Biology Reference
In-Depth Information
even seasonal growth. Different stresses provoke a change in lignin composi-
tion, and this shift depends on the stress which caused it (
Moura et al., 2010
).
The amount and composition of lignin also varies in the cell wall itself.
It has been reported that the middle lamella has a greater quantity of lignin
than the primary and secondary cell wall. In angiosperms, the middle lamella
contains 50-70% lignin, while the secondary cell wall contains 20% lignin.
In gymnosperms, lignin represents 50-70% of the middle lamella whereas the
secondary cell wall is composed of 16-19% lignin. The lignin percentage
differs between hardwoods and softwoods (
Christiernin, 2006a
). As regards
the overall content, lignin represents 20% of dry matter in angiosperms and
30% in gymnosperms. This difference translates into the higher efficiency of
angiosperms, since a lower lignin content and higher carbohydrate content
require less energy and carbon for growth (
Amthor, 2003
). As regards the
lignin composition of the different layers of the gymnosperm cell wall, the
middle lamella is supposed to be rich in p-hydroxyphenyl units, whereas
secondary cell wall layers contain mainly guaiacyl units (
Terashima and
Fukushima, 1989
). The main reported difference in angiosperm lignin com-
position among cell types is between vessels and fibres. Whereas vessels are
composed of G units, fibres are mainly composed of S units (
Li et al., 2001
).
Among the different taxa, the classical view is that angiosperms are com-
posed of a similar proportion of G and S units, whereas gymnosperm cell
walls mainly contain G units, with a lower percentage of H units (derived
from p-coumaryl alcohol monomer), except in compression woods where the
H unit level is significantly elevated (see below). The remaining groups of
vascular plants, Lycophyta and Pteridophyta are supposed to resemble
gymnosperm lignins, that is, composed of G units. However, recent and
more accurate data support the idea of the non-linear evolution of lignin
composition in plant history, as will be discussed later.
Differences between angiosperms and gymnosperms also occur in excep-
tional situations such as reaction wood synthesis. A clear difference between
angiosperms and gymnosperms exists in reaction wood. This type of wood is
produced in order to force leaning stems into the normal position. In angios-
perms, this situation is called tension wood and is characterized by having
fewer xylem vessels elements, a higher proportion of fibres, more cellulose
(including, in some species such as poplar, a filling of the lumena with
cellulose) and less lignin (
Wilson and White, 1986
). Tension wood is also
differentiated by a higher syringyl/guaiacyl ratio (
Aoyama et al., 2001;
Joseleau et al., 2004; Yoshida et al., 2002
). In contrast, compression wood
from gymnosperms is characterized by higher content of lignin and less
cellulose, small tracheids and the absence of S3 layer (
Donaldson et al.,
2004; McDougall, 2000; Timell, 1986
). In addition, compression wood has
