6 Wood Discoloration

114 5 Damages by Viruses and Bacteria
shows beech wood microtome sections whose lignin content have been reduced
from naturally (21%) to about 19% and which subsequently had been used as
the only carbon source for bacteria in liquid cultures. The microtome section in
Fig. 5.2a represents the non-inoculated control. The section in Fig. 5.2b shows
that only the highly lignified middle lamella primary wall region resisted to
the bacterium Cellulomonas flavigena Kellerman and McBeth. However, the
bacteria only consumed the carbohydrates of the pretreated wood. The lignin,
which was dissociated from the pretreated woody cell wall by the bacteria, was
not respired but was refound in the nutrient liquid, suggesting that lignin is
a “ballast” to these bacteria that inhibits the dissimilation of the wood carbohydrates.
The action of the chlorite pretreatment was assumed to result from
the “opening” of the close association between carbohydrates and lignin in the
woody cell wall so that the carbohydrates became accessible to the bacteria.
Decay may have not been due to the reduction of the lignin content, because
bacteria did not attack natural beech wood with 21% lignin content, but degraded
pretreated Scots pine samples with a higher lignin content of about
23% (Schmidt and Bauch 1980).
Several bacteria were isolated from sawn Liriodendron tulipifera lumber already
after 2 months of stacking (Mikluscak and Dawson-Andoh 2004a). After
longer wood exposition under natural conditions, like in soil, or lakes and
marine environment, the lignified cell wall was degraded by mixed populations
and obviously the hurdle of the lignin barrier was cleared (Liese 1950;
Liese and Karnop 1968; Schmidt et al. 1987; Fig. 5.3a). Dependent on the decay
type within the wood cell wall, cavity, erosion, and tunneling bacteria
were distinguished (Singh and Butcher 1991; Nilsson et al. 1992; Singh et al.
1992; Daniel 2003). The two first types resemble the soft-rot types 1 and 2
(Chap. 7.3). The tunneling bacteria are qualified by means of slime sheats to
a gliding movement inside cell wall concavities created by themselves. The
aggregates of the tunneling bacteria subcultured from the woody samples consisted
of different bacterial species (Nilsson and Daniel 1992; Nilsson et al.
1992).
Aureobacterium luteolum Yokota et al. isolated from pond water caused erosions
in the secondary wall in microtome sections of pine sapwood as substrate
in 1 month of incubation, that is, bacterial wood degradation occurred obviously
also by a pure culture under laboratory conditions (Schmidt et al. 1995;
Fig. 5.3c). The result was however not reproducible using another strain of A.
luteolum (Nilsson pers. comm.).
In contrast to the xylem of healthy trees, which was rather “sterile”, wood
samples from forest dieback sites contained several bacteria (Schmidt 1985;
Schmidt et al. 1986). In view of the forest damage by pollution, bacteria (including
RLOs and MLOs) were however assumed to be no causal agents, but
rather, apart from other influences (emissions, climate, location), predisposing
factors, or secondary parasites of the weakened trees.
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