6 Wood Discoloration

100 4 Wood Cell Wall Degradation
the cell walls of xylem tissues, which helps a smooth transportation of water
through vessels and tracheids from roots to branches (Higuchi 2002). The incorporation
of lignin into the cell wall gives trees with heights of 100 m the
chance to remain upright. Lignin gives resistance against disease and wood
decay by microorganisms. Lignin content amounts in softwoods to 26–39%
(average 28%) (compression wood: 35–40%), in hardwoods of the temperatezoneto18–32%(average22%)(tensionwood:15–20%)andintropical
hardwoods to 23–39% (Fengel and Wegener 1989).
The monolignols (p-hydroxycinnamyl alcohols), p-coumaryl, coniferyl, and
sinapyl alcohol, are the primary precursors and building units of all lignins
(Fengel and Wegener 1989; Fig. 4.5). The biosynthetic pathway of monolignols
starts from glucose via shikimic acid over phenylalanine and tyrosine, respectively,
to p-coumaric acid which yields via intermediates p-coumaryl alcohol.
p-coumaric acid is converted via caffeic acid and ferulic acid to coniferyl alcohol.
Ferulic acid is transformed via 5-hydroxyferulic acid and sinapic acid to
sinapyl alcohol (Higuchi 2002).
For lignin polymerization (Li and Eriksson 2005), the monolignols are initially
dehydrogenated by peroxidases and/or laccases to phenoxy radicals. The
radicals then couple non-enzymatically to quinone methides as reactive intermediates.
According to one proposal, dimeric quinone methides are converted
into dilignols by water addition, or by intra-molecular nucleophilic attack by
primary alcohol groups or quinone groups. Dilignols can also undergo enzymatic
dehydrogenation to form the corresponding radicals, which in turn
couple with phenoxy radicals to produce trilignols, etc. In a second mechanism,
enzymatic dehydrogenation is restricted to monolignols. The polymerization
evolves by successive non-radical addition of phenols to the quinone methides.
In a third mechanism, the lignin polymer evolves from the polymerization of
quinone methides.
Most softwood lignins are as guaiacyl lignins (G-lignins) polymers which
are predominantly made of coniferyl alcohol (spruce: C : S : p-C = 94 : 1 : 5).
Hardwood lignins are guaiacyl-syringyl lignins (GS lignins) and consist predominantlyofCandS(beech:C:S:p-C
= 56 : 40 : 4). Guaiacyl-syringylp-hydroxyphenyl
lignins occur in grasses. Lignin quantity and composition
Fig.4.5. Lignin building units. A pcoumaryl
alcohol. B Coniferyl alcohol.
C Sinapyl alcohol
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