Relation between growth stress and lignin concentration in the cell wall: Ultraviolet microscopic spectral analysis

Lignin content in the cell wall was investigated to examine its relation with growth stress, using an ultraviolet microscopic spectrum analyzer. Although a weak correlation existed between the growth stress and lignin concentration in the compound middle lamella, it was believed that the compound middle lamella did not contribute to compressive growth stress generation as there was no correlation between growth stress and lignin concentration in the cell corner part of the intercellular layer. In the secondary wall, larger compressive growth stress was associated with higher lignin concentration especially in the outer part. This finding confirms that lignin contributes positively to the generation of compressive longitudinal growth stresses in the compression wood and more substantially in the outer part of the secondary wall. This experimental result strongly supports our hypothesis of growth stress generation given by the model.This paper was presented at the International Academy of Wood Science Meeting at Vancouver, Canada, July 1997     

Etching of wood surfaces by glow discharge plasma

This research tests the hypothesis that plasma will cause differential etching of wood cell walls because of variation in the susceptibility of aromatic and aliphatic polymers to degradation by plasma. Wood was exposed to glow discharge plasma, and scanning electron microscopy and chromatic confocal profilometry were used to examine etching of cell walls. Plasma etched cell walls and made them thinner, but the middle lamella was more resistant to etching than the secondary wall. Plasma created small voids within the secondary wall, which were separated by thin lamellae connected to the middle lamella and tertiary wall layers. Larger voids were created in cell walls by the etching of bordered and half-bordered pits. Etching of the uppermost layer of cells at wood surfaces occurs first and when large voids are created in the walls of these cells then significant plasma etching of the underlying cells occurs. Etching of wood cell walls can be quantified using confocal profilometry, and using this technique a strong relationship between applied plasma energy and volume of cell wall etched by plasma was observed. It is concluded that all of wood’s polymers can be degraded by plasma even though cell wall layers that are rich in lignin are etched more slowly than other parts of the cell wall.     

Formation and structure of reaction wood in Buxus microphylla var. insularis Nakai

Summary Anatomical differences in xylem between the upper and lower sides formed in the inclined stems of Buxus microphylla with different angular displacement from the vertical were examined microscopically. B. microphylla exhibited a pronounced growth promotion on the lower side of the inclined stems. Formation of tension wood (gelatinous fibers) was not observed. Xylem formed on the lower side showed some interesting features resembling the compression wood formed in gymnosperms. The reaction wood tracheids and vessels showed an excessive lignification in their secondary walls but lacked both helical cavities and an S₃ layer, features that were almost the same as those of primitive gymnosperms. These results indicate that B. microphylla has an ability to form compression wood, suggesting that in the genus Buxus a different mechanism in the conducting elements was developed in the phylogenetic evolution.The first author would like to express his sincere thanks to Dr. T. E. Timell, College of Environmental Science and Forestry, State University of New York, Syracuse, New York, for his invaluable suggestions in connection with this research     

Recent progress in the chemistry and topochemistry of compression wood

Summary A review of the chemistry and topochemistry of compression wood with 200 references. Compression wood contains on the average 30% cellulose, 35–40% lignin, 10% galactan, 9% galactoglucomannan, 8% xylan, and 2% of a 1,3-glucan (laricinan). The cellulose is less crystalline, and the xylan has fewer arabinose side chains than in normal wood. The lignin is composed of guaiacylpropane and p-hydroxyphenylpropane units. It is more condensed, has a higher proportion of carbon-carbon bonds, and contains fewer arylglycerol--aryl ether structures than a normal conifer lignin. The ray cells and the primary wall of the tracheids have the same chemical composition in normal and compression woods. The galactan is largely located in the outer region of the secondary wall. Only 5–10% of the lignin in compression wood tracheids is extracellular. The middle lamella is less lignified than in normal wood, while the S₁ and inner S₂ layers have a lignin concentration of 30–40% which is twice as high as in normal wood. The lignin content of the S₂ (L) layer is equal to or higher than that of the intercellular region along the wall. The review is concluded with a brief reference to areas where present information is incomplete or lacking.A portion of an Academy Lecture of the International Academy of Wood Science, presented at the International Symposium on Wood and Pulping Chemistry (Ekmandagarna 1981), held in Stockholm, Sweden, June 9–12, 1981. Reprints of the unabridged review, published under the title Recent Progress in the Chemistry, Ultrastructure, and Formation of Compression Wood in the preprints of the symposium (SPCI Report 38, Vol. 1, p. 99–147) are available from the author. I wish to express my gratitude to my colleague Professor Robert A. Zabel for generous travel assistance     

