Biology Reference
In-Depth Information
Lignification of latewood tracheids often appears to be incomplete at the onset
of winter, where partially lignified cells can often be observed at the growth ring
boundary. Lignification of latewood tracheids is completed shortly after the
onset of new cell division in the spring.
B. LIGNIN DISTRIBUTION
The distribution of lignin across the tracheid cell wall varies considerably
with each cell wall layer having a characteristic degree of lignification. The
middle lamella/primary wall region, known as the compound middle lamella,
is one of the most highly lignified regions with typically more than 50% lignin
w/w as measured by UV absorbance microscopy or by interference micros-
copy (
Donaldson, 1985; Fergus et al., 1969; Scott and Goring, 1970; Scott
et al., 1969; Wood and Goring, 1974
). In comparison, the S2 layer contains
about 20% lignin w/w. There are no specific measurements for the S1 layer
because of its small size but transmission electron microscopy using perman-
ganate staining often indicates that the S1 layer is slightly less lignified than
the S2 layer (
Donaldson, 1995; Maurer and Fengel, 1991
). The S3 layer is
variable but is generally considered to be more lignified than the S2 layer with
measurements of around 50% lignin w/w (
Bland et al., 1971; Donaldson,
1987; Parham and C ˆ t´, 1971; Scott and Goring, 1970; Timell, 1973
).
Analysis of cell wall fractions separated by density gradient indicates that
secondary wall lignin contains about twice as many phenolic hydroxyl
groups as middle lamella lignin (
Sorvari et al., 1986; Whiting and Goring,
1982a,b; Yang and Goring, 1980
), which also has a higher molecular weight
and higher oxygen content compared to secondary wall lignin (
Sorvari et al.,
1986
). Middle lamella lignin has high levels of H-lignin as shown by both
microautoradiography (
Terashima and Fukushima, 1988
) and cell wall frac-
tionation (
Westermark, 1985
). Condensed G-units are also higher in the
middle lamella relative to the secondary wall (
Westermark, 1985
).
Immunolocalisation of lignin has focussed on different chemical epitopes
within the polymer but relatively few studies have been carried out on conifer
xylem (
Ruel et al., 2002, 2006
). Dibenzodioxocin units are localised in the S2
region of Pinus sylvestris normal wood, and in the S1 and outer S2 regions of
compression wood. In Picea abies, the same epitope is localised in the S3
layer of normal wood and inner S2 region of compression wood, with only
low levels detected in juvenile wood (
Kukkola et al., 2003, 2008
). Antibodies
against milled wood lignin show labelling in the S2 region of P. sylvestris but
with no labelling of the middle lamella (
Kim and Koh, 1997
).
Whereas compression wood occurrence is the primary source of variation in
lignin content in conifers, other factors including Cu deficiency and drought
