Chemical and ultrastructural changes of ash wood thermally modified using the thermo-vacuum process: I. Histo/cytochemical studies on changes in the structure and lignin chemistry

Abstract

Changes in structure and lignin chemistry were investigated in ash wood thermally modified (TMW) by the thermo-vacuum (Termovuoto) process for 3 h at 190–220°C by means of light, fluorescence, and transmission electron (TEM) microscopy combined with histo/cytochemistry. Variation in changes in native cell color in TMWs was positively correlated with differences in lignin content between cell types and cell wall regions in the reference wood. Histochemical staining showed increasing amounts of acidic groups in TMWs with different response to ethanol extraction between secondary cell walls and CMLcc (compound middle lamella/middle lamella cell corner) regions. Fluorescence microscopy of TMWs and references showed a difference in intensity and color emission of lignin autofluorescence, reflecting modification of lignin in TMWs. Changes in histochemistry and fluorescence were prominent at and above 200°C. With TEM, increased intensity of lignin staining and distortion of fiber S1 layers were detected in TMW treated for 3 h at 220°C (TMW3 h, 220°C). TMW3 h, 220°C differed significantly in molecular ultrastructure of fiber cell walls compared to references, such as loss of the lamellar structure and size and distribution of lignin aggregates. The modification in CMLcc structure in ash TMW3 h, 220°C is different from that of softwoods.

Autofluorescence in Plants

“…Lignin fluorescence is sensitive to the molecular environment and so can be manipulated with pH and by treatment with quenching agents such as nitrophenol-labeled carbohydrates [32,41]. Heat treatment [41,42] and infiltration with modifying chemicals [43] can also influence lignin autofluorescence. Lignin fluorescence can, therefore, be used as a biosensor for various research investigations such as measurement of cell wall porosity or detection of infiltrating chemicals in wood modification studies.…”

Enzymatic hydrolysis of the gelatinous layer in tension wood of Salix varieties as a measure of accessible cellulose for biofuels

“…Detailed analyses of individual TW regions and bres after staining was conducted using a Leica DMLB light microscope (Leica Microsystems, Wetzlar, Germany) equipped with an In nity X-32 digital camera (Deltapix, Samourn, Denmark) to con rm presence/absence of TW and G-layers. To visualize lignin distributions, fresh sections were stained using the Wiesner and Mäule reactions [47,48]. For the Wiesner reaction, sections were treated with 2 % v/v phloroglucinol in ethanol and mounted in 6 M hydrochloric acid.…”

Ultrastructure and immunocytochemistry of degradation in spruce and ash sapwood by the brown rot fungus Postia placenta: Characterization of incipient stages of decay and variation in decay process

“…Our previous histochemical study of ash wood showed overall stronger lignin staining by Wiesner reaction (i.e. total lignin) in ray/axial parenchyma cells than fibers which were more like vessels (Kim et al, 2014). However, there is also a possibility of an effect of syringyl (S) lignin on resistance of ray/axial parenchyma cells since unlike softwoods, lignin in hardwood xylem cells is composed of both G and S lignin (Donaldson, 2001).…”

“…The S/G ratio also varies between cell types in hardwoods (Donaldson, 2001). In addition, ash wood also showed high variation in S/G ratio depending on cell type and location within a growth ring (Kim et al, 2014). Schwarze (2007) proposed that the cell wall morphology rather than lignin composition and amount may be responsible for higher resistance of parenchyma cells than fibers.…”