During the growing session of 1995, the total soluble non-protein
nitrogen (TSNN) composition and contents of
mycorrhizal fine roots, xylem sap and phloem exudates of roots from a
coniferous (Picea abies L.(Karst)) and a
deciduous (Fagus sylvatica L.) tree species were analysed at
a field site (‘Höglwald’, Germany) exposed to high
loads of N. In April, TSNN in fine roots of spruce and beech trees amounted
to
16 μmol N g−1 f. wt and
23·3 μmol N g−1 f. wt, respectively. It decreased
to
9·2 μmol N g−1 f. wt and
18·1 μmol N g−1 f. wt, respectively,
after bud break in June. The seasonal maximum of TSNN in fine roots of
spruce was observed in July (32·7 μmol N g−1
f. wt)
followed by a decline of c. 30% until the end of the growing
season in September. TSNN in fine roots of beech trees showed a further
decline between June and July, when its seasonal minimum was determined
(15·6 μmol N g−1 f. wt), and increased to c.
29 μmol N g−1 f. wt until September. In spruce roots
Gln
and Arg were the most abundant TSNN compounds during the entire growing
season.
In roots of beech Asn played an important role alongside Gln and Arg, especially
in April, when it was the most abundant TSNN
compound. Other proteinogenic and non-proteinogenic 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 μmol N ml−1,
respectively. In roots of spruce trees xylem sap TSNN increased after bud
break up to
12·7 μmol N ml−1 in July. At the end of the
growing season
TSNN had declined again to 3·9 μmol N ml−1.
TSNN in the root xylem sap of beech trees decreased after bud break until
July
(2·4 μmol N ml−1 in July) followed by a slight
increase until
September (2·9 μmol 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 prevailed 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 June. In addition to the N compounds mentioned above, a number
of
other proteinogenic and non-proteinogenic amino compounds were found in
root
xylem sap of both species. In either species, nitrate and
ammonium were present in small amounts, contributing <1% and <4%
to TSNN,
respectively. 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 organ.
In phloem exudates of spruce roots, TSNN amounted to
10·7 μmol N g−1 f. wt in April, increased in
June to
23·4 μmol N g−1 f. wt and decreased again until
September to a seasonal minimum of 4·8 μmol N g−1
f. wt. In
contrast to spruce, TSNN content in phloem exudates of beech roots showed
a
seasonal maximum (c. 20 μmol N g−1 f. wt)
in April
with a subsequent decrease in June after bud break (c.
2 μmol N g−1 f. wt). A fourfold
increase in July was followed by a decrease in September, when TSNN in
phloem
exudates of beech roots amounted to 4·3 μmol 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 TSNN during the entire growing
season. In beech trees the seasonal course 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 considerable amounts.
From these results and a previous report on TSNN in above-ground 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 the location of storage
pools are assumed to be responsible for the differences in the seasonal
course of TSNN composition and contents observed between the two tree species.