. wt and 18.1 mu mol N g(-1) f. wt, respect ively, after bud break in June. The seasonal maximum of TSNN in fine r oots of spruce was observed in July (32.7 mu mol N g(-1) f. wt) follow ed by a decline of c. 30% until the end of the growing season in Septe mber. TSNN in fine roots of beech trees showed a further decline betwe en June and July, when its seasonal minimum was determined (15.6 mu mo l N g(-1) f. wt), and increased to c. 29 mu mol N g(-1) f. wt until Se ptember. In spruce roots Gln and Arg were the most abundant TSNN compo unds during the entire growing season. In roots of beech Asn played an important role alongside Gin and Arg, especially in April, when it wa s the most abundant TSNN compound. Other proteinogenic and non-protein ogenic N compounds comprised c. 20-30% of TSNN. Nitrate made up < 1%, and ammonium < 7% of TSNN in the fine roots of both species. In April, TSNN in the xylem sap of roots of spruce and beech trees amounted to 3.4 and 8.6 mu mol N ml(-1), respectively. In roots of spruce trees xy lem sap TSNN increased after bud break up to 12.7 mu mol N ml(-1) in J uly At the end of the growing season TSNN had declined again to 3.9 mu mol N ml(-1). TSNN in the root xylem sap of beech trees decreased aft er bud break until July (2.4 mu mol N ml(-1) in July) followed by a sl ight increase until September (2.9 mu mol N ml(-1)). Arg, Gln and Asp were the most abundant TSNN compounds in the xylem sap of spruce trees contributing together c. 90% to TSNN. The same TSNN compounds prevail ed in the root xylem sap of beech trees in April and July, whereas in June and September Asp was replaced by Asn comprising 57% of TSNN in J une. In addition to the N compounds mentioned above, a number of other proteinogenic and non-proteinogenic amino compounds were found in roo t xylem sap of both species. In either species, nitrate and ammonium w ere present in small amounts, contributing < 1% and < 4% to TSNN, resp ectively. Apparently, inorganic N taken up by the mycorrhizal roots is mainly assimilated in root tissues or by the mycorrhiza and N uptake by the roots is largely adapted to the assimilatory capacity of this o rgan. In phloem exudates of spruce roots, TSNN amounted to 10.7 mu mol N g(-1) f. wt in April, increased in June to 23.4 mu mol N g(-1) f. w t and decreased again until September to a seasonal minimum of 4.8 mu mol N g(-1) f. wt. In contrast to spruce, TSNN content in phloem exuda tes of beech roots showed a seasonal maximum (c. 20 mu mol N g(-1) f. wt) in April with a subsequent decrease in June after bud break (c. 2 mu mol N g(-1) f. wt). A fourfold increase in July was followed by a d ecrease in September, when TSNN in phloem exudates of beech roots amou nted to 4.3 mu mol N g(-1) f. wt. Arg was the most abundant N compound in the phloem of roots from spruce trees and made up c. 60-85 % of TS NN during the entire growing season. In beech trees the seasonal cours e of TSNN correlated with the relative abundance of Arg. Arg comprised 69 and 57%, of TSNN in April and July, respectively, but contributed < 20% in June and September. Besides Arg, other proteinogenic and non- proteinogenic amino compounds could be detected in the phloem of both species. In addition, nitrate and ammonium were present in considerabl e amounts. From these results and a previous report on TSNN in above-g round parts of spruce and beech at the same site, a whole-plant model for the cycling of TSNN in both species is proposed. Differences in th e location of storage pools are assumed to be responsible for the diff erences in the seasonal course of TSNN composition and contents observ ed between the two tree species.