杨木应拉木微区结构可视化及化学成分分析

木材微区结构与木材宏观性质密切相关,杨木应拉木与对应木宏观性质存在较大差别,探究杨木应拉木和对应木微区结构和化学成分,可为了解杨木应力木的宏观性质提供理论根据。借助光学显微镜、荧光显微镜、显微拉曼成像光谱仪、透射电镜对杨木应拉木微区结构进行可视化研究,并借助X射线衍射技术和美国可再生能源实验室方法,分析杨木应拉木的微晶尺寸、结晶度以及化学成分。结果表明:杨木应拉木中应拉区和对应区纤维细胞微区结构差异显著。光学显微镜下显示应拉区木纤维中胶质层清晰可见,荧光显微镜和拉曼显微镜下显示胶质层的木质素浓度比对应区低。透射电镜下显示应拉区木纤维细胞壁结构由初生壁、次生壁和胶质层组成,未见次生壁外层,各层的平均厚度分别为0.61,1.22和2.53μm。对应区木纤维为典型的初生壁和次生壁结构,次生壁各层平均厚度分别为0.33,2.28和0.14μm。杨木应拉区纤维素含量(58.91%)比对应区(41.53%)高,木质素含量和半纤维素含量均比对应区的低,应拉区木质素和半纤维素含量分别为21.99%和12.01%,对应区分别为28.10%和17.08%。杨木应拉区结晶度(48.06%)比对应区(41.01%)高,应拉区晶区宽度为2.66 nm,长度为8.84 nm;对应区晶区宽度为2.65 nm,长度为9.87 nm。     

Comparison of transverse compression creep of Pseudotsuga menziesii and Populus sp. in high-temperature steam environments

Compression creep experiments of Douglas-fir wood (Pseudotsuga menziesii) were performed at high temperature (150°C, 160°C, and 170°C) and under various conditions of steam pressure. The results established that environment conditions had a significant effect on compressive deformation, with the largest deformation obtained under saturated steam conditions. While the temperature significantly affected the compressive deformation of specimens under transient conditions, the temperature within the range studied had little effect on the compressive deformation in saturated steam. Furthermore, in specimens compressed under superheated and transient steam conditions, primary creep behavior was exhibited; while in specimens compressed under saturated steam conditions, creep deformation appeared to enter directly into secondary creep. Moreover, in saturated steam specimens very little creep was observed due to high initial deformation and little potential for additional cell wall buckling. The compressive creep measurements of Douglas-fir were compared with compressive creep of hybrid poplar (Populus deltoides × Populus trichocarpa). Due to lower initial density, and perhaps smaller microfiber angle and lower lignin content of tension wood, the compressive creep modulus of hybrid poplar was lower than Douglas-fir. Therefore, compressive deformation of Douglas-fir, at nearly all examined steam conditions and temperatures, was smaller than compressive deformation of hybrid poplar.     

Distribution of lignin in normal and compression wood of Pinus taeda L.

Summary The distribution of lignin in normal and compression wood of loblolly pine (Pinus taeda L.) has been studied by the technique of lignin skeletonizing. Hydrolysis of the wood carbohydrates with hydrofluoric acid left normal wood tracheids with a uniform distribution of lignin in the S1 and S2 cell wall layers. However, the S3 region of both earlywood and latewood tracheids consistently retained a dense network of unhydrolyzable material throughout, perhaps lignin.Lignin content in compression wood averaged about 7% more than in normal wood and appears to be concentrated in the outer zone of the S2 layer. The inner S2 region, despite helical checking, is also heavily lignified. The S1 layer, although thicker than normal in compression wood tracheids, contains relatively little lignin.Ray cells, at least in normal wood, appear to be lignified to the same extent, if not more so in certain cases, than the longitudinal tracheids. Other locations where lignin may be concentrated include initial pit border regions and the membranes of bordered pits.This report is a detailed excerpt from the Ph. D. dissertation of R. A. P. Financial support provided by the College of Forestry at Syracuse University and the National Defense Education Act is hereby gratefully acknowledged.     

Ray tracheids in Pinus radiata are more highly resistant to soft rot as compared to axial tracheids: relationship to lignin concentration

A visual decay assessment of Pinus radiata wood, which was part of a framing timber in a house in the North Island of New Zealand, indicated the presence of surface decay. Microscopic observations, employing confocal laser scanning microscopy (CLSM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), provided evidence of decay by cavity-forming soft rot (SR) fungi. A comparison of ray tracheids (RT) with axial tracheids (AT) indicated that RT were considerably more resistant to SR than AT. In the heavily degraded regions of wood, where axial tracheid walls contained abundant SR cavities, the walls of RT contained only a few or no cavities. An assessment of lignin concentration in the cell walls by a combination of TEM, confocal fluorescence and UV microscopy provided evidence of greater lignin concentration in the secondary wall of RT as compared to AT, which may explain the observed greater resistance of RT to soft rot. Dedicated to Professor Dr. Dr. h.c. mult. Walter Liese on the occasion of his 80th birthday.     

