Biology Reference
In-Depth Information
Fujita and Harada (1979)
later observed labelling of lignin in the secondary
wall by the same method. Further studies by Terashima and coworkers
demonstrated the formation and localisation of H- and G-lignins using a
variety of labelled precursors including monolignol glucosides. In pine
(
Terashima and Fukushima, 1988; Terashima et al., 1988
) and Ginkgo biloba
(
Fukushima and Terashima, 1991
), H-lignin is formed in the compound
middle lamella and cell corner at an early stage of the lignification process,
whereas G-lignin is first deposited in the middle lamella region, and subse-
quently in the secondary wall.
In pine 'normal wood', lignification occurs in three stages (
Terashima et al.,
1988
): lignification of the cell corner and middle lamella after S1 formation,
slow lignification during secondary wall formation, and the main lignification
after S3 formation. In compression wood, where the pattern of lignin distribu-
tion is significantly different to normal wood, lignification is more distinctly
divided into two stages. Lignification of the middle lamella occurs at an earlier
stage of secondary wall formation with a considerable delay before lignifica-
tion of the secondary wall, which can occur either while the S2 layer is still
forming (
Fukushima and Terashima, 1991
)orafterS2formationiscomplete
(
Fujita and Harada, 1979
). In compression wood, p-hydroxyphenyl units are
deposited in both the compound middle lamella and the S2L region, whereas
in normal wood these units are deposited only in the compound middle lamella
(
Fukushima and Terashima, 1991; Westermark, 1985
).
Lignin deposition appears to occur along the lamellae between cellulose
microfibrils in a circumferential direction more rapidly than towards the
lumen, creating a streaked effect in cell walls where the secondary wall is
undergoing lignification (
Donaldson, 1992
). In the early stages of lignifica-
tion, lignin is deposited at discrete sites in the middle lamella suggesting that
there are initiation or nucleation sites of unknown composition where lignin
polymerisation begins; the finding of ferulates in pine walls (
Carnachan and
Harris, 2000
) suggests that they might fulfil this nucleation role, as they
appear to in grasses (
Ralph et al.,1995
). Subsequent lignification seems to
occur by expansion of these early lignified regions (
Guan et al., 1997
).
Lignin deposition is preceded by deposition of polysaccharides including
both cellulose and hemicelluloses (
Donaldson, 1994; Mast et al.,2009
). Recent
studies have suggested that polysaccharides such as galactan, mannan and
xylan, which have distributions that show a relationship with the degree of
final lignification, may be controlling elements that determine where and
how much lignin is deposited in the various layers of the tracheid cell wall
(
Kim et al., 2010, 2011
).
The pattern of lignin deposition remains the same during the growing season
with a peak in the number of lignifying cells during summer (
Donaldson, 1991
).
