On the distribution and transport of mineral elements in xylem, cambium and phloem of spruce (Picea abies [L.] Karst.)

The distribution of calcium, magnesium and potassium in the cambial region of a 20-year-old Norway spruce was determined by optical emission spectrometry (ICP-OES) in tissue samples ranging from the outer bark up to latewood and earlywood of the outer tree-rings. The highest contents of magnesium and potassium were found in the cambium and, in the case of calcium, in the developing phloem. In more detail we investigated the distribution and incorporation of calcium, magnesium and potassium in a model system of intact two-year-old plants (Picea abies [L.] Karst.). The roots of the seedlings, grown under controlled conditions, were removed from the soil and exposed to labelling solutions containing the enriched stable isotopes Mg-25, K-41, and Ca-44 as tracers. After seven days of labelling the cell walls of earlywood, latewood, cambium and phloem were analysed in stem cross sections of shock-frozen, cryo-substituted tissue by three microprobes: (i) by energy dispersive x-ray analysis (EDXA, 0.3 mu m lateral resolution), (ii) isotope specific point analysis, using the laser microprobe mass analyser (LAMMA, 1.5 mu m lat. res.) and (iii) isotope specific imaging by secondary ion mass spectrometry (SIMS, 1-3 mu m lat. res.). After seven days of labelling approximately 60-75 % of the Ca-content, 50-65 % of the Mg-content and about 25-30 % of the K-content within the cell walls of the shoot originated from the labelling solutions. The upper part(one-year-old) of the shoot axis contained a 10 % smaller fraction of labelled Ca and a 10% larger fraction of labelled Mg in comparison to the more basal regions of the shoot (two-year-old). For all three elements we could not detect any significant radial gradient of the labelled fraction while comparing earlywood, latewood cambium and phloem. We interpret our results as evidence for (i) a relevant bidirectional longitudinal transport and (ii) a radial element exchange between xylem, cambium and phloem, completed within the seven days of labelling.