Tension Wood Fibres Research Articles

Precautions for the Structural Analysis of the Gelatinous Layer in Tension Wood

The gelatinous layer (G-layer) of tension wood fibres in hardwood contributes to the mechanical function of the living tree and has significant consequences on properties of solid wood. Its size, shape and structure observed by optical or electron microscopy exhibits characteristic anatomical features. However, we found that sectioning of non-embedded wood samples results in an uncontrolled swelling of the G-layer. In order to assess this artefact, the shape and thickness of the G-layer was monitored by serial sections from an embedded wood sample, from its trimmed transverse face to that located several hundreds of micrometres deep. The results revealed that the initial cutting before embedding produced a border effect responsible for the swollen nature, which is similar to sections from non-embedded material. After a conventional embedding technique was applied, a section of at least 30 micrometres below the trimming surface is required to observe an un-swollen G-layer.

Poplar genes encoding fasciclin-like arabinogalactan proteins are highly expressed in tension wood.

• Fifteen poplar cDNA encoding fasciclin-like arabinogalactan proteins (PopFLAs) were finely characterized, whereas the presence of arabinogalactan proteins (AGPs) was globally assessed during wood formation. • PopFLAs transcript accumulation was analysed through EST distribution in cDNA libraries, semi-quantitative RT-PCR, microarray experiment and Northern blot analysis. Similarly, AGPs contents were globally quantified by rocket electrophoresis. AGPs accumulation was further examined by Western blotting and immunocytolocalization. • Ten PopFLAs were specifically expressed in tension wood (TW) and not expressed in the cambial zone. Rocket electrophoresis revealed important AGPs accumulation in TW xylem. An anti-AGPs specific antibody recognized two proteins preferentially present in the cell wall-bound fraction from TW. Immunocytochemistry revealed a strong labelling close to the inner part of the G-layer of TW fibres. • PopFLAs are expressed in xylem and many are up-regulated in TW. It is suggested that some PopFLAs accumulating at the inner side of the G-layer may have a specific function in the building of this layer. PopFLAs expression may therefore be linked to the specific mechanical properties of TW.

Tension wood as a model for functional genomics of wood formation.

Wood is a complex and highly variable tissue, the formation of which is developmentally and environmentally regulated. In reaction to gravitropic stimuli, angiosperm trees differentiate tension wood, a wood with specific anatomical, chemical and mechanical features. In poplar the most significant of these features is an additional layer that forms in the secondary wall of tension wood fibres. This layer is mainly constituted of cellulose microfibrils oriented nearly parallel to the fibre axis. Tension wood formation can be induced easily and strongly by bending the stem of a tree. Located at the upper side of the bent stem, tension wood can be compared with the wood located on its lower side. Therefore tension wood represents an excellent model for studying the formation of xylem cell walls. This review summarizes results recently obtained in the field of genomics on tension wood. In addition, we present an example of how the application of functional genomics to tension wood can help decipher the molecular mechanisms responsible for cell wall characteristics such as the orientation of cellulose microfibrils.

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.

An unusual formation of tension wood in a natural forest Acacia sp.

Abstract The determination of microfibril angle (MFA), cellulose crystallite width and microdensity by SilviScan-2 at CSIRO confirmed that several dark bands in the heartwood of an Acacia species were associated with tension wood (TW) fibres. The straight log was taken from a tree of about 30 years of age that had been apparently growing vertically as there were no signs of the usual stimuli for TW formation. The continuous dark bands of about 5 mm width extended radially from the inner heartwood in a spiral pattern and were separated by continuous normal wood bands of similar width. The TW bands continued in the sapwood from the heartwood boundary to the cambium but were not coloured. The dark colour of the TW was probably caused by extractives being polymerised at the heartwood boundary by enzymes in the TW fibres in the sapwood. The MFA and cellulose crystallite width indicated the TW in a band was of similar intensity throughout its length, indicating similar microstructural characteristics at different cambial ages. The initiation of spiral grain formation occurs before that of tension wood in the Acacia samples studied.

Detection in situ and characterization of lignin in the G-layer of tension wood fibres of Populus deltoides.

