From laboratory experiments with seedlings and young trees of Norway
spruce (Picea abies L. Karst.), a cycling
pool of soluble non-protein N compounds is thought to be indicative of
the N-nutritional status of trees. In order
to test whether this assumption can be transferred to mature trees grown
in the field, xylem sap and phloem
exudate were collected from spruce trees in two remote forest stands: (1)
a N-limited stand (Villingen site), and
(2) a stand where trees are sufficiently supplied with N from the soil
(Schluchsee site). Trees at these sites were
c. 80–100 (Villingen site) and c. 40–60 (Schluchsee
site) yr old. In addition to untreated control areas, one entire
watershed area at both sites was subjected to (NH2)2SO4
fertilization to the soil.
In the xylem sap of the spruce roots at both sites Gln, Asp and Arg
were the dominant total soluble non-protein
nitrogen (TSNN) compounds. In the xylem sap of the trunk and the twigs
Arg was virtually absent and Gln plus
Asp dominated TSNN. On average, TSNN in the xylem sap of trees at the Schluchsee
site was 1·5–2-fold higher
than those at Villengen. Highest TSNN contents in the xylem sap were found
during growth and development
of current year tissues, while the lowest TSNN contents were found in summer.
At the Villingen site (NH4)2SO4
fertilization caused an increase in the Gln content in the xylem sap of
all tree sections analysed as well as an increase
in the Arg content in the xylem sap of the roots. At the Schluchsee site
only a small increase in TSNN contents
of the xylem was observed, mostly in the xylem sap of the roots. In phloem
exudates TSNN contents were much
higher in trees on the Schluchsee than on the Villingen site. The seasonal
pattern of TSNN in phloem exudates
was similar to the seasonal pattern found in the xylem sap. During spring
and early summer Gln was the
predominant TSNN compound in phloem exudates, but during late summer and
autumn Arg became
predominant. At the Villingen site (NH4)2SO4
fertilization caused a significant increase in TSNN contents in
phloem exudates of twigs and roots, but at Schluchsee an increase in TSNN
was found only in phloem exudates
of the roots. At both field sites Arg that was not transported to the shoot
by xylem transport, was allocated from
the leaves to the roots by phloem transport and was cycled within the root
system by both xylem and phloem
transport. From these results it is calculated that shoot-to-root signalling
by long-distance transport of amino
compounds can also contribute to the regulation of N-nutrition of mature
spruce trees. Apparently, the internal
cycling of individual N compounds within spruce trees differs considerably.