Journal of Experimental Botany, Vol 50, 1797-1806, Copyright © 1999 by Oxford University Press
S Siebrecht and R Tischner
The capacity of poplar (P. tremula x P.
alba, clone INRA 717 1/B4) to respond to changes in the nutrient
supply with modifications in the xylem exudate collected after decapitation
was investigated with special respect to N-compounds. The composition
(inorganic ions and amino-N) was analysed with respect to (a) the time
after decapitation at different times of day, (b) a change in the nitrogen
concentration from NO-3 to N-free medium, a change
in the nitrogen source from NO-3 to
NH+4, (d) an increase in NO-3
supply from 1 mM to 8 mM, and (e) the withdrawal of
K+ supply. (a) The ion concentration in the xylem
exudate was not affected up to 15 min after decapitation of the plants.
Later a continuous increase in the concentration was observed. This
increase was large if decapitation was performed in the middle of the light
period and small at night-time. In both types of experiments (b,c) the
NO-3 concentration dropped immediately after the
transfer, indicating the close connection between
NO-3 uptake and xylem loading. (b) After transfer to
N-free medium poplar did not balance the charge in the xylem by increasing
the concentration of other inorganic anions or decreased xylem loading of
cations within 3 d of treatment. The N-status of the xylem exudate was
reduced within 15 min. After transfer of the
NO-3-grown plants to NH+4, as
the sole N-source, the charge compensation in the xylem exudate was
maintained by reducing the loading of cations, and 3 d later by an enhanced
xylem loading of mainly SO24-. The N-status in the
xylem exudate was maintained by an immediate increase in glutamine
concentration (2-fold in 15 min). (d) Increasing the
NO-3 supply to 8 mM had no effect on the ionic
composition or the N-status of the xylem exudate. (e) The withdrawal of
K+ from the medium for 11 d resulted in a limitation
of the S- and N-supply of the plant, causing a decrease in the N-status of
the xylem sap. The data are discussed with respect to charge compensation
after changes in the nutrient supply and to the maintenance of the nitrogen
status in the xylem sap.Keywords: Poplar, xylem
exudate, nitrogen metabolism
ARTICLES
Changes in the xylem exudate composition of poplar Populus tremula x P. alba)-dependent on the nitrogen
Albrecht v.Haller Institut, Untere Karspule 2, 37073 Gottingen, Germany; Corresponding author; E-mail: rtischn@gwdg.de
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