Formation and Structure of Lignin in Monocotyledons. III. Heterogeneity of Sugarcane (Saccharum officinarumL.) Lignin with Respect to the Composition of Structural Units in Different Morphological Regions

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Abstract
Heterogeneity of sugarcane lignin with respect to the composition of structural units in different morphological regions was studied by microautoradiography and some degradative analyses. Structure of the lignin differs among fiber, vessel and parenchyma. The lignin in the secondary wall of fiber is composed of syringyl (S)-, guaiacyl (G)- and p-hydroxyphenyl (H)-propane units with accompanying phenolic acid residues, and the proportion of these monolignols is S > G > H. The lignin in vessels of protoxylem contains more G and H units than S units, and that in vessels of metaxylem is similar to that in fibers. Phenolic acid constituent in sugarcane cell wall includes sinapic acid in addition to p-coumaric and ferulic acids. Ferulic acid deposits at the very early stage of lignification, and p-coumaric and sinapic acids increasingly deposit with the progress of lignification. Therefore, the ratio of p-coumaric acid or sinapic acid to ferulic acid increases with lignification. Parenchyma wall involves larger amount of phenolic acids than vascular bundle does. The lignin in parenchyma is difficult to isolate by Björkman procedure, because there is greater possibility to form cross-linkage among cell wall polymers through phenolic acids. The cross linkages involving phenolic acid ester also explains why the sugarcane lignin is easily degraded.