Mechanical constraints on lignin deposition during lignification

Summary The ultrastructure of lignifying cell walls in Pinus radiata D.Don was investigated using potassium permanganate staining and transmission electron microscopy. Lignin deposition occurred at numerous discrete sites within various cell wall regions, suggesting the presence of some initiating agent at these sites. In the middle lamella region, lignin deposition occurred by addition of protolignin monomers to spherical particles of lignin. Lignification was completed by expansion of these spherical particles, initially forming irregular patterns of lignification followed by infilling of adjacent areas. In contrast, lignification in the secondary wall occurred by deposition of protolignin monomers onto the ends of expanding lignin lamellae between cellulose microfibrils leading to greatly elongated patches of lignin due to the greater rate of deposition along the microfibril axis compared to that across it. It is concluded that the cellulose matrix in which lignin deposition occurs, in the secondary wall, can exert a mechanical influence which limits the rate of lignin deposition in the direction perpendicular to the microfibril axis.     

In situ measurement of tensile elastic moduli of individual component polymers with a 3D assembly mode in wood cell walls

We attempted to measure in situ the tensile elastic moduli of individual component polymers with a three-dimensional (3D) assembly mode in the cell walls of Sugi (Cryptomeria japonica D. Don) without isolating the polymers. To prepare wood tangential slices [50 × 6 × 0.2 mm (L × T × R)] consisting of lignin with a 3D assembly mode in the cell walls, cellulose and hemicellulose were removed using the method of Terashima and Yoshida (2006) to obtain methylated periodate lignin slices. To prepare wood slices consisting of polysaccharide with a 3D assembly mode in the cell walls, lignin was removed using the method of Maekawa and Koshijima (1983) to obtain holocellulose slices. Static tensile test was applied to determine the elastic moduli of 3D lignin and 3D polysaccharide slices. The followings were revealed. The elastic modulus of the 3D lignin slices was 2.8 GPa, regardless of the microfibril angle (MFA) in the slices. The elastic moduli of the 3D polysaccharide slices with MFAs of 14°, 23°, 34°, and 42° were 18, 12, 9, and 4 GPa, respectively. The former shows that the lignin with a 3D assembly mode behaves as an isotropic substance in the cell walls, while the latter suggests that the 3D polysaccharide slice shows marked anisotropic structure in the cell wall. Despite the fact that cellulose content increased after lignin removal, values of substantial elastic modulus of the cell wall slightly decreased regardless of MFA. Following two possible reasons were pointed out for explaining this phenomenon. First, lignin removal caused an artifactual deterioration in the polysaccharide slices at the level of macromolecular aggregate. Second, rigid and fusiform-shaped cellulose crystallites are dispersed in the soft matrix of amorphous polysaccharide, and those are loosely connected to each other by the intermediary of matrix polysaccharide. Those suggest that the rigid cellulose crystallite can optimize its strong mechanical performance in the polysaccharide framework of the wood cell wall in combination with the ligninification.     

Modelling moisture-related mechanical properties of wood Part I: Properties of the wood constituents

Summary By starting with simple concepts of the molecular structure and building up through the various levels of organisation in the wood cell wall it is possible to construct a model that simultaneously predicts the variation with moisture content change of both the longitudinal Young's modulus and longitudinal shrinkage of wood. To do this it is first necessary to define the stiffness and swelling characteristics of the lignin, hemicellulose and cellulose constituents of the wood as moisture content changes. It is suggested here that it is the bound fraction of the sorbed water that is responsible for the changes in swelling stress as well as for change in stiffness in the lignin and hemicellulose. The magnitudes of the stiffness of each of the constituents appear to be quite closely circumscribed by experimental values for longitudinal Young's modulus and shrinkage of wood and it is apparent that the stiffness characteristics of the in situ constituents are compatible with available experimental evidence for extracted lignin and hemicellulose and for native cellulose.     

