Intact two-year-old Norway spruce trees were used to study the mineral element distribution within the shoot, in particular in the phloem, cambium and xylem, as well as the uptake of tracers for Mg, K and Ca via the root. The roots of seedlings, grown under controlled conditions were removed from the soil and exposed to labelling solutions containing stable isotopes as tracers. Isotope specific analysis was performed using the laser-microprobe-mass-analyzer (LAMMA, single point analysis, 1 µm lateral resolution) on thin-sections and secondary-ion-mass-spectrometry (SIMS, 24 µm lat. res.) for isotope specific element imaging on exposed surfaces of shock-frozen, cryo-substituted material. Quantitative element analysis of plant fractions was performed by optical-emission-spectrometry (ICP-OES).
The tracers were applied in a split-root double labelling experiment with two corresponding stable isotopes of Mg and Ca (25Mg or 26Mg, 42Ca or 44Ca) and addition of K (41K), thus distinguishing the two sources of uptake. Roots were split into halves and labelled in separate chambers for 7 days (side A: 25Mg and 42Ca; side B: 26Mg and 44Ca). The differential labelling in the split-root system allowed us to monitor the uptake and transport into the shoot in the presence (added to one part of the root system) and absence of Al, under otherwise identical conditions within one plant.
Both (i) the uptake of K, Mg, and Ca from the solution into the shoot, and (ii) a significant release of these elements from the root and from the shoot into the solution were detectable. In cross-sections of the shoots the cell walls of earlywood, latewood, cambium and phloem were analyzed. After 7-days of labelling approximately 6070% of the Mg- and Ca-content and about 2530% of the K-content within the cell walls originated from the labelling solutions. The upper (one-year-old) part of the shoot axis contained a slightly lower (5%) labelled fraction for Mg and Ca as compared to the lower (two-year-old) part. There was no significant radial gradient for the labelled element fractions in the earlywood, latewood, cambium and phloem and no change in total element concentration before and after labelling. Localizing analysis in shoots of four-month-old Norway spruce seedlings confirmed these findings. We interpret this as evidence of a bidirectional radial and longitudinal element transfer between xylem, cambium and phloem.
Key words: element uptake, microprobe analysis, spruce, stable isotopes.
Correspondence: Arnd J. Kuhn, Institut für Biologische Informationsverarbeitung, Forschungszentrum Jülich GmbH (KFA), D-52425 Jülich, Germany
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