Heterogeneity in formation of lignin—XI: An autoradiographic study of the heterogeneous formation and structure of pine lignin

Summary Selective labeling of p-hydroxyphenyl-, guaiacyl-and syringylpropane moieties in protolignin was achieved by administration of corresponding ³H-labeled monolignol glucosides to differentiating xylem of pine. The growing process of the protolignin macromolecule in the specific morphological region was visualized by application of high resolution microautoradiography to the selectively labeled wood tissue.p-Hydroxyphenyl lignin is formed mainly in the compound middle lamella and cell corner in an early stage of cell wall differentiation. There are two peaks of deposition of guaiacyl lignin in the compound middle lamella at an early stage and in the secondary wall at a late stage. The content of condensed guaiacyl units is higher in the middle lamella than in the secondary wall lignin. Syringyl lignin is formed mainly in the inner layer of the secondary wall in a late stage as a minor structural moiety. During the formation of the cell wall, protolignin grows under definite biological regulations to a heterogeneous macromolecule which consists of various structural moieties arranged in a regular manner. The origin of the heterogeneous structure was explained as a result of the biogenesis of protolignin in the cell wall.     

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.     

Topochemistry of delignification in Douglas-fir wood with soda,soda-anthraquinone and karft pulping as determined by SEM-EDXA

Summary Delignification studies on 0.5 m sections of Douglas-fir earlywood tracheids pulped by soda, soda-anthraquinone (soda/AQ) and kraft pulping processes were performed by determining bromine concentrations in various morphological regions with SEM-EDXA technique. Soda/AQ pulping was much more selective in removing lignin from the middle lamella regions than either soda or kraft pulping. However, up to 50% delignification, more lignin was removed from the secondary wall by soda or kraft, compared to soda/AQ pulping. The kinetics of lignin removal in the various morphological regions were established. Addition of AQ and sodium sulfide resulted in an earlier transition from a slow initial to a rapid bulk delignification, particularly in the middle lamella, and in an enhanced bulk delignification in the secondary wall. Anthraquinone was also found to promote residual delignification in the secondary wall, where sodium sulfide was not effective. The opposite was observed for the bulk delignification in the middle lamella, where only sodium sulfide addition improved the rate significantly. The great differences observed in the bulk delignification rates between middle lamella and secondary wall in soda pulping as well as their response to additives suggest structural differences between middle lamella and secondary wall lignins.Paper No. 6712 in the Journal Series of the North Carolina Agricultural Research Service, Raleigh, North Carolina. The paper, an excerpt from the Ph. D. dissertation of Shiro Saka, was selected for the second place Wood Award for 1980 cosponsored by Forest Products Research Society and the Kirby Forest Industries, Inc., Houston, Texas, and presented at the American Chemical Society Annual Meeting, Las Vegas, Nevada, August, 1980     

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.     

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.     

The ultrastructure of cellulose from wood

Summary During the delignification of wood several processes overlap one another. One of these is the penetration of the wood samples by the delignifying solution. Penetration tests under different conditions show that there is almost no difference in the penetration depth of wood samples penetrated by diffusion only and of wood samples treated with pressure. In both cases the pits are closed during the flow of fluids. Vacuum treated samples show better penetration and most of the pits in these samples remain open. The distribution of fluids within the cell walls takes place in the intercellular spaces, in small openings in the compound middle lamella and in the secondary wall 1 as well as in very fine pores in the secondary wall 2. The penetration of a wood sample is greatly facilitated if the sample is mechanically injured. A slowly proceeding delignification with ethylene sulphite shows that the delignification obviously starts in the S 1 layer and proceeds from there towards the compound middle lamella and the S 2 layer. In the first stages of lignin removal the compound middle lamella is also attacked, the attack beginning at the border of the pit chambers.     

Ultraviolet microspectrophotometric investigation of the distribution of lignin in Prunus jamasakura differentiated on a three-dimensional clinostat

To examine the effect of gravity on lignin content and deposition in plant cells, we used ultraviolet (UV) microspectrophotometry and chemical methods to investigate the secondary xylem of Prunus jamasakura grown on a three-dimensional (3D) clinostat, which simulates microgravity. The stem of the 3D-clinostat specimens elongated with bending and the width of their secondary phloem increased. The UV absorbance of the 3D-clinostat specimens at 278 nm was higher than that of the control specimens, which were grown on the ground, in the wood fiber cell corner middle lamella, compound middle lamella, and fiber secondary wall; the UV absorbance in the vessel secondary wall did not differ between the specimens. The lignin content in the stem, including the bark, of the 3D-clinostat specimens, as determined using an acetyl bromide method, was less than that of the control specimens. In the specimens that differentiated on a 3D clinostat, the amount of lignin in the wood fibers increased, while the proportion of the lignified xylem in the stem decreased relative to control values.     