基因工程对林木生长表型与细胞壁组分及构造的影响研究

通过基因工程技术培养出木质素含量低、纤维素含量高和糖转化效率高以及优质的木材,对于将其定向应用于制浆造纸、生物炼制、木质建筑及装饰材料方面具有重要的研究意义。文章详细阐明了木质素和纤维素基因调控技术对转基因林木生长表型、细胞壁化学组分含量及其微区分布、组织细胞形态及细胞壁超微构造影响的研究进展,并对转基因林木今后的重点发展方向进行了展望,以期为我国定向培育优质速生人工林提供理论依据。     

Comparative study of anatomy and lignin distribution in normal and tension wood of Salix gordejecii

Fibre morphology, anatomy and ultrastructure in cell wall of Salix gordejecii normal wood were examined by transmission electron microscopy (TEM). S. gordejecii tension wood can be recognized anatomically by the presence of gelatinous (G) fibres, which contain a conspicuously thickened inner cell wall layer. TEM images showed that cell wall of S. gordejecii normal wood was typically divided into three layers including the primary wall (P), the middle lamellar (ML) and the secondary wall (S₁, S₂ and S₃). Lignin distribution was determined by using confocal laser scanning microscopy (CLSM) and transmission electron microscopy with energy dispersive X-ray analysis (TEM-EDXA). Confocal images (530 nm) of S. gordejecii normal wood showed strongly lignified CCML, and weakly lignified ML and S2 layer. Weakly lignified fibres (F) and strongly lignified vessels (V) were also detected by using CLSM. Results obtained from confocal microscopy were further confirmed by using TEM-EDXA, indicating that the ratio of lignin concentration in CCML, ML and S2 is 1.72 (1321):1.31 (1006):1 (768). Lignin distribution in tension wood is similar to that in normal wood, except for the non-lignified G layer.     

SEM and shrinkage analyses of Southern Pine wood following pyrolysis

Summary Samples of Southern Pine (Pinus spp.) charred to 250°, 300°, 350°, 400° or 600°C in a flowing nitrogen atmosphere at rates of 1°, 10° and 50°C/min were examined using the scanning electron microscope to quantify changes in cross-sectional tracheid dimensions. The disappearance of discrete cell wall layers was time-dependent as opposed to strictly temperature-dependent. Tracheid diameters decreased in response to temperature and charring rate. Double cell wall thickness values also decreased in response to charring. This shrinkage was nearly isotropic. Below 300°C, latewood cells were more stable than earlywood cells. Above 300°C, the reverse was true. This was attributed to the probable circumferential arrangement of cellulose, hemicellulose, and lignin within the cell wall. The highest rate of thermal degradation was between 300° and 350°C.The sample material was prepared by F. C. Beall and D. H. Slocum at the University of TorontoThe authors are Assistant Professor of Forestry, Professor of Biological Sciences and Professor of Forestry, respectively, at the University of Missouri-Columbia, Columbia     

Heterogeneity in formation of lignin

Summary The formation of lignin in the cell wall of compression wood of Pinus thunbergii was examined by selective radio-labeling of specific structural units in the lignin and visualization of the label in the different morphological regions by microautoradiography. Deposition of lignin in the tracheid cell wall of compression wood occurred in the order: p-hydroxyphenyl, guaiacyl and syringyl lignin, which is the same order as observed in normal wood. However, the period of lignification in the compression wood was quite different from those of normal and opposite woods. The p-hydroxyphenyl units were deposited mainly in the early stage of cell wall formation in compound middle lamella in normal and opposite woods, while in compression wood, they were formed in both the compound middle lamella and the secondary wall. The most intensive lignification was observed during the formation of the S₂ layer, proceeding from the outer to inner S₂ layers for a long period in compression wood. In the normal or opposite woods, in contrast, the lignification became active after formation of S₃ had begun, then proceeded uniformly in the secondary wall and ended after a short period.A part of this report was originally presented at the 1989 International Symposium on Wood and Pulping Chemistry at Raleigh, NC, U.S.A.     

