Distribution and Structure of Lignin

Abstract

Lignification starts at cell corner middle lamella when the secondary wall formation of tracheid starts, and then proceeds toward compound middle lamella in gymnosperm. Lignification of compound middle lamella continuously proceeds during S₁ and S₂ formation stages, and ceases in S₃ formation stage. Lignification of secondary wall starts at cell corner regions of outer S₁ after S₁ formation starts, and extends around the perimeter of outer S₁. Then, lignification proceeds centripetally toward the lumen of tracheid lagging behind cell wall thickening. Secondary wall lignification actively occurs after S₃ formation. Finally, secondary wall shows almost uniform distribution of lignin. Lignin content of secondary wall ranges between 0.20 to 0.25 g/g, though that of compound middle lamella has more than 0.50 g/g. Lignification process of vessel element and fiber in angiosperm are similar to that of tracheid in gymnosperm. Lignification of vessel cell wall proceeds very quickly, although that of fiber cell wall very slowly. Vessel secondary wall contains more guaiacyl lignin, though fiber secondary wall is rich in syringyl lignin. Lignin content of the former is 0.22-0.27 g/g, and the latter 0.12-0.16 g/g. Heterogeneity of the chemical structure of lignin is also summarized.