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     

Lignification in poplar tension wood lignified cell wall layers

The lignification process in poplar tension wood lignified cell wall layers, specifically the S(1) and S(2) layers and the compound middle lamella (CML), was analysed using ultraviolet (UV) and transmission electron microscopy (TEM). Variations in the thickness of the gelatinous layer (G-layer) were also measured to clarify whether the lignified cell wall layers had completed their lignification before the deposition of G-layers, or, on the contrary, if lignification of these layers was still active during G-layer formation. Observations using UV microscopy and TEM indicated that both UV absorbance and the degree of potassium permanganate staining increased in the CML and S(1) and S(2) layers during G-layer formation, suggesting that the lignification of these lignified layers is still in progress during G-layer formation. In the context of the cell-autonomous monolignol synthesis hypothesis, our observations suggest that monolignols must go through the developing G-layer during the lignification of CML and the S(1) and S(2) layers. The alternative hypothesis of external synthesis (in the rays) does not require that monolignols go through the G-layer before being deposited in the CML, or the S(1) and S(2) layers. Interestingly, the previous observation of lignin in the poplar G-layer was not confirmed with the microscopy techniques used in the present study.     

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.     

Role of the gelatinous layer on the origin of the physical properties of the tension wood

To discuss the role of the gelatinous layer (G-layer) on the origins of the physical properties peculiar to the tension wood fiber (TW fiber), the deformation process of an isolated TW fiber caused by a certain biomechanical state change was formulated mathematically. The mechanical model used in the present formulation is a four-layered hollow cylinder having the compound middle lamella (CML), the outer layer of the secondary wall (S1) and its middle layer (S2), and the G-layer (G) as an innermost layer. In the formulation, the reinforced matrix mechanism was applied to represent the mechanical interaction between the cellulose microfibril (CMF) as a framework bundle and the amorphous substance as a matrix skeleton in each layer. The model formulated in the present study is thought to be useful to investigate the origins of extensive longitudinal drying shrinkage, large tensile growth stress, and a high axial elastic modulus, which are rheological properties peculiar to the TW. In this article, the detailed process of the mathematical formulation is described. In a subsequent article, some TW properties from a 70-year-old Kohauchiwakaede (Acer sieboldianum Miq.) will be analyzed using the newly developed model.     

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     

The distribution of lignin in sprucewood as determined by ultraviolet microscopy

The distribution of lignin in black spruce has been determined quantitatively by the study of 0.5 m transverse sections in a UV microscope. The average lignin concentration in the compound middle lamella was about twice that in the secondary wall. The lignin concentration of the middle lamella at the cell corners of adjacent tracheids was nearly four times that in the secondary wall but the volume of the secondary wall was much greater than the volume of the middle lamella. Thus, for earlywood, 72% of the total lignin was in the secondary wall leaving only 28% in the compound middle lamella and cell corner middle lamella regions. The corresponding values for latewood were 82% and 18% respectively. Use of oblique longitudinal sections of 0.1 m thick permitted the resolution of the compound middle lamella. The lignin concentration in the true middle lamella was found to be equal to that in the cell corner middle lamella and the primary wall lignin content to be about twice that in the secondary wall.

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Zusammenfassung Die Verteilung des Lignins in Fichtenholz wurde quantitativ durch Untersuchung von 0,5 m dicken Querschnitten unter dem UV-Mikroskop bestimmt. Die mittlere Ligninkonzentration war in der Mittelschicht etwa doppelt so hoch wie in der Sekundärwand. Die Ligninkonzentration der Mittelschicht war in den an die Tracheiden anstoßenden Zellecken annähernd viermal höher als in der Sekundärwand, wogegen das Volumen der Sekundärwand wesentlich größer war als das der Mittelschicht. Dagegen befand sich beim Frühholz 72% des gesamten Lignins in der Sekundärwand und nur 28% fanden sich in der Mittelschicht selbst und in ihren Zelleckbereichen. Die entsprechenden Werte für Spätholz betragen 82% bzw. 18%. Die Anwendung von schrägen Längsschnitten von 0,1 m Dicke erlaubte die Auflösung der Mittelschicht. Die Ligninkonzentration in der Mittllamelle war gleich groß wie in der in den Zellzwickeln befindlichen Mittellamelle und der Ligningehalt der Primärwand war etwa doppelt so groß wie derjenige in der Sekundärwand.

