RESUMO
The mechanical performance of bamboo is highly dependent on its structural arrangement and the properties of biomacromolecules within the cell wall. The relationship between carbohydrates topochemistry and gradient micromechanics of multilayered fiber along the diametric direction was visualized by combined microscopic techniques. Along the radius of bamboo culms, the concentration of xylan within the fiber sheath increased, while that of cellulose and lignin decreased gradually. At cellular level, although the consecutive broad layer (Bl) of fiber revealed a relatively uniform cellulose orientation and concentration, the outer Bl with higher lignification level has higher elastic modulus (19.59-20.31 GPa) than that of the inner Bl close to the lumen area (17.07-19.99 GPa). Comparatively, the cell corner displayed the highest lignification level, while its hardness and modulus were lower than that of fiber Bl, indicating the cellulose skeleton is the prerequisite of cell wall mechanics. The obtained cytological information is helpful to understand the origin of the anisotropic mechanical properties of bamboo.
RESUMO
MAIN CONCLUSION: 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.
