Scanning UV microspectrophotometry as a tool to study the changes of lignin in hydrothermally modified wood

Thermal modification (TM) of wood has occupied a relatively narrow but stable niche as an alternative for chemical wood protection. There are different technological solutions for TM and not all details of their effects on wood tissue have been understood. The one-stage hydrothermal modification (HTM) at elevated vapour pressure essentially changes the wood’s composition and structure. In the present paper, the changes in three hardwood lignins (alder, aspen, and birch) were observed within the cell wall by means of cellular UV microspectrophotometry. The lignin absorbances in the compound middle lamella (CML) of unmodified wood are 1.7- to 2.0-fold higher than those in the fibre S2 layer. The woods were modified in the temperature range from 140 to 180°C, while in the lower temperature range (140°C/1 h), the UV absorbances are little affected. Essential changes occur in the range of 160–180°C and the UV data reflect these by absorbtion changes, while the absorbances at 278 nm rise with factors around 2 more in the S2 layer than in the CML. The absorbance increments are interpreted as polycondensation reactions with furfural and other degradation products of hemicelluloses with the lignin moiety of the cell wall.

Distribution of lignin in vascular bundles of coconut wood (Cocos nucifera) by cellular UV-spectroscopy and relationship between lignification and tensile strength in single vascular bundles

The topochemical distribution of lignin in vascular bundles separated from different radial positions of Mexican coconut wood stems (Cocos nucifera) has been studied with focus on the relationship between the degree of lignification and tensile strength properties. The cellular lignin distribution was analyzed by UV-microspectrophotometry (UMSP) scanning at 280 nm of sections of vascular bundles (VBs) of 1 μm thickness. The fibers of the VBs with high tensile strength reveal a relatively low UV-absorbance at 280 nm (A280 nm 0.39), whereas the VBs with low tensile strength display the highest A280 nm (0.59). The S2 of fiber walls are characterized by the typical lamellar structure with increasing lignin contents from the cell lumen towards the compound middle lamellae (CML). The A280 nm data of CMLs are higher (0.67 to 0.87) than those of the S2 wall layers. Overall, the A280 nm values of S2 of fibers walls within single VBs of coconut are in the range of 0.36 to 0.59.

Topochemistry of heat-treated and N-methylol melamine-modified wood of koto (Pterygota macrocarpa K. Schum.) and limba (Terminalia superba Engl. et. Diels)

To broaden the knowledge about the chemical changes at the cell wall level of differently modified tropical hardwoods, heat-treated and N-methylol melamine (NMM)-treated samples of koto (Pterygota macrocarpa) and limba (Terminalia superba) were prepared. UV microspectrophotometry (UMSP) was applied at 278 and 240 nm as specific wavelengths to analyze chemical alterations of the samples caused by heat and NMM treatment, respectively. The absorbance of koto exceeded that of limba before and after treatment, potentially due to the higher extractive content of the former. Regardless of the wood species, the absorbance of the samples increased with increasing intensity of the NMM treatment. Additionally, the absorbance of lignin within the spectrum of 230–350 nm was altered due to the NMM treatment. The functionality of applying specific wavelengths for the analysis of different modification methods of wood was proven. However, the comparison with literature did not show differences in the absorbance, which could be assigned to the characteristics of tropical hardwoods.

Cellular and topochemical characteristics of secondary changes in bark tissues of beech (Fagus sylvatica)

Anatomical and histometrical investigations were carried out on bark tissues (non-collapsed and collapsed phloem, and periderm) of beech (Fagus sylvatica L.) trees growing at a forest site near Ljubljana, Slovenia (400 m a.s.l.). Secondary changes in bark, especially sclereid formation and lignification, were followed at cellular and sub-cellular levels by light microscopy (LM), UV microspectrophotometry (UMSP), and transmission electron microscopy (TEM). The average width of the whole bark was 5960 μm; non-collapsed phloem on average occupied 4%, collapsed phloem 89%, and periderm 7% of the bark. Secondary changes in mature phloem were characterized by a collapse of sieve tubes, inflation of axial parenchyma, and development of sclereids. The percentage of sclereid areas and the stage of their development elevated with increasing distance from the cambium. The most pronounced increment in the sclereid proportion was observed in the second- and third-fifth of the bark. Sclereid cell walls were thick and poly-lamellated and had similar spectral characteristics but distinctly higher UV absorbance values than xylem fibers. By means of a combination of LM, TEM and UMSP techniques, the structure and secondary changes in the bark could be precisely identified.

