Severe Compression Wood Research Articles

Peculiar traits of wood in a leaning stem of Scots pine (Pinus sylvestris L.)

Abstract The paper analyses biometrical and anatomical traits of wood in a leaning stem of pine trees. For study purpose, five pine trees (Pinus sylvestris L.) with a visibly leaning stem were chosen. Wood samples were taken at three different stem heights, that is, below the stem curvature, at the curvature and above the curvature. Microscopic specimens were prepared and used for the following measurements: annual rings width, tracheids diameter and tracheids wall thickness. The measurements were performed for wood located on the lower side of the leaning stem and on the opposite side. Cytochemical staining was performed to identify the occurrence of laricinan. The results showed tracheids with a rounded shape and thick cell walls, helical cavities and intercellular spaces in wood located at curvature height on the lower side of the leaning stem. These traits indicate a severe compression wood that allowed pine trees to change their stem position in relation to the vector of gravity.

Location and characterization of lignin in tracheid cell walls of radiata pine (Pinus radiata D. Don) compression woods

Tilted stems of softwoods form compression wood (CW) and opposite wood (OW) on their lower and upper sides, respectively. More is known about the most severe form of CW, severe CW (SCW), but mild CWs (MCWs) also occur widely. Two grades of MCWs, MCW1 and MCW2, as well as SCW and OW were identified in the stems of radiata pine (Pinus radiata) that had been slightly tilted. The four wood types were identified by the distribution of lignin in the tracheid walls determined by fluorescence microscopy. A solution of the fluorescent dye acridine orange (AO) (0.02% at pH 6 or 7) was shown to metachromatically stain the tracheid walls and can also be used to determine lignin distribution. The lignified walls fluoresced orange to yellow depending on the lignin concentration. Microscopically well-characterized discs (0.5 mm diameter) of the wood types were used to determine lignin concentrations and lignin monomer compositions using the acetyl bromide method and thioacidolysis, respectively. Lignin concentration and the proportion of p-hydroxyphenyl units (H-units) relative to guaiacyl (G-units) increased with CW severity, with <1% H-units in OW and up to 14% in SCW. Lignin H-units can be used as a marker for CW and CW severity. Similar discs were also examined by Raman and FTIR micro-spectroscopies coupled with principal component analysis (PCA) to determine if these techniques can be used to differentiate the four different wood types. Both techniques were able to do this, particularly Raman micro-spectroscopy.

Tracheid cell-wall structures and locations of (1 → 4)-β-d-galactans and (1 → 3)-β-d-glucans in compression woods of radiata pine (Pinus radiata D. Don)

BackgroundCompression wood (CW) forms on the underside of tilted stems of coniferous gymnosperms and opposite wood (OW) on the upperside. The tracheid walls of these wood types differ structurally and chemically. Although much is known about the most severe form of CW, severe CW (SCW), mild CWs (MCWs), also occur, but less is known about them. In this study, tracheid wall structures and compositions of two grades of MCWs (1 and 2) and SCW were investigated and compared with OW in slightly tilted radiata pine (Pinus radiata) stems.ResultsThe four wood types were identified by the distribution of lignin in their tracheid walls. Only the tracheid walls of OW and MCW1 had a S3 layer and this was thin in MCW1. The tracheid walls of only SCW had a S2 layer with helical cavities in the inner region (S2i). Using immunomicroscopy, (1 → 4)-β-D-galactans and (1 → 3)-β-D-glucans were detected in the tracheid walls of all CWs, but in only trace amounts in OW. The (1 → 4)-β-D-galactans were located in the outer region of the S2 layer, whereas the (1 → 3)-β-D-glucans were in the inner S2i region. The areas and intensities of labelling increased with CW severity. The antibody for (1 → 4)-β-D-galactans was also used to identify the locations and relative amounts of these galactans in whole stem cross sections based on the formation of an insoluble dye. Areas containing the four wood types were clearly differentiated depending on colour intensity. The neutral monosaccharide compositions of the non-cellulosic polysaccharides of these wood types were determined on small, well defined discs, and showed the proportion of galactose was higher for CWs and increased with severity.ConclusionThe presence of an S3 wall layer is a marker for very MCW and the presence of helical cavities in the S2 wall layer for SCW. The occurrence and proportions of (1 → 4)-β-D-galactans and (1 → 3)-β-D-glucans can be used as markers for CW and its severity. The proportions of galactose were consistent with the labelling results for (1 → 4)-β-D-galactans.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-016-0884-3) contains supplementary material, which is available to authorized users.

