6 Wood Discoloration

70 3 Physiology
more thermotolerant than the corresponding mycelium. The basidiospores of
S. lacrymans were killed by 32 h at 60 ◦ Cor1hat100 ◦ C (Hegarty et al. 1986).
However, 4 h at 65 ◦ C reduced the germination rate from 30 to 8% (Hegarty
et al. 1987). The heat resistance of basidiospores has also to be considered in
view of eradication of indoor wood-decay fungi by heat treatment.
As spore forming bacteria may survive 100 ◦ C, nutrient media for laboratory
experiments are sterilized at 121 ◦ C and 210 kPa pressure in autoclaves.
Alternatively, fractionated sterilization at 100 ◦ C (tyndallization) may be used
(heating at 100 ◦ C on three successive days for 30 min to destroy vegetative
cells; between the three heat phases incubation at room temperature to allow
germination of survived spores). Heat-sensitive nutritives can by sterilized by
membrane filtration using filter membranes with a pore size of 0.1–0.45µm.
Insensitive laboratory equipment like glass material becomes sterile by 1 h of
dry heat at 180 ◦ C. Wood samples for decay experiments may be sterilized by
ethylene oxide in special devices.
In many fungi, spores and also mycelia are resistant to cold. Thus, fungal
cultures in international strain collections are conserved, except by lyophilization,alsoinliquidnitrogenat−196
◦ C and not like it is usually done in small
laboratories in the refrigerator on agar (or also on small wood pieces: Delatour
1991).
3.5
pH Value and Acid Production by Fungi
The pH value influences germination of spores, mycelial growth, enzyme activity
(wood degradation), and fruit body formation. The optimum for wood
fungi is often in slightly acid environment of pH 5–6 and for wood bacteria at
pH 7. Basidiomycetes have an optimum range of pH 4–6 and a total span of
about 2.5–9 (Thörnqvist et al. 1987). Ascomycetes, particularly soft-rot fungi,
may tolerate more alkaline substrates to about pH 11. Thus, the pH values from
3.3–6.4 in the wood capillary water of living trees and in aqueous extracts of
wood and bark samples from trees of the temperate zones and from trading
timbers (Sandermann and Rothkamm 1959; Rayner and Boddy 1988; Fengel
and Wegener 1989; Landi and Staccioli 1992; Roffael et al. 1992a, 1992b) correspond
with the pH demands of wood fungi. Over the tree cross section, pH
differences can occur, that is for example the heartwood of oaks and Douglas
fir is more acid than the sapwood. Furthermore, an initial pH value can be
changed in the context of microbial succession, because bacteria may acidify
or alkalize the substrate by their metabolites (fatty acid production in acid
wetwood or methane or ammonia formation in alkaline wetwood; Chap. 5.2).
Outside about pH 2 and 12, respectively, microbial activity is commonly prevented.
The acid pH-extreme of Aspergillus niger is 1.5 (Reiß 1997). There
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