Fiber Characteristics and Mechanical Properties of Oxytenanthera abyssinica

Unlike the culm hollow structure of most bamboo species, Oxytenanthera abyssinica has a unique solid or semi-solid culm, which may endow it with superior mechanical performance. In this study, the variation in fiber morphology and micro-mechanical properties across the radial regions of bamboo culm was examined by optical microscopy, scanning electron microscopy, X-ray diffraction, and nanoindentation. Results showed that the mean values of vascular bundle frequency and fiber tissue proportion were 1.76 pcs/mm2 and 21.04%, respectively, both of which increased gradually from inner to outer. The mean length, diameter, and length-diameter ratio of the fiber were 2.10 mm, 21.54 μm, and 101.41 respectively. The mean indentation modulus of elasticity (IMOE) and hardness were 21.34 GPa and 545.88 MPa. The IMOE exhibited a significant increase from the inner to the middle region, and little change was observed from the middle to the outer region. There were slight fluctuations in hardness along the radial direction. The mean crystallinity and microfibril angle(MFA) of the fibers was 68.12% and 11.26 degrees, respectively. There is a positive correlation between cellulose crystallinity and the IMOE and hardness, while there is a negative correlation between the MFA and the IMOE and the hardness.

The Structural Origins of Wood Cell Wall Toughness

The remarkable mechanical stability of wood is primarily attributed to the hierarchical fibrous arrangement of the polymeric components. While the mechanisms by which fibrous cell structure and cellulose microfibril arrangements lend stiffness and strength to wood have been intensively studied, the structural origins of the relatively high splitting fracture toughness remain unclear. This study relates cellulose microfibril arrangements to splitting fracture toughness in pine wood cell walls using in situ electron microscopy and reveals a previously unknown toughening mechanism: the specific arrangement of cellulose microfibrils in the cell wall deflects cracks from the S2 layer to the S1/S2 interface, and, once there, causes the crack to be repetitively arrested and shunted along the interface in a zig-zag path. It is suggested that this natural adaptation of wood to achieve tough interfaces and then deflect and trap cracks at them can be generalized to provide design guidelines to improve toughness of high-performance and renewable engineering materials.

Ethylene Signaling Is Required for Fully Functional Tension Wood in Hybrid Aspen

Tension wood (TW) in hybrid aspen trees forms on the upper side of displaced stems to generate a strain that leads to uplifting of the stem. TW is characterized by increased cambial growth, reduced vessel frequency and diameter, and the presence of gelatinous, cellulose-rich (G-)fibers with its microfibrils oriented parallel to the fiber cell axis. Knowledge remains limited about the molecular regulators required for the development of this special xylem tissue with its characteristic morphological, anatomical, and chemical features. In this study, we use transgenic, ethylene-insensitive (ETI) hybrid aspen trees together with time-lapse imaging to show that functional ethylene signaling is required for full uplifting of inclined stems. X-ray diffraction and Raman microspectroscopy of TW in ETI trees indicate that, although G-fibers form, the cellulose microfibril angle in the G-fiber S-layer is decreased, and the chemical composition of S- and G-layers is altered than in wild-type TW. The characteristic asymmetric growth and reduction of vessel density is suppressed during TW formation in ETI trees. A genome-wide transcriptome profiling reveals ethylene-dependent genes in TW, related to cell division, cell wall composition, vessel differentiation, microtubule orientation, and hormone crosstalk. Our results demonstrate that ethylene regulates transcriptional responses related to the amount of G-fiber formation and their properties (chemistry and cellulose microfibril angle) during TW formation. The quantitative and qualitative changes in G-fibers are likely to contribute to uplifting of stems that are displaced from their original position.

Do tall tree species have higher relative stiffness than shorter species?

PREMISE OF THE STUDY

In 1757 Leonhard Euler demonstrated that to avoid bending tall columns needed to be stiffer but not stronger than shorter columns of equal diameter and material density. Many researchers have concluded that trees have a fixed stiffness to basic density ratio, and therefore, trees adjust for increasing height by adding mass to adjust stem form. But the wood science literature points to considerable variance in stiffness with respect to green wood density.

METHODS

Using the vast global repository of green wood mechanical properties, we compared relative stiffness and relative strength between taller and shorter species. For North American trees, we examined stem moisture distribution.