Analysis of static bending-induced compression wood formation in juvenile Picea omorika (Pančić) Purkynĕ

Young P. omorika trees subjected to static bending showed a severe compression wood response as characterized by fluorescence spectroscopy/microscopy, which decreased in severity with height correlated with a decrease in bending moment. This investigation is aimed at understanding the reaction wood response in a slow-growing conifer species under conditions of severe and long-term bending stress. Compression wood (CW) formation was studied in stems of juvenile P. omorika after trees were subjected to static bending by wiring at an angle of about 90 degrees, for 1 year. The applied static bending would correspond to the impact of winter snow loads or snow falls on juvenile conifers. Stem sections were collected during one growing season and examined by fluorescence microscopy, and fluorescence spectroscopy including deconvolution analysis. Trees exposed to bending produced large amounts of severe CW but very low amounts of opposite wood (OW) during the experimental season indicating a dramatic change in biomass distribution compared to control trees. Indicators of cell wall structure changes, such as fluorescence emission spectra, peak intensities, and shifts in the positions of the long-wavelength spectral components, decreased from the stem base to the top of the stem, in line with a calculated decrease in bending moment.

Near Infrared Spectroscopic and Hyperspectral Imaging of Compression Wood in Pinus Radiata D. Don

Near infrared (NIR) spectroscopy has been used to predict the severity of compression wood in samples of radiata pine ( P. radiata D. Don) using a subjective microscopic assessment of compression wood as the reference method. The calibration statistics are only moderate ( R2 = 0.84, r2 = 0.6) which may be due to the subjectivity of the traditional method. Spatial resolution of compression wood in board cross-sections and discs from logs has been achieved using an improvised “NIR microscope” based on a Zeiss trinocular microscope and, more recently, using a NIR hyperspectral line camera. Compression wood was identifiable on the surface of discs across the continuum from normal wood to severe compression wood.

Mechanical performance and cellulose microfibrils in wood with high S2 microfibril angles

Corewood and compression wood, both with high S2 microfibril angles, are the worst parts of the tree most in need of improvement in wood quality. This study focuses on the characteristics of cellulosic reinforcement in wood with high S2 microfibril angles lying between 35° and 60°, as well as the probable influence of these characteristics on wood longitudinal mechanical properties. Stiffness and tensile strength were measured on 221 thin radiata pine wood strips. The results indicate that at high angles, the average values of wood stiffness and tensile strength are low with relatively large variations. The variations of stiffness and strength are weakly correlated with the variation of S2 microfibril angles, regardless of wood type. Further study involved examining the characteristics of cellulose microfibrils within the S2 layer using dual axis electron tomography for samples with the same microfibril angle but differing mechanical performance. From the fact that the severe compression wood possessed fewer cellulose microfibrils and had abundant dislocations, and that at many kinking points these cellulose microfibrils were broken down into shorter dislocation segments, it was concluded that the characteristics of cellulose microfibril reinforcement are responsible for the inferior stiffness and tensile strength in the severe compression wood compared with normal corewood with the same high S2 microfibril angle.

Distribution of (1-&gt;4)- -galactans, arabinogalactan proteins, xylans and (1-&gt;3)- -glucans in tracheid cell walls of softwoods

Polysaccharides were located in the walls of normal and compression wood tracheids of Pinus radiata (radiata pine), Picea sitchensis (Sitka spruce) and Picea abies (Norway spruce) by transmission electron microscopy using immunogold labelling with monoclonal antibodies to (1-->4)-beta-galactan (LM5), (1-->3)-beta-glucan, arabinogalactan proteins (AGPs) (MAC207) and heteroxylans (LM10 and LM11). In fully differentiated compression wood tracheids, (1-->4)-beta-galactan was found in the S2((L)) layer and, to a smaller extent, at the interface between the compound middle lamella and the S1 layer. (1-->4)-beta-Galactan appeared to be displaced from, or modified in, the S1 layer during cell wall formation. (1-->3)-beta-Glucan (callose) was confined to the helical cavities in the inner S2 layer of severe compression wood. MAC207 AGP glycan epitope was found exclusively in the S1 and S3 layers of normal wood tracheids and in the S1 and inner S2 layers of compression wood tracheids. Binding of LM10, which specifically recognizes unsubstituted or low-substituted xylans, occurred at similar locations to the MAC207 epitope, whereas binding of LM11, which recognizes more highly substituted as well as unsubstituted xylans, occurred throughout the tracheid walls with the exception of the primary wall. Immunogold labelling showed that the different wall layers of softwood tracheids have different polysaccharide compositions which change abruptly during cell wall formation.