The occurrence of lignin in the additional gelatinous (G-) layer that differentiates in the secondary wall of hardwoods during tension wood formation has long been debated. In the present work, the ultrastructural distribution of lignin in the cell walls of normal and tension wood fibres from poplar (Populus deltoides Bartr. ex Marshall) was investigated by transmission electron microscopy using cryo-fixation-freeze-substitution in association with immunogold probes directed against typical structural motifs of lignin. The specificity of the immunological probes for condensed and non-condensed guaiacyl and syringyl interunit linkages of lignin, and their high sensitivity, allowed detection of lignin epitopes of definite chemical structures in the G-layer of tension wood fibres. Semi-quantitative distribution of the corresponding epitopes revealed the abundance of syringyl units in the G-layer. Predominating non-condensed lignin sub-structures appeared to be embedded in the crystalline cellulose matrix prevailing in the G-layer. The endwise mode of polymerization that is known to lead to these types of lignin structures appears consistent with such an organized cellulose environment. Immunochemical labelling provides the first visualization in planta of lignin structures within the G-layer of tension wood. The patterns of distribution of syringyl epitopes indicate that syringyl lignin is deposited more intensely in the later phase of fibre secondary wall assembly. The data also illustrate that syringyl lignin synthesis in tension wood fibres is under specific spatial and temporal regulation targeted differentially throughout cell wall layers.

ROLES OF AUXIN AND GIBBERELLIN IN GRAVITY-INDUCED TENSION WOOD FORMATION IN AESCULUS TURBINATA SEEDLINGS

The lowest nodes of 6-week-old Aesculus turbinata seedlings were treated with uniconazole-P, an inhibitor of gibberellin (GA) biosynthesis, or a mixture of uniconazole-P and GA3 in acetone solution. To the seedling stems, an inhibitor of auxin transport (NPA) or inhibitors of auxin action (raphanusanin or MBOA) were applied in lanolin paste. The seedlings were tilted at a 45° angle and kept for 10 weeks before histological analysis. Decreases in both normal and tension wood formation followed the application of uniconazole-P. The application of GA3 together with uniconazole-P partially negated the effect of uniconazole-P alone. The application of NPA inhibited tension wood formation at, above, and below the lanolin-treated portions. The treatment of raphanusanin or MBOA also resulted in decreases in tension wood formation at the treated portions. The inhibitory effects of these chemicals applied on the upper side of tilted stems or around the entire stem were greater than on the lower side. The application of uniconazole-P in combination with raphanusanin, MBOA or NPA showed synergistic effects on the inhibition of tension wood formation. The results suggest that both auxin and GA regulate the quantitative production of tension wood fibers and are essential to tension wood formation.

Stem tangential strain on the tension wood side of Fagus crenata saplings

The tangential strain on the inner bark surface of Fagus crenata sapling stems was continuously measured using strain gauges. The total strain increased daily, increasing at night and decreasing during the day. When tension wood was induced by artificial inclination, the strain increased more on the upper side than on the lower side; and the increment in the strain at night was larger on the upper side than on the lower. The change in tangential strain on the inner bark surface arose from changes in the water content and the volume of differentiating cells. Differentiating tension wood fibers appear to contain more water and to expand more at night than differentiating normal wood fibers. We can determine whether tension wood is formed from the tangential strain during growth.

Field Emission SEM Studies on Softwood Tracheids and Hardwood Fibres – A Review of Activities at the EMPA Wood Laboratory

Over the past ten years transverse fracture surfaces of tensionloaded softwoods, hardwoods and reaction wood have been studied using a high resolution FE SEM. Due to the fracture process the cell wall becomes loosened and its components can be distinctly observed (as opposed to smooth cross sections of cut wood). Softwood tracheids (compression wood, too) and hardwood fibres showed predominantly radial orientations(perpendicular to the other layers) of the cellulosic fibrils of the secondary wall layer 2 (S2). Similar structures were found on sections of different hardwoods degraded by white rot fungi. Contrary to that finding there is no discernible preferential orientation of the cellulose fibrils in the S2 or G layer of pronounced tension wood cells. In our opinion functional advantages of such structures in the strengthening tissue are quite clear. The sandwich-like structure is important for the buckling resistance of the tracheids and fibre cell walls and therefore also to the bending stiffness of the whole tree. Tension wood fibres of hardwood trees solely exposed to longitudinal tension do probably not need a compression stiffening, explaining why transverse fibril agglomerations are absent here. In many other studies a polylaminated concentric arrangement of the cellulose/ ligninhemicellulose matrix in the S2 layer is postulated and clearly documented. Taking our results into account, we conclude that concentric and/or radial arrangements (perpendicular to the CML) in the S2 of softwood tracheids and hardwood fibres coexist for good ontogenetic, physiological, and mechanical reasons.

