| Being a bearing structure of the whole plant cell wall, bamboo fiber decides the plant body of physical, mechanical and chemical properties. Of which the unique multilamellate structure consist of broad layer and narrow layer shift leads to excellent mechanical performance. Thus further study on the ultra-microstructural and microfibril orientation of fiber cell wall is of important theoretical and practical significance both for the understanding of bamboo anatomical structure and high value-added utilization of bamboo and bionic materials design.The intermediate of internode from moso bamboo(Phyllostachys edulis(Carrière) J.Houz.)was selected. The multilayer structure and the orientation of cellulose microfibril of bamboo cell wall were observed by polarization microscopy(PLM), field emission gun-scanning electron microscopy(FEG-SEM) and atomic force microscope(AFM). Especially, the AFM was used for studying the microfibril orientation of bamboo fiber cell wall, the fine structure of the cell wall and microfibril ultrastructure. And then, the unit cell, crystallinity, size and polymorphy of cellulose microfibril in bamboo were studied by synchrotron radiation-wide angle x-ray scattering(SR-WAXS). The microfibril angle of bamboos were analyzed with X-ray diffraction(XRD), SR-WAXS and synchrotron radiation-small angle x-ray scattering(SR-SAXS), and a variety of methods were used to calculate the microfibril angle of bamboo. The final purpose was to obtain the model of bamboo fiber cell wall and ultrastructure of cellulose microfibril.The conclusions are as follows:(1) Microfibril orientations of fiber cells were tested by PLM, FEG-SEM and AFM. Microfibril angle(MFA) of each layer was roughly measured by PLM. The MFA of broad layers was relatively low, which was almost parallel to cell axis; the one of narrow layers was relatively high,which was nearly perpendicular to cell axis. It was more likely to observe wall layer number by PLM, with 2 ~ 3 broad layers of fiber cells accounted for 66% in fibrous sheath. The MFA of the local cell wall in delignified longitudinal sections can be clearly measured by FEG-SEM. Generally, AFM was a properly method that can be used in not only measuring the thickness of primary wall, intercellular layer and narrow layer, but also can be used in getting the MFA of cell wall.(2) The orientation of cellulose microfibril can be observed by AFM phase image of delignification of tangential section. The MFA were approximately 90°,60°,30° and 10°. The oblique section of bamboo fiber cell wall can be observed by AFM,but the complete structure of cell wall cannot be obtained by three-dimensional reconstruction of image; however, AFM was still used for the study on ultrastructure of the cell wall and microfibril. An unknown compound was discovered in fiber cell wall, which was connected with the pit. The internal structures of cell wall were made up of microfibril layer with good directional arrangement(cell wall sublayer), and between each sublayer was filled lignin particles with average diameter of about 18 nm. The periodic structure of sublayer was about 20 nm. Microfibril were made of single fibril or several elementary ones, and majority section of microfibril were normally composed of 3 ~ 4 elementary fibrils of which thickness distribution arranged from 15 to 20 nm. Based on above results and previous studies, a new fiber cell wall model was proposed that is with 2 growth cycle layers and sublayers was proposed. The microfibril orientations of primary wall were in a random arrangement, and the angle between microfibril and principal axis of cells is large. The thickness of primary wall mainly ranged from 50 nm to 80 nm. The MFA of the outermost layer of the secondary wall was 80° ~ 90° and the thickness was100 nm ~ 1μm. In a growth cycle layer, the MFA of outside growth cycle layer was large, about 80°, which gradually inward transformed to a broad layer with smaller MFA. Many sublayer constituted transition zones and MFA were decreased rapidly from outside to inside transition zones, from 70°, 60°, 50°, 40°, to 30°. MFA of broad layers were small which was between 5° and 20°.Transition layer had different thickness throughout the fiber cells. The thickness in the middle cells was thinner while on both ends of the spindle fiber cell was thicker, from 100 nm to 400 nm. The uniformity of microfibril orientation in cell cavity was poor and the MFA was between 30° ~ 60°.(3) The cellulose Iβ structure was identified as monoclinic unit cell with a monoclinic unit cell of a = 8.35?, b(fiber axis) = 10.38?, c = 8.02?, β = 84.99° in bamboo. Two glucose residues were arranged for the unit cell, which were also possessed by adjacent four cells. Crystals in bamboo can be determined from two-dimensional diffraction patterns in both the equatorial direction and meridional direction. Crystallinity index of bamboo were calculated from diffraction intensity data in two different methods, peak height method and peak deconvolution method. Results showed that the peak height method produces approximately 15% to 20% higher CI than that of the peak deconvolution method. The width and length were calculated to be 2.69 nm and 16.15 nm respectively. the cellulose Iβ was dominant in bamboo.(4) When cells were rotated along the longitudinal direction, results were not significant changed with 0.6T method which had limitation in calculating the average microfibril angle of bamboo. The(020) and(040) diffraction peaks of bamboo were shown respectively from XRD and SR-WAXS, and the curve profile from SR-WAXS was better than the one from XRD. The average microfibril angle of bamboo were 9.19° and 7.91° with(002) diffraction peaks and the average microfibril angle of bamboo were 11.02°, 6.37° and 11.35°(Including background) with(040) diffraction peaks. These results indicated that the microfibril angle of bamboo was low. Microfibril orientations of fibers cell were mainly parallel to the axial direction with 0° ~ 15°, and the MFA of parenchyma cells were about 60°. |
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