Biology Reference
In-Depth Information
II. LIGNIFICATION IN CONIFERS
Lignin is a heterogeneous cell wall polymer created though the oxidative
coupling of p-hydroxycinnamyl alcohols (monolignols) or related com-
pounds (
Ralph et al., 2004
). The polymer is particularly abundant in cells
that undergo secondary cell wall thickening such as tracheids (
Harris, 2006
).
Lignin fulfils vital functions in vascular plants as it reinforces plant cell walls,
facilitates water transport, provides compressive strength to conducting
tissues, and acts as a mechanical barrier to pathogens (
Boudet, 2007
).
The biochemical composition of tracheid cell walls differs substantially
from those of vessel elements and wood fibres that occur in hardwoods,
particularly with regard to non-cellulosic polysaccharides and lignin
(
Harris, 2006
). Lignin in conifers is primarily derived from the monolignols
p-coumaryl and coniferyl alcohols, producing p-hydroxyphenyl (H) and
guaiacyl (G) units in lignin, and lacks the sinapyl alcohol-derived syringyl
(S) units that are commonly found in hardwoods (
Harris, 2006
). Lignin
composition and content can vary considerably within a tree, primarily
reflecting changes in wood composition. Compression wood occurrence is
the primary source of variation in lignin content and composition in conifers
(
Fig. 1
). Compression wood is a type of reaction wood formed in response to
gravitropic stimuli in conifers and is characterised by high lignin content and
a high H:G-ratio compared to normal wood (
Nanayakkara et al., 2009
).
A. LIGNIN DEPOSITION
Deposition of lignin begins around the time of completion of secondary wall
formation when an S3 layer can be detected by polarised light microscopy
(
Donaldson, 1992; Kutscha and Schwarzmann, 1975; Takabe et al., 1981;
Wardrop and Bland, 1959
). The early stages of lignification begin in the middle
lamella/primary wall region at cell corners, spreading around the circumfer-
ence of the cell and then towards the lumen. Lignification continues until
apoptosis is complete. In Pinus taeda, the concentration of lignin in the middle
lamella reaches approximately 50% of its maximum before lignification of the
secondary wall is detected (
Saka and Thomas, 1982
). An S3 layer was detected
prior to the start of secondary wall lignification, which is a more gradual
process than the relatively rapid lignification of the middle lamella. The state
of lignification may vary among adjacent cells suggesting that lignification is
controlled on an individual cell basis (
Donaldson, 1992; Yumoto et al.,1982
).
Using tritiated ferulic acid combined with microautoradiography,
Saleh
et al. (1967)
demonstrated labelling of lignin, first in the primary wall at the
cell corners and in the middle lamella of radial and then tangential walls.
