Abstract:

Objective: In order to deepen our understanding of the cell wall biosynthesis and mechanical failure of rattan cane (Calamus simplicifolius, an important commercial rattan species in China), cell wall ultrastructure and polysaccharides composition of the middle lamella (cell corner middle lamella, ccml, and compound middle lamella, cml) were investigated, which will provide theoretical instruction for the toughening of rattan cane and manufacture of rattan based composites. Method: The rattan cane sections delignified by acid sodium chlorite was visualized by TEM(transmission electron microscope) and confocal Raman microscopy in order to reveal the ultrastructure and polysaccharides composition in middle lamella areas. Result: TEM images revealed that the fiber and parenchyma wall was divided into middle lamella (ccml and cml), primary wall and alternating broad and narrow layered secondary wall. After partial delignification, fibrillar structures at the ccml of fiber were observed, while the cml was detached from the adjacent secondary wall. The cml area of parenchyma was swelling and distorted. High spatial resolution confocal Raman microscopy was used to visualize the lignin, cellulose, xylan and pectin distribution. The secondary wall and middle lamella of fiber and parenchyma was easily differentiated by integrating over the band regions from 2 789-3 000 cm^-1 (C-H stretching). By integrating over the lignin band at 1 598 cm^-1, it was found that the lignin located at middle lamella areas between fiber and parenchyma was more difficult to be removed relative to that between fibers, while there existed large number of lignin within the secondary wall, probably due to the linkage between lignin and polysaccharides by ether and ester bonds. By integrating over the cellulose, xylan and pectin band at 380 cm^-1, 1 318 cm^-1 and 854 cm^-1, it was found that cellulose and xylan were uniformly distributed within the fiber and parenchyma secondary wall, while pectin mainly accumulated within the middle lamella areas, especially the cml between parenchyma. Conclusion: The ultrastructure and polysaccharides composition of the middle lamella in rattan cane was investigated by combining in TEM and confocal Raman microscopy. Entangled fibrillar structure was visualized in the delignified ccml of fiber, while parenchyma displayed swelling cml and enlarged gap between adjacent cells. Furthermore, Raman images revealed that there was abundant pectin and small amounts of xylan in the middle lamella, while cellulose homogeneously deposited within in the secondary wall.

Key words: Calamus simplicifolius, middle lamella, ultrastructure, polysaccharides composition