A histological investigation of Oriental beech wood decayed by Pleurotus ostreatus and Trametes versicolor

Chemical, light and electron microscopic studies were carried out on wood of Oriental beech (Fagus orientalis Lipsky) decayed by the white‐rot fungi Pleurotus ostreatus and Trametes versicolor for 30, 60 and 120 days according to the modified European standard EN 113. Mass loss as well as lignin, cellulose and carbohydrate content were determined before and after fungal attack. There were no significant differences of wood mass loss and chemical composition between both fungi at the end of incubation. After each incubation period, small specimens were stained for microscopic studies. The micromorphology of fungal cell wall degradation was rather similar for both fungi. Both decreased the cell wall thickness to the same extent. The accumulation of hyphae as well as the rupture of cell walls was also similar. The occurrence of hyphae, cavities in the pits and vessel walls followed nearly the same patterns. The parenchyma cells were completely destroyed. Altogether, both fungi produced a simultaneous white rot in Oriental beech wood.     

Nanostructural assembly of cellulose,hemicellulose, and lignin in the middle layer of secondary wall of ginkgo tracheid

Physical, chemical, and biological properties of wood depend largely on the properties of cellulose, noncellulosic polysaccharides, and lignin, and their assembly mode in the cell wall. Information on the assembly mode in the main part of the ginkgo tracheid wall (middle layer of secondary wall, S2) was drawn from the combined results obtained by physical and chemical analyses of the mechanically isolated S2 and by observation under scanning electron microscopy. A schematic model was tentatively proposed as a basic assembly mode of cell wall polymers in the softwood tracheid as follows: a bundle of cellulose microfibrils (CMFs) consisting of about 430 cellulose chains is surrounded by bead-like tubular hemicellulose-lignin modules (HLM), which keep the CMF bundles equidistant from each other. The length of one tubular module along the CMF bundle is about 16 ± 2 nm, and the thickness at its side is about 3–4 nm. In S2, hemicelluloses are distributed in a longitudinal direction along the CMF bundle and in tangential and radial directions perpendicular to the CMF bundle so that they are aligned in the lamellae of tangential and radial directions with regard to the cell wall. One HLM contains about 7000 C₆-C₃ units of lignin, and 4000 hexose and 2000 pentose units of hemicellulose.     

Analysis on residue formation during wood liquefaction with polyhydric alcohol

Liquefactions of cellulose powder, steamed lignin, alkali lignin, and their mixtures were carried out to analyze the reaction process of wood using polyhydric alcohol. The liquefaction of wood proceeded immediately and wood components were converted to N,N-dimethylformamide (DMF)-soluble components. After that, the condensation reaction occurred with increasing reaction time. However, none of cellulose powder, steamed lignin, and alkali lignin condensed by themselves during their liquefaction. The mixture of cellulose and lignin was also liquefied, and condensed after a long reaction time. The results of analysis showed that the behavior of the mixture resembled that of wood with respect to molecular weight distribution and the main functional groups. Lignin was converted to DMF-soluble compounds in the initial stage of wood liquefaction, followed by cellulose gradually being converted into soluble compounds. After that, condensation reactions took place among some parts of depolymerized and degraded compounds from cellulose and lignin, and were converted into DMF-insoluble compounds. It was concluded that the rate-determining step of wood liquefaction was the depolymerization of cellulose. Furthermore, it was suggested that the condensation reaction was due to the mutual reaction among depolymerized cellulose and degraded aromatic derivatives from lignin or due to the nucleophilic displacement reaction of cellulose by phenoxide ion.Part of this report was presented at the 52nd Annual Meeting of the Japan Wood Research Society, Gifu, April 2002     

Heterogeneity of lignin concentration in cell corner middle lamella of white birch and black spruce

