Changes in the xylem exudate composition of poplar (Populus tremula x P-alba) -: dependent on the nitrogen and potassium supply

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 NO3- to N-free medium, (c) a change in the nitrogen source from NO3- to NH4+, (d) an increase in NO3- 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 NO3- concentration dropped immediately after the transfer, indicating the dose connection between NO3- 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. (c) After transfer of the NO3--grown plants to NH4+, 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 SO42-. The N-status in the xylem exudate was maintained by an immediate increase in glutamine concentration (2-fold in 15 min), (d) Increasing the NO3- supply to 8 mM had no effect on the ionic composition or the M-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.