Comparative studies on lignin distribution by UV microscopy and bromination combined with EDXA

Summary In order to elucidate a previously reported discrepancy in the ratio of the lignin concentration in the middle lamella to that in the secondary wall as determined by ultraviolet (UV) microscopy and bromination combined with EDXA, the ultraviolet absorptivity of the lignin and the lignin reactivity towards bromination were compared for black spruce wood (Picea mariana Mill.). In addition, UV microscopy and EDXA techniques were applied to the determination of lignin distribution in the tracheids in order to establish the relationship between the two techniques. The results indicated that, although the ultraviolet absorptivity in different morphological regions is essentially the same, the secondary wall lignin was 1.70 times more reactive towards bromination than the middle lamella lignin. By applying the value of 1.70 as a correction to the EDXA results, the estimated lignin distribution by EDXA was in fairly good agreement with that from UV microscopy.The authors would like to thank Dr. J.-F. Revol and Dr. M. Tsuji for their assistance during this study     

Weathering characteristics of hardwood surfaces

Summary Four hardwoods, red oak, white oak, yellow-poplar, and sweetgum, were exposed to outdoor weathering and to artificial ultraviolet (UV) light with wavelengths of >220 and >254 nm. Discoloration and loss of brightness were observed from all specimens regardless of their exposure conditions. White oak and sweetgum changed color at a slower rate than did red oak and yellow-poplar. SEM micrographs showed that all wood species exhibited surface deterioration after 30 days exposure to sunlight or 500 hrs to UV light. Loss of middle lamella, separation of procumbent cells, and damage of pit structures were observed on transverse sections for all species. ESCA studies revealed a high oxygen content at the wood surfaces indicating severe oxidation of wood exposed either outdoors or to artificial UV light. The generation of new chromophoric groups such as carbonyls, carboxylic acids and quinones, and the loss of lignin at the oxidized surface were demonstrated experimentally by infrared studies.This study was supported by cooperative research funds provided by the U.S. Forest Products Laboratory, Madison, Wisconsin. The authors wish to acknowledge this support     

The application of ultraviolet microscopy to the distribution of lignin in wood description and validity of the technique

A method has been developed for the determination of lignin distribution in the wood cell wall by ultraviolet microscopy. The method incorporates some important advances on previus applications of UV microscopy to the study of lignin distribution. Ultrathin cross-sections of wood are obtained by the sample preparation and sectioning techniques of electron microscopy. The specimens are examined in monochromatic ultraviolet light using quartz reflection optics. The microscope image is photographically recorded and the negative is subsequently subjected to densitometric analysis. Each stage of the analytical procedure has been critically assessed to determine its validity and limitations. The method is ideally suited to the study of the removal of lignin from the wood cell wall during cooking and possesses other important applications in wood technology.     

Lignin distribution in spruce (Picea abies) determined by mercurization with SEM-EDXA technique

Summary The lignin distribution between the middle lamella and the cell wall of spruce fibers has been determined by a new technique based on a mercurization of the lignin and a concomitant determination of mercury by the SEM-EDXA technique. The ratio of lignin in the middle lamella at the cell corners to the lignin in the secondary wall was 2.5±0.6 for latewood and 2.4±0.6 for earlywood. This gives a lignin content of 55–58% in the true middle lamella in the cell corners. The reactivity to mercuric acetate of different wood elements was determined in separate experiments. Fractions enriched in ray cells, middle lamella, and compression wood all reacted at the same rate as the whole wood; about one mole of mercury was incorporated per mole of lignin (C₉-unit).     

Modelling the structure of the softwood cell wall for computation of mechanical properties

A means to quantitatively construct two layer models of the wood cell-wall utilising basic density and mean microfibril angle data is discussed. It is assumed that the lignin distribution is uniform in the secondary wall layers, that there is a fixed polysaccharide ratio throughout the wall and that variation in wall thickness arises only from variation in S2 layer thickness. It is shown that the relative thickness of those cell wall layers in which the cellulose is transversely oriented (M+P, S1 and S3) have a significant effect on longitudinal shrinkage and that variance between computed and measured shrinkage values is reduced when compared with earlier models if both basic density and mean microfibril angle are taken into account.