Tension wood fibers are related to gravitropic movement of red mangrove (Rhizophora mangle) seedlings.

Freshly collected viviparous seedlings (propagules) were collected from wild plants of Rhizophora mangle and planted in vertical or horizontal positions. A total of 80 seedlings were examined anatomically at various ages and orientations. After rooting, seedlings reoriented from horizontal to vertical by extreme bending in the hook region of the hypocotyl directly above the basal 1 cm where roots formed. Hypocotyl bending occurred over many months. Trends in position and relative abundance of tension fibers (also called gelatinous fibers) over time were followed. The erection of the seedling was related to increased secondary xylem and the number of tension wood fibers on the upper side of the hook region. However, linear regressions had low coefficient of determination (r(2)) values, presumably related to seedlings with high variability.

Relationship between transverse shrinkage and tension wood from three provenances of Eucalyptus globulus Labill

was studied using 12 mm increment cores taken from straight, vertical and dominant trees of three provenances from a plantation near Tarpina, South Australia. Tension wood fibre percentages were determined from microscopic examination of stained transverse sections and transverse shrinkage was measured from cores dried to 17% equilibrium moisture content. Shrinkage before and after steam reconditioning, as well as associated collapse, were related to the proportions of tension wood. Contrary to normal expectations, high levels of tension wood produced greater shrinkage in the sapwood than in the heartwood in material from all provenances. Collapse was significantly greater in material from the Jeeralang and Western Tasmania provenances than in the King Island provenance. While the mean proportion of tension wood was highest (4.3%) in material from Jeeralang and least from King Island (2.4%), no significant difference among the three provenances was evident for proportions of tension wood fibres when considering only the last year's growth. Variation in the width of the gelatinous layer (G-layer) was recorded for the tension wood fibres. Fibres with thin walled G-layers were usually scattered among normal fibres and collapsed regions containing such material generally recovered after reconditioning. On the other hand, regions containing the more typical tension wood fibres, with thick walled G-layers, exhibited high levels of shrinkage and collapse, and poor recovery after reconditioning.

Relationships between Density, Shrinkage, Extractives Content and Microfibril Angle in Tension Wood from Three Provenances of 10-Year-Old Eucalyptus globulus Labill

Summary Density and microfibril angle (MFA) of tension wood and normal wood were assessed in the sapwood and heartwood, from three provenanaces of 10-year-old Eucalyptus globulus Labill. Density was measured using a modified saturation method that also enabled the calculation of the extractives lost during saturation. Microdensity and MFA were determined by SilviScan 2, a rapid X-ray densitometry and X-ray diffraction system developed at CSIRO. Significant differences were found in density and extractives between provenances and also density between the sapwood and adjacent heartwood from each provenance. This result may explain some of the drying differences between provenances found in an earlier study (Washusen and Ilic 2000). Sapwood samples with high percentages of tension wood fibres had high density and a significant positive correlation was found between microdensity and tension wood fibre percentage. MFA was found to be very low in normal wood in the sapwood, where most tension wood was found, so tension wood could not be identified by MFA. The positive association between tension wood and wood density suggests that caution should be taken when selecting trees for high wood density in tree improvement programs.

REACTION TISSUES IN GNETUM GNEMON A PRELIMINARY REPORT

Gnetum gnemon exhibits Rouxʼs model of tree architecture, with clear differentiation of orthotropic from plagiotropic axes. All axes have similar anatomy and react to displacement in the same way. Secondary xylem of displaced stems shows little eccentricity of development and no reaction anatomy. In contrast, there is considerable eccentricity in extra-xylary tissue involving both primary and secondary production of apparent tension-wood fibres (gelatinous fibres) of three main kinds. Narrow primary fibres occur concentrically in all axes in the outer cortex as a normal developmental feature. In displaced axes gelatinous fibres are developed abundantly and eccentrically on the topographically upper side, from pre-existing and previously undetermined primary cortical cells. They are wide with lamellate cell walls. In addition narrow secondary phloem fibres are also differentiated abundantly and eccentrically on the upper side of displaced axes. These gelatinous fibres are narrow and without obviously lamellate cell walls. Eccentric gelatinous fibres thus occupy a position that suggests they have the function of tension wood fibres as found in angiosperms. This may be the first report in a gymnosperm of fibres with tension capability. Gnetum gne-mon thus exhibits reaction tissues of unique types, which are neither gymnospermous nor angiospermous. Reaction tissues seem important in maintaining the distinctive architecture of the tree.

