Bernadette Nanayakkara

Quantitative chemical indicators to assess the gradation of compression wood

Abstract A chemistry-based parameter has been sought for determining the gradation of compression wood (CW), i.e., the severity, in tissues of Pinus radiata wood. Fluorescence microscopy was the reference for characterisation of the tissues containing CW. The collected material contained CW of varying severity, beginning with normal wood (NW containing no CW), continuing with material with some features of CW (CW of mild severity, MCW) and ending up with a material with pronounced features of CW (CW of high severity, SCW). Matching opposite wood (OW) was also included in the study. The chemical analyses included lignin determination, sugar analysis in the acid hydrolysate, thioacidolysis, 31P-NMR spectroscopic analysis and steric exclusion chromatography of thioacidolysis products. As the severity of CW changed progressively from NW through MCW to SCW, all chemical parameters changed concurrently. In particular, levels of galactose and lignin increased, while those of glucose and mannose decreased. The amounts of p-hydroxyphenyl β-ethers released by chemical degradation and uncondensed p-hydroxyphenyl C-9 units also increased at elevated CW severity levels. The amounts of galactose and the p-hydroxyphenyl content of the lignin correlated linearly with lignin for CW samples. The chemical differences between CW and OW in the stem, branch and seedling were similar, i.e., they are independent of the morphological origin of the sample. Parameters based on the p-hydroxyphenyl unit content appear the most suitable chemical indicators of CW severity, as they are least sensitive to the samples morphological origin and their response to CW severity is high.

Near infrared hyperspectral imaging applied to mapping chemical composition in wood samples

This paper describes a method for the two-dimensional mapping of chemical composition on the transverse face of cross-sectional discs from trees. The method uses an imaging spectrograph coupled to a near infrared (NIR) camera (900–1700 nm) to obtain NIR hyperspectral data sets which are processed using partial least squares regression to visualise the distribution and variation of lignin, galactose and glucose in Pinus radiata discs with R2/standard error of performance values of 0.84/1.48 (lignin), 0.87/0.68 (galactose) and 0.87/0.95 (glucose). The hardware design and software control are described along with a method for calibration based on one dimension spatially resolved predictions of chemical composition from conventional NIR spectroscopy. The NIR imaging system was designed as a rapid and cost-effective means of mapping chemical composition over the entire disc at a spatial resolution of ∼4mm2/pixel. The resulting maps of chemical composition clearly indicate, at high spatial resolution, the extent of heterogeneity that occurs in logs.

Xylem parenchyma cell walls lack a gravitropic response in conifer compression wood

AbstractMain conclusionCell wall fluorescence and immunocytochemistry demonstrate that xylem parenchyma cell walls do not show changes in structure and composition related to gravitropic response comparable to those of tracheids, even when they have lignified secondary cell walls. Tracheid cell walls in compression wood have altered composition and structure which generates the strain responsible for correction of stem lean as part of the gravitropic response of woody plants. Xylem parenchyma cell walls vary among conifer species and can be lignified secondary walls (spruce) or unlignified primary walls (pine). It can be expected that xylem parenchyma with lignified secondary cell walls might show features of compression wood comparable to those of tracheids that have a similar type of cell wall. A comparison of xylem parenchyma cell walls in normal and compression wood in species with lignified and non-lignified parenchyma cell walls provides a unique opportunity to understand the process of reaction wood formation in conifers. Using both UV/visible fluorescence microscopy of cell wall fluorophores and immunocytochemistry of galactan and mannan epitopes, we demonstrate that xylem parenchyma cell walls do not show the changes in composition and structure typical of compression wood tracheids. Adjacent cells of different types but with similar cell wall structure can undergo cell wall developmental changes related to support or defence functions independent of their neighbours. Tracheids are sensitive to gravitropic signals while xylem parenchyma cells are not.

Effects of induced drought and tilting on biomass allocation, wood properties, compression wood formation and chemical composition of young Pinus radiata genotypes (clones)

Abstract Eight genotypes (clones) of Pinus radiata were subjected to drought and stem inclination to assess genotype response to common stressors. While drought stress reduced diameter growth, height growth and total biomass accumulation, root to shoot (R/S) ratios were unaffected. Drought-stressed plants had significantly lower average acoustic velocity, but longitudinal shrinkage (LS) and density were not different from those of the control plants. Radial diameter growth and R/S ratios were unaffected by tilting. Inclined stems had significantly lower acoustic velocity, and significantly higher LS and density than control stems. Acoustic velocity had a strong negative correlation with LS (r2=0.79). Compression wood (CW) content was much higher in tilted plants, compared to control and drought treatment plants. The CW of tilted trees had different chemistry than that of the CW of drought and control plants. Genotypes differed significantly in the amount of CW formed as a response to tilting, demonstrating that the formation and extent of CW is genetically influenced. Mechanical perturbation in conjunction with acoustic methods for assessing stiffness would be a useful approach for early-age selection of genotypes less prone to form CW.