KEY RESULTS

For all regions of the world, taller species on average possessed greater stiffness, but not strength, than shorter species of equal basic specific gravity. We looked for a possible universal mechanism that might allow taller tree species to adjust stiffness without affecting xylem specific gravity and concluded that the evidence points to a decrease in cellulose microfibril angle in structural cell walls combined with possible increases in holocellulose percentage. The evidence is strongest for conifers. We also showed that tall conifers have the ability to adjust the distribution of xylem moisture to maximize conduction while minimizing column load.

CONCLUSIONS

Our research reveals that taller trees have developed internal stem adjustments to minimize diameter increase while attaining ever-greater heights, thus enabling these taller species to reduce energy expended on biomass accumulation while gaining greater access to solar radiation.

Cellulose structure and lignin distribution in normal and compression wood of the Maidenhair tree (Ginkgo biloba L.)

We studied in detail the mean microfibril angle and the width of cellulose crystals from the pith to the bark of a 15-year-old Maidenhair tree (Ginkgo biloba L.). The orientation of cellulose microfibrils with respect to the cell axis and the width and length of cellulose crystallites were determined using X-ray diffraction. Raman microscopy was used to compare the lignin distribution in the cell wall of normal/opposite and compression wood, which was found near the pith. Ginkgo biloba showed a relatively large mean microfibril angle, varying between 19° and 39° in the S2 layer, and the average width of cellulose crystallites was 3.1-3.2 nm. Mild compression wood without any intercellular spaces or helical cavities was observed near the pith. Slit-like bordered pit openings and a heavily lignified S2L layer confirmed the presence of compression wood. Ginkgo biloba showed typical features present in the juvenile wood of conifers. The microfibril angle remained large over the 14 annual rings. The entire stem disc, with a diameter of 18 cm, was considered to consist of juvenile wood. The properties of juvenile and compression wood as well as the cellulose orientation and crystalline width indicate that the wood formation of G. biloba is similar to that of modern conifers.

Plant material features responsible for bamboo's excellent mechanical performance: a comparison of tensile properties of bamboo and spruce at the tissue, fibre and cell wall levels

BACKGROUND AND AIMS

Bamboo is well known for its fast growth and excellent mechanical performance, but the underlying relationships between its structure and properties are only partially known. Since it lacks secondary thickening, bamboo cannot use adaptive growth in the same way as a tree would in order to modify the geometry of the stem and increase its moment of inertia to cope with bending stresses caused by wind loads. Consequently, mechanical adaptation can only be achieved at the tissue level, and this study aims to examine how this is achieved by comparison with a softwood tree species at the tissue, fibre and cell wall levels.

METHODS

The mechanical properties of single fibres and tissue slices of stems of mature moso bamboo (Phyllostachys pubescens) and spruce (Picea abies) latewood were investigated in microtensile tests. Cell parameters, cellulose microfibril angles and chemical composition were determined using light and electron microscopy, wide-angle X-ray scattering and confocal Raman microscopy.

KEY RESULTS

Pronounced differences in tensile stiffness and strength were found at the tissue and fibre levels, but not at the cell wall level. Thus, under tensile loads, the differing wall structures of bamboo (multilayered) and spruce (sandwich-like) appear to be of minor relevance.

CONCLUSIONS

The superior tensile properties of bamboo fibres and fibre bundles are mainly a result of amplified cell wall formation, leading to a densely packed tissue, rather than being based on specific cell wall properties. The material optimization towards extremely compact fibres with a multi-lamellar cell wall in bamboo might be a result of a plant growth strategy that compensates for the lack of secondary thickening growth at the tissue level, which is not only favourable for the biomechanics of the plant but is also increasingly utilized in terms of engineering products made from bamboo culms.

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.

Palaeo-adaptive properties of the xylem of Metasequoia: mechanical/hydraulic compromises

The xylem of Metasequoia glyptostroboides Hu et Cheng is characterized by very low density (average specific gravity = 0.27) and tracheids with relatively large dimensions (length and diameter). The microfibril angle in the S2 layer of tracheid walls is large, even in outer rings, suggesting a cambial response to compressive rather than tensile stresses. In some cases, this compressive stress is converted to irreversible strain (plastic deformation), as evidenced by cell wall corrugations. The heartwood is moderately decay resistant, helping to prevent Brazier buckling. These xylem properties are referenced to the measured bending properties of modulus of rupture and modulus of elasticity, and compared with other low-to-moderate density conifers. The design strategy for Metasequoia is to produce a mechanically weak but hydraulically efficient xylem that permits rapid height growth and crown development to capture and dominate a wet site environment. The adaptability of these features to a high-latitude Eocene palaeoenvironment is discussed.