Summary Raman microprobe spectroscopy was used to study the concentration of lignocellulosics in the cell corner middle lamella. Spectra obtained from 1.6 m regions, from 30 cell corner middle lamellae of both birch and spruce, showed the presence of lignin. However, the relative concentration of lignin to cellulose varied considerably. These results corroborate the view expressed in previous reports of the need for caution in using the lignin concentration values of cell corner middle lamella as a internal reference for studying the variation of lignin concentration in other morphological regions of the cell wall, such as secondary cell wall layers.The use of trade or firm names in this publication is for reader information and does not imply endorsement by the U.S. Department of Agriculture of any product or service     

Characterization of bamboo cellulose-based green composite film by NMMO technology

Abstract Bamboo cellulose, lignin and starch have been molecularly blended to form homogeneous composite films by NMMO-technology. The structural properties of the films were investigated with FTIR, WAXD and AFM technologies, respectively. The results show that bamboo cellulose, lignin and starch have been molecularly blended through NMMO-technology. There was a two-phase system consisting of ternary composite components as one phase and pores as the other on the surface of the composite film. Because of the existence of homogeneous phase structure formed by the rearrangement of the natural polymer molecules, the film shows good properties originating from the mutual supplement of different natural components. __________ Translated from Journal of Bamboo Research, 2007, 26(3): 32–36 [译自: 竹子研究汇刊]     

Micromorphological characteristics of bamboo (<Emphasis Type="Italic">Phyllostachys pubescens</Emphasis>) fibers degraded by a brown rot fungus (<Emphasis Type="Italic">Gloeophyllum trabeum</Emphasis>)

The decay pattern in bamboo fibers caused by a brown rot fungus, Gloeophyllum trabeum, was examined by microscopy. The inner part of the polylaminate secondary wall was degraded, while the outer part of the secondary wall remained essentially intact. Degradation in bamboo fiber walls without direct contact with the fungal hyphae was similar to wood decay caused by brown rot fungi. Degradation in polylaminate walls was almost confined to the broad layers whereas the narrow layers appeared resistant. The p-hydroxylphenyl unit lignin in middle lamella, particularly in the cell corner regions, was also degraded. The degradation of lignin in bamboo fibers was evidenced by Fourier transform infrared spectra. The present work suggests that the decay of bamboo fiber walls by G. trabeum was influenced by lignin distribution in the fiber walls as well as the polylaminate structures.     

Decomposition behavior of woody biomass in water-added supercritical methanol

The chemical conversion of Japanese beech (Fagus crenata Blume) in water-added supercritical methanol was studied for a wide range of water content using a batch-type reaction vessel to obtain chemicals from lignocellulosics. It was consequently found that addition of water enhanced the decomposition of wood cell wall components; cellulose, hemicelluloses, and lignin. In cases of high water content, however, it resulted in low solubility of lignin-derived products causing an increase in the mass of the residue. The water content was thus optimized to be around 10% (v/v) for the decomposition of wood. Concomitantly, the yields and selectivity of the chemicals from wood could be regulated by the addition of water, especially for the lignin-derived products. As a result, the monomeric compounds of lignin, coniferyl alcohol and sinapyl alcohol, were recovered as their γ-methyl ethers in the presence of water in higher yields than those obtained without addition of water.     