Mapping within-stem variation of chemical composition by near infrared hyperspectral imaging

A near infrared (NIR) imaging spectrograph was used to generate maps of chemical composition distribution on the surface of transverse wood discs taken from tree stems. The measured chemical components were lignin, galactose, glucose and mannose as well as cellulose and hemicelluloses, which were calculated from monomeric sugars. These components were determined using NIR-based chemistry models, which had been developed specifically for the imaging spectrograph. Explained test-set variation for key constituents ranged between 60% (galactan) and 78% (lignin). Day-to-day variability was 1–2% (standard deviation/range) depending on the chemical property. Various operational parameters such as room temperature, sample temperature, sample surface preparation and sample thickness were found to have a non-negligible, but manageable, influence on predicted results. The influence of room and sample temperatures could be reduced by incorporating temperature changes into the chemistry model. Extractives, transported to, and concentrated at, the disc surface during drying, needed to be physically removed from the surface to avoid an unpredictable influence on chemical results. Wood fibre angles at the disc surface needed to be aligned in a consistent manner to the camera. NIR information was found to derive from a sample depth of up to 10 mm. This distance was consequently chosen as the minimal sample thickness.

Comparison of hydrogenolysis with thioacidolysis for lignin structural analysis

Abstract Mild hydrogenolysis has been compared with thioacidolysis as a method for degrading lignins in situ and in isolated form before analysis by gas chromatography/mass spectrometry and quantitative 31P nuclear magnetic resonance (NMR) spectroscopy. Both degradation methods gave similar levels of β-aryl ether-linked phenylpropane units that were released as monomers. Degradation by hydrogenolysis generally gave lower levels of total phenylpropane units when analyzed by 31P NMR, especially in the case of lignins with high levels of condensed units. Overall, these results indicate that mild hydrogenolysis could offer an alternative to thioacidolysis for probing lignin structure.

Understanding the degree of condensation of phenolic and etherified C-9 units of in situ lignins.

A novel approach for the quantification of the degree of condensation at the C(5) position of etherified and phenolic phenylpropane (C-9) units of in situ lignin is described. This is achieved by degrading unmethylated and methylated wood by thioacidolysis and analyzing the resultant product mixtures by quantitative (31)P NMR spectroscopy. Applying this new method to compression wood and normal wood from Pinus radiata showed that, whereas 41-47% of etherified guaiacyl C-9 units are condensed at the C(5) position, almost all phenolic guaiacyl C-9 units exist as uncondensed moieties. Analysis of milled wood lignin (MWL) isolated from the same wood by (31)P NMR spectroscopy before and after thioacidolysis showed that the phenolic guaiacyl C-9 units were more condensed than those in the in situ lignin. This is likely due to partial cleavage of the more condensed etherified linkages during the lignin isolation, leading to a relative increase in condensed phenolic guaiacyl C-9 units.

Fluorescence imaging of cambial zones to study wood formation in Pinus radiata D. Don.

Key messageA technique whereby whole mounts of delicate tissues of differentiating xylem are imaged directly by polishing and block-face imaging of embedded microcores. Autofluorescence and image analysis aids identifying the stages of xylogenesis.AbstractStem microcores from fast-growing trees, such as Pinus radiata (D. Don) with wide zones of cambium and differentiating xylem and very wide growth rings, pose a challenge for microscopy, as they are difficult to handle and easily damaged compared to slower growing species. A novel procedure has been developed which captures high-resolution images directly from the block face of large samples embedded in plastic resin without the need for sectioning or staining. Microcores of differentiating xylem of P. radiata growing in the central North Island of New Zealand were embedded in a low viscosity acrylic resin. The surface of the entire resin block was abraded and polished to expose cross sections of the wide zone of wood formation in these fast-growing trees without damage or distortion. Autofluorescence imaging was performed using a confocal laser scanning microscope. This avoided the need for staining and allowed the determination of the beginning of lignification based on lignin autofluorescence. Image analysis was used to determine the widths of: (a) the cambium, cell expansion, and wall-thickening zone (CET) and (b) the wall lignification zone (LT). A fast-growing tree had wider CET and LT zones than a slow-growing tree. This was due to the fast-growing tree producing more tracheids than the slow-growing tree, rather than by the production of larger tracheids.

Screening of juvenile Pinus radiata wood by means of Py-GC/MS for compression wood focussing on the ratios of p-hydroxyphenyl to guaiacyl units (H/G ratios)

Abstract Analysis of thin slivers of juvenile (2-year-old) pine wood by pyrolysis-gas chromatography-mass spectrometry (Py-GC/MS) was explored as a fast and high-throughput method to assess compression wood (CW) via the ratio of the p-hydroxyphenyl to guaiacyl units (H/G ratio). Various chemical parameters such as contents of lignin, galactose, and p-hydroxyphenyl (H) units have been related to the CW in the tissue. The H/G ratio was calculated from all identified H and G lignin-derived pyrolysis products. The H/G ratio ranged between 0.019 and 0.052 for normal wood (NW) and between 0.133 and 0.227 for CW. There was a weak correlation between CW H/G ratio and Klason lignin content (R2=0.4), but it was not correlated to the galactose content. Lignin- and carbohydrate-derived pyrolysis products were subjected to principal component (PC) analysis. The first two PCs discriminate the CW from the NW, accounting for about 41% of the total variance in the dataset. The model developed based on lignin-derived peaks explained 54% of the variance.