     

Generation process of growth stresses in cell walls: Relation between longitudinal released strain and chemical composition

Summary To advance the discussion on the evolution mechanism of tree growth stresses, the relation between released strain and the chemical components was investigated experimentally. The expansive released strain in the longitudinal direction assumed large values as the lignin content increased in the compression wood, but there was no relation between released strain and lignin content in the normal wood region. The contractive released strain assumed large values as the cellulose content and its crystallinity increased. Their correlation was very high and clear. From these facts, it is considered that the lignin deposition plays an important role in the generation of the growth stresses in compression wood but is not important in normal or tension wood regions. Cellulose microfibrils contract along their longitudinal axis during cell maturation, and the stress included by the contraction creates a longitudinal growth stress in normal and tension woods.The authors thank Prof. R. R. Archer, University of Massachusetts, for his valuable advice. A part of this research was supported by a grant under the Monbusho International Scientific Research Program     

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     

Prediction of the buckling stress of intermediate wooden columns using the secant modulus

We made several buckling tests of wooden columns with intermediate slenderness ratios () and examined the empirical formulas. On the basis of the examination, we formulated an equation for predicting the buckling stress () of an intermediate wooden column. Sitka spruce (Picea sitchensis Carr.) and buna (Japanese beech, Fagus crenata BI.) were used for the studies. A compressive load was applied on the specimen supported with pin ends, and the buckling stress was predicted by the tangent modulus theory and two empirical equations: those of Tetmajer and Newlin-Gahagan. The predicted - relations were compared with the test results, and the applicability of these predictions were examined. Based on the comparisons, we formulated an equation that can predict the - relations of materials with various stress-strain characters in the plastic strain range.     

Seasonal changes in lignin distribution during tracheid development in Pinus radiata D. Don

Summary Lignin distribution in developing tracheids of Pinus radiata was studied throughout the growth' season using quantitative interference microscopy. The pattern of lignification remained constant although the number of lignifying cells varied reaching a maximum in summer. Lignification of the secondary wall of latewood tracheids was incomplete at the onset of winter. Each stage of lignification was preceded by deposition of carbohydrates with lignification of the middle lamella starting after S1 formation and lignification of the secondary wall starting after S3 formation. Lignification of the middle lamella was completed before the start of lignin deposition in the secondary wall. In one of the trees examined, the secondary wall lignified concurrently with the middle lamella and this was associated with a low lignin concentration in the middle lamella of mature cells. The secondary wall reached a mature lignin concentration of 21–22% v/v except in one specimen containing severe compression wood which reached 28% v/v. The cell corner middle lamella reached a mature lignin concentration of 74–87% v/v.     

On the detachment of the gelatinous layer in tension wood fiber

The detachment of the gelatinous layer (G-layer), often observed on microtome cross sections, has led some authors to believe that the G-layer cannot act as the driving force of longitudinal shrinkage in tension wood. The aim of this study was to observe the detachment of the G-layer along fibers. Green wood blocks were cut transversely into two samples. One sample was kept in water and the other was oven-dried. With one face being common to both samples, the detachment of the G-layer was studied on the same fibers. Observations were performed after blocking deformation by embedding. This revealed that the detachment of the G-layer is an effect produced by the act of cutting the transverse face of the wood block to be embedded. At distances greater than 100 µm from this primary surface of the sample, no detachment was observed. Drying shrinkage shows little or no effect on this detachment. The result seems to explain well why the detachment of the G-layer occurs during sectioning using conventional sliding microtomy. These observations prove the adhesion of the G-layer in massive wood and confirm the active role of the G-layer in tension wood properties.     

Proteomic analysis of the G-layer in poplar tension wood

Angiosperm trees bend their stems by forming tension wood at the upper side of leaning stems. Most tension wood has a cellulose-rich G-layer in the innermost surface of the fiber cell wall. Strong tensile stress is considered to occur in the G-layer. This study undertook to identify the proteins involved in G-layer formation and function through a proteomic analysis of G-layer-localized protein. G-layers of poplar were loosened by sonication and isolated as doughnut-shaped pieces of thinly sliced transverse sections. The proteins, once extracted with urea/detergent solution, were separated by two-dimensional polyacrylamide gel electrophoresis, and 110 spots were subjected to liquid chromatography tandem mass spectrometry (LC/MS/MS). A database search for these spots’ mass spectrum patterns identified 72 proteins. In addition, all peptide digestion mixtures of G-layer proteins were separated by strong cation exchange chromatography and 39 proteins were identified using LC/MS/MS analysis. Proteins involved in wall formation, such as lignin biosynthesis-related protein, xyloglucan endotransglucosylase, and fasciclin-like arabinogalactan protein, were notably detected in the G-layer.     

