6 Wood Discoloration

5.2 Bacteria 113
doch and Campana 1983; Zimmermann 1983; Schink and Ward 1984; Kučera
1990; Klein 1991; Walter 1993; Xu et al. 2001).
Bacteria may be also associated with the development of false frost cracks in
oak, ash, elm, poplar, and Silver fir. These radial shakes develop progressively
from the stem interior, being initiated either from old cambial injuries or from
pockets of fungal heart rot (Shigo 1972; Butin and Volger 1982). Occurrence,
distribution, and enzyme activities of the bacteria isolated from pedunculate
oak trees supported the assumption that bacteria may be involved in the
weakening of the woody tissue in the area of the ray parenchyma cells so that
mechanical factors like frost subsequently push the shake in the predamaged
tissue (Schmidt et al. 2001).
Several bacteria isolated from wet-stored stem wood were able to degrade
pectin, hemicelluloses, and cellulose when these cell wall components had been
supplied as isolated compounds (Schmidt and Dietrichs 1976). With regard to
lignin, lignin derivatives or DHPs up to 1 kDa were attacked (Vicuña 1988).
In view of bacterial wood degradation, bacteria attacked within partially
lignified plant organs, like a shoot or a needle, only non-lignified tissue. The
cell walls of the phloem cells of the vascular bundles were degraded, but those
of the xylem part resisted. Inside woody tissue, bacteria preferentially feed
soluble sugars, the content of parenchyma cells and attack non-lignified pit
membranes (Liese 1970). In tension wood fibers, bacteria only consumed the
cellulosic G-layer (Schmidt 1980). After a mild delignifying pretreatment of
wood samples with sodium chlorite, however, bacteria caused mass loss up
to 70% (Schmidt 1978), as the carbohydrates were now accessible. Figure 5.2
Fig.5.2. Beech wood microtome sections with slightly reduced lignin content without (a)
and after culture with Cellulomonas flavigena (b)
www.taq.ir