Wood Anatomical features and chemical composition of Prosopis kuntzei from the paraguayan chaco

Anatomical features for Prosopis kuntzei Harms were studied by light and scanning electron microscopy. The wood is mainly diffuse-porous with indistinct growth ring boundaries. Vessel diameter ranges between 11 to 193 μm. The thick-walled fibres average 1275 μm in length. Parenchyma bands are 66 to 1066 μm apart. Heartwood extractives were studied in the vessels, rays and part of the fibres by means of scanning UV microspectrophotometry. The pyrolitic lignin content is 30.7%. The percentage of polyphenolic compounds, such as flavonoids, hydrolysable tannins and proanthocyanidins, is comparatively high at 5.8%. Total extract contents were determined after organic solvent extractions (23.2%) and water extractions (24.9%). The FTIR spectroscopy showed nearly identical spectra for the methanol and water extracts, with characteristic absorption bands for aromatic substances at 1615 and 1520 cm-1. The spectrum of the acetone extract differs only due to an additional but distinct absorption in the carbonyl range at 1695 cm-1. GC/MS analyses revealed that in the acetone and methanol extracts, tetrahydroxy-flavan-3-ols (isomers of catechin and epicatechin) were the main constituents with a ratio of 25.3 and 27.6%.

Intervessel Pit Structure and Histochemistry of Two Mangrove Species as Revealed by Cellular UV Microspectrophotometry and Electron Microscopy: Intraspecific Variation and Functional Significance

Intervessel pits play a key role in trees' water transport, lying at the base of drought-induced embolism, and in the regulation of hydraulic conductivity via hydrogels bordering pit canals. Recently, their microstructure has been the focus of numerous studies, but the considerable variation, even within species and the histochemistry of pit membranes, remains largely unexplained. In the present study, intervessel pits of the outermost wood were examined for Avicennia marina, of dry and rainy season wood separately for Rhizophora mucronata. The thickness of the pit membranes was measured on transmission electron micrographs while their topochemical nature was also analyzed via cellular UV microspectrophotometry. Pit membranes of R. mucronata were slightly thicker in dry season wood than in rainy season wood, but their spectra showed for both seasons a lignin and a yet unidentified higher wavelength absorbing component. It was suggested to be a derivative of the deposits, regularly filling pit canals. The vestures of A. marina chemically resembled pit membranes rather than cell walls.

Anatomy, Cell Wall Structure and Topochemistry of Water-Logged Archaeological wood aged 5,200 and 4,500 years

Evaluating the state of deterioration of water-logged archaeological wood is necessary in order to select treatments for its conservation and storage, particularly in the case of valuable archaeological artefacts. For this purpose archaeological wood of ash (Fraxinus sp.) and oak (Quercus sp.) buried in water-logged conditions at prehistoric settlements on the Ljubljansko barje (Ljubljana moor), Slovenia, aged approx. 5,200 and 4,500 years, was investigated by means of light microscopy (LM), transmission electron microscopy (TEM) and cellular UV-microspectrophotometry (UMSP). LM and TEM revealed that the ash wood aged 5,200 years was the least preserved. The secondary walls of fibres, vessels and parenchyma cells were considerably thinner than in normal wood, indicating distinct degradation. TEM and UMSP additionally revealed strong delignification of the remaining parts of the secondary walls of all cell types. The compound middle lamellae appeared structurally intact, but had lower UV-absorbance than normal wood of the same species. The cell corners were topochemically unchanged, as shown by high analogue UV-absorbance. The UV-absorbance maxima at a wavelength of 278 nm corresponded to those of hardwood lignins. The oak heartwood was generally better preserved than the ash wood. Within each species, the 4,500- year-old samples generally appeared better preserved than those 5,200 years old.