Subcellular quantitative determination of K and Ca in phloem, cambium, and xylem cells of spruce (Picea abies [L.] Karst.) during earlywood and latewood formation

The present investigation was directed to calibrated subcellular quantitative determination of K and Ca in phloem, cambium, and xylem cells of spruce (Picea abies [L.] Karst.) by energy dispersive X-ray spectroscopy (EDXS) in combination with transmission electron microscopy (TEM). The element content was studied during earlywood and latewood formation throughout primary and secondary wail formation and lignification. In the experimental stage, increment cores were extracted from three 100-year-old forest trees and either shock-frozen, embedded, and used for semithin cuttings, or prepared for optical emission spectroscopy (ICP-OES) for bulk analysis from tissue sections. The structural dynamics in cell formation of phloem, cambium, and xylem was expressed in terms of the radial cell diameter and the cell wall area. The lignification of the same cells was studied by subcellular UV-spectroscopy, During earlywood formation, differentiating tracheids area strong physiological sink for K. Particularly during cell enlargement, a strong increase of the symplasmatic K content (>1 ng per cell) was detected. An increase of the Ca content of differentiating cells (0.4-1 ng,a per cell) was found during secondary wall formation, whereas the Ca content of the cell wall decreased remarkably during its lignification (0.1-0.3 ng per cell). From these experiments it can be concluded that K is a driving force in cell enlargement of differentiating tracheids (primary wall phase), whereas Ca is of importance especially for wall synthesis in the secondary phase of xylem cell development and lignification.