Lignin deposition during earlywood and latewood formation in Scots pine stems

Lignin deposition at consecutive secondary wall thickening stages of early and late xylem cells during annual ring wood formation in Scots pine (Pinus sylvestris L.) stems was studied. Lignin patterns, isolated by thioglycolic acid method, consisted of alcohol-soluble (LTGA-I) and alkali-soluble (LTGA-II) fractions. The sum of two fractions, being the total lignin content, gradually increased in the course of lignification. However, the increments of lignin amount at each development stage of early and late tracheids were different. The intensity of lignin deposition increased in the course of earlywood tracheid maturation and decreased toward the end of latewood cell differentiation. The deposition of two lignin fractions in each layer of forming wood also occurred oppositely. The increment of LTGA-I descended, whereas that of LTGA-II increased from the beginning to the end of early xylem lignification. In contrast, LTGA-I increment dropped, whereas LTGA-II rose during late xylem lignification. Gel permeation chromatography showed that the lignins, formed at the beginning of lignification, were more homogeneous and had higher molecular weight compared with the lignins at the end of cell differentiation. Besides, the content of cellulose, estimated as the residue after lignin isolation, and of cell wall substances, presented as cell wall cross-section areas, at consecutive maturation stages of early and late xylem cells have been found to be different. The data show that lignin deposition occurred in different conditions and with opposite dynamics during early and late xylem formation.     

Microstructural and microchemical studies of wood of Swietenia mahagoni Linn. decayed by Fomes durissimus Lloyd

Histopathological studies of wood of Swietenia mahagoni Linn. decayed by Fomes durissimus Lloyd reveal the presence of mycelia in all the wood clements and considerable thinning of cell walls and tissue disintegration due to utilization of both lignin and ccllulosc at advanccd stages of decay. Microchcmical tcsts and quantitative analysis of lignin nnd ccllulosc corroborated the results of histopathological studies indicating that the fungus mainly consumes thc lignin along with some amount of cellulose.     

Modelling longitudinal elastic an shrinkage properties of wood

Summary A model was developed for estimating elastic and shrinkage properties of a softwood cell wall from the properties of its polymeric constituents: cellulose, hemicellulose and lignin. The theory of composite materials was used. Based on a literature survey, models of latewood, earlywood and compressionwood of a softwood cell wall structure were made. The model takes into account the helical winding of the microfibrils in the cell wall and it estimates the behaviour of a balanced laminated double-cell wall in which rotation is restrained by adjacent cells. The calculated elastic and shrinkage properties were compared with earlier test results and good agreement was found.     

Ultraviolet resistance and other physical properties of softwood polymer nanocomposites reinforced with ZnO nanoparticles and nanoclay

Wood polymer nanocomposites (WPNCs) based on nano-ZnO and nanoclay were prepared by impregnation of melamine formaldehyde–furfuryl alcohol copolymer, 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU), a cross-linking agent and a renewable polymer obtained as a gum from the plant Moringa oleifera under vacuum condition. Fourier transform infrared spectroscopy (FTIR) and X-ray diffractometry (XRD) studies were employed for the characterization of modified ZnO and WPNCs. The change in crystallinity index (CrI) value of the cellulose in wood and the distribution of ZnO nanoparticles in composites were determined using FTIR and XRD. Scanning electron microscopy and Transmission electron microscopy showed the presence of nanoparticles and nanoclay in the cell lumen or cell wall of wood. An enhanced UV resistance property was shown by the treated wood samples as judged by lower weight loss, carbonyl index, lignin index, cellulose CrI values, and mechanical property loss compared to the untreated wood samples. Wood polymer composites treated with 3 phr each of nanoclay, ZnO, and the plant gum showed an improvement in mechanical properties, flame-retarding properties, thermal stability, and lower water uptake capacity.     

Lignin distribution in wood from a progeny trial of genetically selected Pinus radiata D. Don

Summary Cell wall lignin distribution was assessed in Pinus radiata wood using quantitative interference microscopy. Three groups of trees were examined. Five trees, offspring of NZ clone 850-55, and five trees of NZ 850-55 crossed with several parents of the Guadelupe provenance, were compared with five trees of unselected P. radiata. Lignin concentration in the cell corner middle lamella region was significantly lower in both offspring groups of NZ 850-55 when compared with the unselected control trees. No difference in S2 lignin concentration was observed among the three groups. This result represents the first indication that variations in lignin distribution are genetically controlled in pines.The author gratefully acknowledges the assistance of Dr. C. J. A. Shelbourne, Mr. G. D. Young, FRI, and Mr. R. S. Foster, Papro (NZ), during the course of this work