Both G units and S units deposited in the whole lignification process of xylem fiber. The topochemical variations in newly formed xylem and phloem of Populus × euramericana were investigated by combined microscopic techniques. During xylem formation, earlier cell wall deposition in vessel and afterwards in the neighboring fiber was observed in situ. Raman images in xylem fiber emphasized that cell wall deposition was an ordered process which lignification started in cell corner following carbohydrates deposition. Higher deposition speed of carbohydrates was revealed at the beginning of the cell wall differentiation, and the syringyl (S) units deposition was more pronounced compared with guaiacyl (G) units at the earlier stage of lignification. The comparative analysis of cell wall composition in phloem fiber indicated that phloem formed earlier than xylem and the distribution of lignin monomers varied significantly with phloem fiber location. Furthermore, an interesting phenomenon was found that the outermost phloem fiber near the periderm displayed a multilayered structure with alternating broad and narrow layer, and the broad lamellae showed higher concentration of carbohydrates and S lignin. The cytological information including cell wall composition and lignin structure of xylem and phloem might be helpful to understand the wood growth progresses and facilitate utilization of woody plants.
Keywords: Cell wall deposition; Lignin monomers; Phloem; Populus × euramericana; Raman microscopy; Xylem.