Systematic and functional anatomy of seedlings in mangrove Rhizophoraceae: vivipary explained?

The hypocotyl of viviparous seedlings of mangrove Rhizophoraceae (tribe Rhizophoreae) has a common suite of anatomical features but a range of anatomical structures that may be diagnostic at the generic or even specific level. This information is summarized in the form of a dichotomous key. A common and previously unreported anatomical feature is the presence of tension wood fibres developed eccentrically in secondary xylem at the distal pole of seedlings grown in both artificial and natural conditions. These fibres are apparently the source of the mechanism whereby an initially horizontal seedling becomes erected rapidly once it is rooted distally, even though this appears to be achieved at a considerable mechanical disadvantage. The morphological result is a distinct «hook» at the distal end of the established seedling. This mechanism allows rapid erection of a horizontally stranded seedling, such that the plumule is raised, snorkel-like, above immediate tidal influence. This may be necessary, since gas exchange through the surface of the seedling is not possible via the thick, cutinized and stomata-free epidermis of the hypocotyl. We suggest that the need to produce an elongated propagule, with this self-erecting capability favours the evolution of vivipary in these plants because it permits the earliest development of the plumule in air rather than water. The suitability of mangrove seedlings for further experimental and anatomical study is emphasized. The study also exemplifies how anatomy can be placed in a dynamic context.

Systematic and functional anatomy of seedlings in mangrove Rhizophoraceae: vivipary explained?

Abstract The hypocotyl of viviparous seedlings of mangrove Rhizophoraceae (tribe Rhizophoreae) has a common suite of anatomical features but a range of anatomical structures that may be diagnostic at the generic or even specific level. This information is summarized in the form of a dichotomous key. A common and previously unreported anatomical feature is the presence of tension wood fibres developed eccentrically in secondary xylem at the distal pole of seedlings grown in both artificial and natural conditions. These fibres are apparently the source of the mechanism whereby an initially horizontal seedling becomes erected rapidly once it is rooted distally, even though this appears to be achieved at a considerable mechanical disadvantage. The morphological result is a distinct «hook» at the distal end of the established seedling. This mechanism allows rapid erection of a horizontally stranded seedling, such that the plumule is raised, snorkel-like, above immediate tidal influence. This may be necessary, since gas exchange through the surface of the seedling is not possible via the thick, cutinized and stomata-free epidermis of the hypocotyl. We suggest that the need to produce an elongated propagule, with this self-erecting capability favours the evolution of vivipary in these plants because it permits the earliest development of the plumule in air rather than water. The suitability of mangrove seedlings for further experimental and anatomical study is emphasized. The study also exemplifies how anatomy can be placed in a dynamic context.

Regulation by uniconazole-P and gibberellins of morphological and anatomical responses of Fraxinus mandshurica seedlings to gravity.

The present study deals with roles of gibberellins (GAs) in gravitropic responses of woody stems of horizontally-positioned, 2-year-old seedling of Fraxinus mandshurica Rupr. var. japonica Maxim. The application of uniconazole-P, an inhibitor of GAs biosynthesis, to stem nodes at various concentrations significantly inhibited not only righting the stems but also wood formation. The application of GAs (GA3 and GA4) in combination with uniconazole-P negated the inhibitory effect of the treatment of uniconazole-P alone. The GAs alone did not affect wood formation, however, both GAs increased wood formation on both the upper and lower sides of horizontal stems with an increase in the concentration of combined uniconazole-P. In all of the seedlings, tension wood was formed on the upper side of stems. The application of uniconazole-P alone at various concentrations did not inhibit the formation of tension wood fibres.

The Possible function of Buttresses in Caryocar Nuciferum (Caryocaraceae) in Guyana: Ecological and Wood Anatomical Observations

This paper describes the microscopic structure and morphology of stern and buttresses of swamp-grown Caryocar nuciferum L. and discusses the function of buttresses. Buttresses are mainly found at the opposite side of the leaning direction of a tree and thus could function as tension members. In contrast to the stern wood, which exhibits a moderate amount of tension wood fibres with a gelatinous layer, the wood of the buttresses on the tension side and the compression side of the leaning tree is characterised by thick-walled tension wood fibres. In addition, the number of vessels in the buttresses is substantially higher than that in the stern wood. The preferential direction of the buttresses and the anatomical differences in the various parts of the tree are discussed.