Quantification of compression wood severity in tracheids of Pinus radiata D. Don using confocal fluorescence imaging and spectral deconvolution

Confocal fluorescence microscopy was used to examine the spectral characteristics of lignin autofluorescence in secondary cell walls of normal and compression wood from Pinus radiata. Using UV excitation, fluorescence spectra of normal and compression wood sections showed significant differences, especially in the outer secondary cell wall of tracheids, with a shift in maxima from violet to blue wavelengths between normal and compression wood. A comparison of normal wood, mild and severe compression wood, showed that the wavelength shift was intermediate in the mild compression wood compared to the severe compression wood, thus offering the possibility of quantifying the severity by measuring ratios of fluorescence at violet and blue wavelengths. Fluorescence induced by blue light, rather than UV, was less well differentiated amongst wood types. Spectral deconvolution indicated the presence of a minimum of five discrete lignin fluorophores in the cell walls of both normal and compression wood tracheids. Comparison with lignin model compounds suggest that the wavelength shift may correspond in part to increased levels of p-hydroxy type lignin in the compression wood samples. The combination of confocal fluorescence imaging and related spectral deconvolution therefore offers a novel technique for characterising cell wall lignin in situ.

Relationship between development of compression wood and gene expression

In gymnosperms, tissue that differs from normal wood develops on the lower side of an inclined stem or branch and is referred to as compression wood. Compression wood is characterized by thicker tracheid walls, higher lignin content, and shorter tracheid length, among other features. No definable boundary exists between normal wood and compression wood, and compression wood is classified continuously from mild to severe. Therefore, kinds and abundances of transcripts are expected to be considerably different between mild and severe compression wood. We prepared Chamaecyparis obtusa saplings grown at various angles of inclination (0–50°) to obtain normal, mild, and severe compression wood samples. We examined the relationship between the degree of development of compression wood and relative transcript abundance of previously screened genes. Lignin content and relative abundance of a transcript with high similarity to laccase increased with increasing inclination angle of the saplings, i.e. development of compression wood. In contrast, tracheid length decreased with increasing development of compression wood, and in samples where the tracheid length was short, relative abundance of a transcript with high similarity to ascorbate oxidase decreased.

Detection and Classification of Norway Spruce Compression Wood in Reflected Light by Means of Hyperspectral Image Analysis

A methodology has been developed based on reflected light to detect compression wood in stem cross sections of Norway spruce (Picea abies [L.] Karst.). In addition to quantify the spatial distribution of compression wood, the chronological pattern of its formation is recorded by cross linking the pixel classification to the tree ring sequence. An imaging spectrometer is used to record the spectral characteristics in the visible light and near infrared of the cross-sectional surface. Cross-sectional areas are classified by hyperspectral image analysis into severe compression wood, moderate compression wood, normal wood, and background/cracks. The classification is performed by the Spectral Angle Mapper algorithm, which compares the standardized spectrum of each pixel with reference spectra stored in a spectral library. The reference spectra are obtained from selected training areas of the different compression wood severity classes identified by cell characteristics under a light microscope. The tree ring boundaries are located in a grey scale image which shows the spatial information at wavelength 435 nm and the annual radial increment is measured. The classification accuracy is tested by a confusion matrix and cross-analysed with High-Frequency Densitometry.

Longitudinal shrinkage behaviour of compression wood in radiata pine

The behaviour of longitudinal shrinkage was investigated in the corewood of a swept, 17-year-old New Zealand radiata pine stem. Wood categories in terms of normal wood, mild compression wood and severe compression wood were identified microscopically using autofluorescence of lignin. Average longitudinal shrinkage was collated according to corewood location and wood category within corewood in the leaning and the vertical parts of the stem, and then maximum radial difference of longitudinal shrinkage within growth ring was examined. The results show that the average longitudinal shrinkage is significant (2.4%) in the corewood of the leaning part of the stem. Among wood categories, severer compression wood displays the highest (2.9%) average longitudinal shrinkage. In the context of this study, growth rings may consist of one of three types of wood: (1) only normal wood; (2) a single compression wood type; and (3) mixed-type wood. Where multiple compression woods co-existed with normal wood, the maximum radial difference of longitudinal shrinkage within the growth ring was found to be 4.0%. A strong correlation (R 2 = 0.90) between average MFA and average longitudinal shrinkage suggests a significant influence of the average MFA on average longitudinal shrinkage across the three growth ring types.

