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 respo nd to changes in the nutrient supply with modifications in the xylem exudat e collected after decapitation was investigated with special respect to N-c ompounds. The composition (inorganic ions and amino-N) was analysed with re spect to (a) the time after decapitation at different times of day, (b) a c hange in the nitrogen concentration from NO3- to N-free medium, (c) a chang e 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 concent ration in the xylem exudate was not affected up to 15 min after decapitatio n of the plants, Later a continuous increase in the concentration was obser ved. 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, indicati ng the dose connection between NO3- uptake and xylem loading, (b) After tra nsfer to N-free medium poplar did not balance the charge in the xylem by in creasing the concentration of other inorganic anions or decreased xylem loa ding 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 N H4+, 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 enhance d xylem loading of mainly SO42-. The N-status in the xylem exudate was main tained by an immediate increase in glutamine concentration (2-fold in 15 mi n), (d) Increasing the NO3- supply to 8 mM had no effect on the ionic compo sition 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 p lant, causing a decrease in the N-status of the xylem sap. The data are dis cussed with respect to charge compensation after changes in the nutrient su pply and to the maintenance of the nitrogen status in the xylem sap.