Orientation of microfibrils and microtubules in developing tension-wood fibres of Japanese ash (Fraxinus mandshurica var. japonica)

The orientation of cellulose microfibrils (MFs) and the arrangement of cortical microtubules (MTs) in the developing tension-wood fibres of Japanese ash (Fraxinus mandshurica Rupr. var. japonica Maxim.) trees were investigated by electron and immunofluorescence microscopy. The MFs were deposited at an angle of about 45° to the longitudinal axis of the fibre in an S-helical orientation at the initiation of secondary wall thickening. The MFs changed their orientation progressively, with clockwise rotation (viewed from the lumen side), from the S-helix until they were oriented approximately parallel to the fibre axis. This configuration can be considered as a semihelicoidal pattern. With arresting of rotation, a thick gelatinous (G-) layer was developed as a result of the repeated deposition of parallel MFs with a consistent texture. Two types of gelatinous fibre were identified on the basis of the orientation of MFs at the later stage of G-layer deposition. Microfibrils of type 1 were oriented parallel to the fibre axis; MFs of type 2 were laid down with counterclockwise rotation. The counterclockwise rotation of MFs was associated with a variation in the angle of MFs with respect to the fibre axis that ranged from 5° to 25° with a Z-helical orientation among the fibres. The MFs showed a high degree of parallelism at all stages of deposition during G-layer formation. No MFs with an S-helical orientation were observed in the G-layer. Based on these results, a model for the orientation and deposition of MFs in the secondary wall of tension-wood fibres with an S1 + G type of wall organization is proposed. The MT arrays changed progressively, with clockwise rotation (viewed from the lumen side), from an angle of about 35–40° in a Z-helical orientation to an angle of approximately 0° (parallel) to the fibre axis during G-layer formation. The parallelism between MTs and MFs was evident. The density of MTs in the developing tension-wood fibres during formation of the G-layer was about 17–18 per μm of wall. It appears that MTs with a high density play a significant role in regulating the orientation of nascent MFs in the secondary walls of wood fibres. It also appears that the high degree of parallelism among MFs is closely related to the parallelism of MTs that are present at a high density.

Ultrastructural Investigation of Tension Wood Fibre in Fraxinus mandshurica Rupr. var. japonica Maxim.

The ultrastructure of the fibre wall in Fraxinus mandshurica Rupr. var. japonica Maxim. was investigated by electron microscopy. The trees had been inclined artificially at an angle of 30° to the vertical at the beginning of the initiation of cambial growth in early spring. The secondary walls of tension wood fibres were of the outer (S 1) layer and gelatinous (G) layer type. The microfibrils in the gelatinous (G) layer were oriented as a steep Z-helix relative to the fibre axis with a deviation that ranged from 0° to 25° but was mainly between 5° and 10°. The cross-sectional surface of tension wood fibres revealed the relatively strong attachment of the G-layer to the S 1 layer. The G-layer stained weakly with potassium permanganate. The S 1 layer of tension wood fibres stained less strongly than that of the normal and opposite wood fibres. These results indicate that the tension wood in F. mandshurica var. japonica is not typical and is somewhat anomalous. The secondary walls of normal and opposite wood fibres were composed of two layers, S 1 and S 2, and lacked an S 3 layer. Microfibrils in the S 3 layer of juvenile stems were extremely variable in orientation and were sparsely distributed without forming a layer. By contrast, a very thin S 3 layer was present in the wood fibres of mature stems. The variations in the formation of the S 3 layer in the fibre walls were probably due to the differences in the cambial age of the stems of F. mandshurica Rupr. var. japonica.

Formation and deposition of tannins in Quebracho colorado (Schinopsis balansae Engl.)

In Quebracho colorado heartwood tannins are located in the lumina of most parenchyma cells, fibres and vessels. Formation and deposition of the tannins were studied by electron microscopy. In parenchyma cells phenolic compounds first occur at the inner and outer side of the membranes of vacuoles. Later, these vacuoles are completely filled with dark stained compounds and fuse together. From parenchyma cells the phenolic substances migrate into the lumina of fibres and vessels through pits. In pits they penetrate into the compound middle lamella (CML) and diffuse along the CML. Tannins were also found in the cell walls and in fissures within the CML and G-layer of tension wood fibres.