In situ detection of cell wall polysaccharides in sitka spruce (Picea sitchensis (Bong.) Carrière) wood tissue

Wood cell wall polysaccharides can be probed with monoclonal antibodies and carbohydrate-binding modules (CBMs). Binding of monoclonal antibodies to β-1-4-xylan, β-1-4-mannan, β-1-3-glucan, and α-1-5-arabinan structures were observed in native Sitka spruce (Picea sitchensis (Bong.) Carrière) wood cell walls. Furthermore CBMs of different families, differing in their affinities for crystalline cellulose (3a) and amorphous cellulose (17 and 28), were shown to bind to the native wood cell walls with varying intensities. Resin channel forming cells exhibited an increased β-1-4-xylan and a decreased β-1-4-mannan content. Focusing on severe compression wood (CW) tracheids, β-1-3-glucan was found towards the cell lumen. In contrast, α-1-5-arabinan structures were present in the intercellular spaces between the round tracheids in severe CW, highlighting the importance of this polymer in cell adhesion.

Effects of Coumarate 3-Hydroxylase Down-regulation on Lignin Structure

Down-regulation of the gene encoding 4-coumarate 3-hydroxylase (C3H) in alfalfa massively but predictably increased the proportion of p-hydroxyphenyl (P) units relative to the normally dominant guaiacyl (G) and syringyl (S) units. Stem levels of up to approximately 65% P (from wild-type levels of approximately 1%) resulting from down-regulation of C3H were measured by traditional degradative analyses as well as two-dimensional 13C-1H correlative NMR methods. Such levels put these transgenics well beyond the P:G:S compositional bounds of normal plants; p-hydroxyphenyl levels are reported to reach a maximum of 30% in gymnosperm severe compression wood zones but are limited to a few percent in dicots. NMR also revealed structural differences in the interunit linkage distribution that characterizes a lignin polymer. Lower levels of key beta-aryl ether units were relatively augmented by higher levels of phenylcoumarans and resinols. The C3H-deficient alfalfa lignins were devoid of beta-1 coupling products, highlighting the significant differences in the reaction course for p-coumaryl alcohol versus the two normally dominant monolignols, coniferyl and sinapyl alcohols. A larger range of dibenzodioxocin structures was evident in conjunction with an approximate doubling of their proportion. The nature of each of the structural units was revealed by long range 13C-1H correlation experiments. For example, although beta-ethers resulted from the coupling of all three monolignols with the growing polymer, phenylcoumarans were formed almost solely from coupling reactions involving p-coumaryl alcohol; they resulted from both coniferyl and sinapyl alcohol in the wild-type plants. Such structural differences form a basis for explaining differences in digestibility and pulping performance of C3H-deficient plants.

Structure?function relationships of four compression wood types: micromechanical properties at the tissue and fibre level

The mechanisms behind compressive stress generation in gymnosperms are not yet fully understood. Investigating the structure–function relationships at the tissue and cell level, however, can provide new insights. Severe compression wood of all species lacks a S3 layer, has a high microfibril angle in the S2 layer and a high lignin content. Additionally, special features like helical cavities or spiral thickenings appear, which are not well understood in terms of their mechanical relevance, but need to be examined with regard to evolutionary trends in compression wood development. Thin compression wood foils and isolated tracheids of four gymnosperm species [Ginkgo biloba L., Taxus baccata L., Juniperus virginiana L., Picea abies (L.) Karst.] were investigated. The tracheids were isolated mechanically by peeling them out of the solid wood using fine tweezers. In contrast to chemical macerations, the cell wall components remained in their original condition. Tensile properties of tissue foils and tracheids were measured in a microtensile apparatus under wet conditions. Our results clearly show an evolutionary trend to a much more flexible compression wood. An interpretation with respect to compressive stress generation is discussed.

WITHIN-TREE VARIATION IN ANATOMICAL PROPERTIES OF COMPRESSION WOOD IN RADIATA PINE

Two trees of radiata pine, one showing severe lean, the other growing almost vertically, were assessed for the presence and anatomical properties of compression wood, including anatomy, lignin distribution, microfibril angle, basic density, radial and tangential lumen diameter and cell wall thickness. Both trees contained significant amounts of compression wood although the severity and amount of compression wood was greater in the leaning tree. Changes in lignin distribution seem to be characteristic of the mildest forms of compression wood with reduced lignification of the middle lamella representing the earliest change observed from normal wood. An increase in microfibril angle was associated with both mild and severe compression wood although examples of severe compression wood with the same or smaller microfibril angles than opposite wood, or with very small microfibril angles, were found. When segregated into mild and severe compression wood the average difference in microfibril angle was 4° and 8° respectively compared with opposite wood. Within-ring distribution of microfibril angle was different in severe compression wood compared to opposite wood with higher angles in the latewood. Severe compression wood showed a 22% increase in basic density compared to mild compression wood and opposite wood. The increased density was accounted for in terms of a 26% increase in tracheid wall thickness throughout the growth ring, offset by a 9% increase in radial lumen diameter, slightly greater in the latewood. There were no significant changes in density or cell dimensions in mild compression wood compared with opposite wood.

