On the Cytochemistry of Cell Wall Formation in Poplar Trees

The ultrastructure of cell walls and the mechanisms of cell wall formation are still not fully understood. The objective of our study was therefore to obtain additional fine structural details on the deposition of cell wall components during the differentiation of xylem cells in hybrid aspen ( Populus tremula L. × P. tremuloides Michx.) we used as a model tree. At the electron microscope level, PATAg staining revealed a successive deposition of polysaccharides with increasing distance from the cambium. Staining with potassium permanganate and UV microspectrophotometry showed that the cell walls were lignified, with some delay to the deposition of polysaccharides. Immunogold labelling of three lignin types in developing cell walls varied with progressive deposition of cell wall layers. Condensed lignin subunits were localized in corners of cells adjacent to the cambium prior to S1 formation, whereas non‐condensed lignin subunits became labelled only in later stages ‐ in secondary walls near cell corners and simultaneously with the completion of S1 formation. As S2 polysaccharide deposition progressed, the labelling extended towards the lumen. Labelling of peroxidases revealed their presence in cell corner regions of young xylem cells, still lacking a secondary wall, implying that peroxidases are incorporated into the developing cell wall at early developmental stages. A weak labelling of middle lamella regions and secondary walls could also be seen at later stages. The results are discussed in relation to current knowledge on the succession of polysaccharide and lignin deposition in woody cell walls.