Nature’s hierarchical materials

The term microfibril angle (MFA) in wood science refers to the angle between the direction of the helical windings of cellulose microfibrils in the secondary cell wall of fibres and tracheids and the long axis of cell. Technologically, it is usually applied to the orientation of cellulose microfibrils in the S2 layer that makes up the greatest proportion of the wall thickness, since it is this which most affects the physical properties of wood. This review describes the organisation of the cellulose component of the secondary wall of fibres and tracheids and the various methods that have been used for the measurement of MFA. It considers the variation of MFA within the tree and the biological reason for the large differences found between juvenile (or core) wood and mature (or outer) wood. The ability of the tree to vary MFA in response to environmental stress, particularly in reaction wood, is also described. Differences in MFA have a profound effect on the properties of wood, in particular its stiffness. The large MFA in juvenile wood confers low stiffness and gives the sapling the flexibility it needs to survive high winds without breaking. It also means, however, that timber containing a high proportion of juvenile wood is unsuitable for use as high-grade structural timber. This fact has taken on increasing importance in view of the trend in forestry towards short rotation cropping of fast grown species. These trees at harvest may contain 50% or more of timber with low stiffness and therefore, low economic value. Although they are presently grown mainly for pulp, pressure for increased timber production means that ways will be sought to improve the quality of their timber by reducing juvenile wood MFA. The mechanism by which the orientation of microfibril deposition is controlled is still a matter of debate. However, the application of molecular techniques is likely to enable modification of this process. The extent to which these techniques should be used to improve timber quality by reducing MFA in juvenile wood is, however, uncertain, since care must be taken to avoid compromising the safety of the tree.

Cellulose microfibril angle in the cell wall of wood fibres

Microfibril angle (MFA) is perhaps the easiest ultrastructural variable to measure for wood cell walls, and certainly the only such variable that has been measured on a large scale. Because cellulose is crystalline, the MFA of the S2 layer can be measured by X-ray diffraction. Automated X-ray scanning devices such as SilviScan have produced large datasets for a range of timber species using increment core samples. In conifers, microfibril angles are large in the juvenile wood and small in the mature wood. MFA is larger at the base of the tree for a given ring number from the pith, and decreases with height, increasing slightly at the top tree. In hardwoods, similar patterns occur, but with much less variation and much smaller microfibril angles in juvenile wood. MFA has significant heritability, but is also influenced by environmental factors as shown by its increased values in compression wood, decreased values in tension wood and, often, increased values following nutrient or water supplementation. Adjacent individual tracheids can show moderate differences in MFA that may be related to tracheid length, but not to lumen diameter or cell wall thickness. While there has been strong interest in the MFA of the S2 layer, which dominates the axial stiffness properties of tracheids and fibres, there has been little attention given to the microfibril angles of S1 and S3 layers, which may influence collapse resistance and other lateral properties. Such investigations have been limited by the much greater difficulty of measuring angles for these wall layers. MFA, in combination with basic density, shows a strong relationship to longitudinal modulus of elasticity, and to longitudinal shrinkage, which are the main reasons for interest in this cell wall property in conifers. In hardwoods, MFA is of more interest in relation to growth stress and shrinkage behaviour.

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Microfibril Angle: Measurement, Variation and Relationships – A Review

https://www.repository.cam.ac.uk/bitstream/1810/279810/1/Authors%20copy.pdf

Towards the design of high-performance plant fibre composites

Linkage disequilibrium (LD) mapping using natural populations results in higher resolution of marker-trait associations compared to family-based quantitative trait locus (QTL) studies. Depending on the extent of LD, it is possible to identify alleles within candidate genes associated with a trait. Analysis of a natural mutant in Arabidopsis has shown that mutations in cinnamoyl CoA reductase (CCR), a key lignin gene, affect physical properties of the secondary cell wall such as stiffness and strength. Using this gene, we tested whether LD mapping could identify alleles associated with microfibril angle (MFA), a wood quality trait affecting stiffness and strength of wood. We identified 25 common single-nucleotide polymorphism (SNP) markers in the CCR gene in Eucalyptus nitens. Using single-marker and haplotype analyses in 290 trees from a E. nitens natural population, two haplotypes significantly associated with MFA were found. These results were confirmed in two full-sib families of E. nitens and Eucalyptus globulus. In an effort to understand the functional significance of the SNP markers, we sequenced the cDNA clones and identified an alternatively spliced variant from the significant haplotype region. This study demonstrates that LD mapping can be used to identify alleles associated with wood quality traits in natural populations of trees.

https://www.genetics.org/content/genetics/171/3/1257.full.pdf

Polymorphisms in Cinnamoyl CoA Reductase ( CCR ) Are Associated With Variation in Microfibril Angle in Eucalyptus spp.

SilviScan-2 was used to estimate and map microfibril angle (MFA) and density in twenty-nine 15-year-old Eucalyptus nitens trees. Over 4000 MFA measurements were made at about 100/hour. Each measurement represented a weighted average for about 50000 fibres. After an initial decrease near ground level, density increased with height in the stem. MFA decreased with height in the stem, reached a minimum around 30 to 50% of tree height, then increased towards the top. In the radial direction, density first decreased for a few years, then increased towards the bark. MFA was in the range 20 to 30 degrees near the pith at all heights and generally decreased towards the bark over most of the height of the stem. The lowest MFA values (approx. 10 degrees) were found close to the bark at 30 to 50% of stem height. Both density and MFA varied more rapidly near ground level, increasing the uncertainty of correlations between breast-height properties and whole tree properties. On some samples, high-resolution (0.2 mm) MFA scans were performed to confirm an earlier finding that MFA is strongly inversely correlated with density over a few growth rings but not over larger distances. X-ray diffraction was also used for estimating the orientation of the fibres within the samples. The radial variation in fibre orientation generally decreased with distance from the ground. Breast-height sample properties were moderate to good predictors of whole tree properties. Better correlations were obtained using samples from 5.5 m. Whole tree average density and MFA were found to be uncorrelated, indicating that tree improvement strategies could be designed to simultaneously optimise these properties.

Sharee Stringer

Papers

Variation of microfibril angle, density and fibre orientation in twenty-nine Eucalyptus nitens trees