Stem straightness and compression wood in a 22-year-old stand of container-grown Scots pine trees

The distribution of compression wood in relation to eccentric growth and development of stem straightness was studied in a 22-year-old Scots pine (Pinus sylvestris L.) stand in central Sweden that was established with container-grown seedlings. Stem straightness was measured on the same 440 trees in 1986 and 1997. The number of stems with straight base sections increased from 60% in 1986 to 89% in 1997. Measurements of 72 sample trees in 2001 showed that 96% of the trees had developed straight stem bases. External geometry data of the logs was obtained with a Rema 3D log scanner. A sub-sample of 16 trees was randomly selected for analysis of compression wood distribution and eccentricity measurements. From each tree, 11 discs were cut at every 60 cm along the stem. All discs, except one, contained compression wood. Compression wood and pith eccentricity was most pronounced near the stem base but not significantly correlated to basal sweep. Severe compression wood content was correlated to pith eccentricity and bow height. In general, correlations were better for the basal sections of the logs. Even though most trees were straight, they contained large amounts of compression wood. It is evident that eccentric growth and compression wood formation play major roles in the development of stem straightness. In several stems, a spiral compression wood distribution pattern was found. Reasons for this are discussed.

FOURIER TRANSFORM METHODS IN IMAGE ANALYSIS OF COMPRESSION WOOD AT THE CELLULAR LEVEL

Compression wood affects the overall quality of construction timber and paper quality. We have investigated the microscopic features of lumen shape and tracheid shape for compression wood studies and detection in softwoods. In this paper, we describe a method for directly analyzing tracheid and lumen shape over an entire image. The method uses the Fast Fourier Transform (FFT) and reduces the two-dimensional image data to one-dimensional data, from which lumen and tracheid shape can be evaluated. We illustrate the method by comparison of compression wood images to normal wood images. The results of detecting severe compression wood were successful, while the detection of weak compression wood was not satisfactory.

Micromorphological Characteristics of Compression Wood Degradation in Waterlogged Archaeological Pine Wood

Summary The degradation characteristics of waterlogged archaeological compression wood excavated in South Korea were examined by transmission electron microscopy (TEM) and confocal laser scanning microscopy (CLSM). Degradation of Pinus compression wood occurred mainly in the inner part of the S2 layer. In contrast, the outer part of the S2 layer remained relatively intact. CLSM and TEM showed the erosion type of bacterial attack to be dominant in the secondary cell walls of both severe and mild compression wood. However, in some cases the middle lamella was also degraded, which suggests that other forms of microbial attacks, such as bacterial tunnelling, were also present. Bacterial erosion in the severe compression wood was mainly confined to the inner part of the S2 layer whereas in the mild compression wood it also extended into the outer part of the S2 and the S1 layer. The extent of erosion correlated to the differences in the amount and distribution of lignin, particularly in the outer S2 layer between the severe and mild compression wood cells. These features are compared with the degradation of normal Pinus wood.

Ultrastructure of tracheid cell walls in radiata pine (Pinus radiata) mild compression wood

The structure of tracheid cell walls in mild compression wood of Pinus radiata D. Don was examined by confocal laser scanning fluorescence microscopy (CLSM) and transmission electron microscopy (TEM). Fluorescence microscopy using lignin autofluorescence revealed cell to cell variations in lignification of the cell corner middle lamella (CCML) and outer S2 (S2L) regions supporting observations made at higher resolution with TEM. The mild compression wood sample examined lacked the characteristic intercellular spaces of most grades of compression wood and thus represents the least severe grade of compression wood. TEM observations revealed evidence for radial striations in the S2 region probably representing regions of alternating high and low lignin concentration, which may be the precursor to the helical cavities found in severe compression wood. These radial striations typically showed a sinuous and branched structure, extending partly into the S2L region, where the orientation of lignin lamellae was predominantly tangential following the arrangement of the cellulose microfibrils. At high magnification lignin occurred in a patchy distribution overlaying the typical lamellate arrangement associated with normal wood. Significant variability in the lignification of the CCML region was also observed sometimes giving a patchy or granular appearance. The lignification of the CCML and the S2L regions was generally comparable, while S1 and S2 layers were less lignified. A residual S3 layer was sometimes present in the form of a very thin layer slightly more lignified than the adjacent S2 layer.Key words: compression wood, Pinus radiata D. Don, ultrastructure, confocal laser scanning microscopy, transmission electron microscopy, lignin distribution.

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

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.