Effects of quenching on relaxation properties of wet wood

A new relaxation property is discussed on the basis of creep behavior of wet wood specimens pretreated with heating at various temperatures followed by quenching. The treated samples showed more marked relaxation than that of an untreated sample. The relationship between relaxation time and heating history was represented by an equation ln() = –( _f – k ₁)T + [ln( _g) + k ₂], where ln() is the logarithmic relaxation time of wet samples after quenching, T is the difference between the heating temperature and the glass transition temperature (T _g), ln( _g) is the logarithmic relaxation time at T _g, is a constant, _f is the coefficient of thermal bulk expansion, and k ₁ and k ₂ are constants. It was concluded from the analysis of experimental results that the change in the relaxation property caused by heating and the following quenching is due to the temporary free volume created by freezing of molecular chain motion of wood components, most probably lignin, during quenching.This work was presented at the 52nd Annual Meeting of Japan Wood Research Society, Gifu, April 2002     

On the origin of growth stresses in trees

Summary A mechanism for growth stress generation is studied which involves a contractive strain in the microfibril direction and swelling strain in the transverse direction in the developing wall of wood cells. A cylindrically anisotropic elastic model is used to calculate the accumulation of residual stresses in the S₂ wall as it is formed. An explicit relation between the shrinkage/swelling strains in the growth increment of the cell wall and the resulting axial and circumferential stresses induced in the cell is derived. For gymnosperm cells the transition from tensile stress in normal wood cells to compressive stress in compression wood cells is found with increasing microfibril angle.     

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.     

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).     

The diffusion, size and location of added silver grains in the cell walls of Swedish pine, Pinus sylvestris

The size and location of silver particles in K-glycerate/AgNO₃ impregnated Swedish pine, green wood as well as high temperature dried, have been studied using TEM micrographs. The diameter of the silver particles was found to be 2–20 nm in the impregnated green wood and as large as 1000 nm (major axis) for the ellipsoid-shaped silver clusters in the impregnated dried wood. Studying the projected area of the silver particles in impregnated green wood indicated that there are a lot of particles (40%) in the compound middle lamella with fewer particles in the S₂ (6–8%), S₁ (4%) and S₃ (2%) layers. The average distance between the silver particles, 50 nm (S₂-layer), in impregnated green wood shows that the impregnant is distributed in the cell wall at the microfibrilar level. Experimental results show that the fastest diffusion path into the cell wall is from the lumen over the pit membrane through the compound middle lamella and not from the lumen through the secondary wall layer S₃. Received 11 January 1998     

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     

Patterns of longitudinal and tangential maturation stresses in Eucalyptus nitens plantation trees

Context

Tree orientation is controlled by asymmetric mechanical stresses set during wood maturation. The magnitude of maturation stress differs between longitudinal and tangential directions, and between normal and tension woods.

Aims

We aimed at evaluating patterns of maturation stress on eucalypt plantation trees and their relation with growth, with a focus on tangential stress evaluation.

Methods

Released maturation strains along longitudinal and tangential directions were measured around the circumference of 29 Eucalyptus nitens trees, including both straight and leaning trees.

Results

Most trees produced asymmetric patterns of longitudinal maturation strain, but more than half of the maturation strain variability occurred between trees. Many trees produced high longitudinal tensile stress all around their circumference. High longitudinal tensile stress was not systematically associated with the presence of gelatinous layer. The average magnitude of released longitudinal maturation strain was found negatively correlated to the growth rate. A methodology is proposed to ensure reliable evaluation of released maturation strain in both longitudinal and tangential directions. Tangential strain evaluated with this method was lower than previously reported.

Conclusion

The stress was always tensile along the longitudinal direction and compressive along the tangential direction, and their respective magnitude was positively correlated. This correlation does not result from a Poisson effect but may be related to the mechanism of maturation stress generation.     

Topochemical investigation into the delignification of Eucalyptus globulus chips during semi-chemical sulfite pulping

The course of delignification of Eucalyptus globulus fibers during neutral semi-chemical sulfite pulping (NSSC) was studied by universal microspectrophotometry (UMSP 80, ZEISS). UV-investigation into a cellular level enables the topochemical analyses of delignification within individual cell wall layers during cooking. Cooks were carried out in a laboratory seven liter digester with liquor circulation and electrical heating device. Chip samples were taken throughout the cooking for chemical and UV microscopic analyses. UV microscopy analysis revealed for Eucalyptus globulus chips a preferred lignin removal during NSSC cooking in cell corner and compound middle lamella regions.