Chemical Imaging of Poplar Wood Cell Walls by Confocal Raman Microscopy

Abstract

Confocal Raman microscopy was used to illustrate changes of molecular composition in secondary plant cell wall tissues of poplar (Populus nigra 3 Populus deltoids) wood. Two-dimensional spectral maps were acquired and chemical images calculated by integrating the intensity of characteristic spectral bands. This enabled direct visualization of the spatial variation of the lignin content without any chemical treatment or staining of the cell wall. A small (0.5 mm) lignified border toward the lumen was observed in the gelatinous layer of poplar tension wood. The variable orientation of the cellulose was also characterized, leading to visualization of the S1 layer with dimensions smaller than 0.5 mm. Scanning Raman microscopy was thus shown to be a powerful, nondestructive tool for imaging changes in molecular cell wall organization with high spatial resolution.

Raman Spectroscopic Characterization of Wood and Pulp Fibers

“…Such information can be obtained with a Raman micro scope that provides images based on a Raman band characteristic for the component of interest [18]. In cell wall Raman imaging, inherent scattering characteristics of cellulose and lignin are used to visualize their distribution and no external labels are necessary [8,9]. In confocal Raman microscopy, point-by-point imaging couples the mobility of an auto mated scanning stage with the high numerical aperture of the microscope objective to take the spectrum at each sample point.…”

“…Most recent applications of confocal Raman microscopy to woody cell walls consisted of chemical imaging of poplar (including the tension wood) and spruce woods [8,9] as well as topochemical studies of beech wood [7]. In the case of spruce [9], a state-of-the-art 633-nm laser-based confocal Raman microscope was used to determine the distribution of cell wall components in the cross section of black spruce wood in situ.…”

“…Such progress has made the macro-and microlevel Raman investigations of wood and fibers much more valuable. For instance, in the area of microinvestigations, chemical imaging of fiber cell walls has become a reality and images of cellulose and lignin distributions in the plant cell wall have provided useful information on the compositional and organizational characteristics of woody tissues [7][8][9].…”

“…Currently easier to use and increasingly available, Raman microscopy is being used more frequently. In the investigations of wood and pulp fibers, Raman stud ies range from investigating tensile deformation of single wood fibers [55,56] to detailed investigations, at the subcellular level, of composition and ultrastructure of native woody tissue [7][8][9].…”

Visualization of structural changes in cellulosic substrates during enzymatic hydrolysis using multimodal nonlinear microscopy

“…Time of flight secondary ion mass spectroscopy (Jeremic et al 2014), infrared absorption (Gierlinger et al 2008) and Raman scattering techniques (Gierlinger and Schwanninger 2006) allow studying chemical information at submicrometre spatial resolution. Imaging speed of micro-spectroscopy techniques is often limited by the amount of spectral information one wants to retrieve.…”