IR Mapping and Imaging of Trees

The chemical image associated with syringyl lignin in longitudinal poplar sections suggested a higher lignin content in the outer bark than in the inner bark and a lower content in the cambium between the inner bark and the xylem [99, 100]. The crystallinity index of cellulose, which effects physical, mechanical, and chemical properties of cell walls, can be examined spatially resolved [100-102]. 

The fibers encircling the vessels in Populus tremula show a chemotype with higher lignin content compared to fibers in fiber-rich regions [103]. In addition, an increased hgnin content was found in the narrow band of parenchyma cells at the annual ring [103]. This probably reflects the thick primary cell wall and the additional wall layer described for this cell type. 

Synchrotron IR imaging of cross sections of Populus trkhocarpa stems revealed the distribution of aromatic compounds, fatty oils and waxes, acyl esters, proteins, lignin, and polysaccharides [104]. Acyl esters showed a close spatial distribution with cell wall polysaccharides. 

Genetic modification of the lignin biosynthesis by downregulation of the cinnamoyl-coenzyme A reductase in poplar P. tremula x Populus alba) decreased the lignin content and increased breakdown or remodeling of noncellulosic cell wall polymers as supported by FT-IR analysis [105]. 

In combination with polarization modulation, synchrotron IR microspectroscopy of single spruce fibers Picea abies L.) provided information on hydrogen bonding, orientation, and accessibility of structural units to heavy water and dimethylacetamide applied in a custom-built microfluidic cuvette [106]. A tabulated overview about the assignments of hydrogen bonds and their orientation is also provided by the authors. 

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