6 Wood Discoloration

26 2 Biology
to September. In cities in temperate regions, the spore concentration of Cladosporium,
mainly C. herbarum, often rises up to 10,000–15,000 spores/m 3
air with peaks of more than 50,000 spores/m 3 (Nolard 2004). Air turbulence
during stable areas of high pressure may result in daily rhythms, the concentration
rising during the midday (Reiß 1997). Interiors with high dust content
(e.g., the wood-processing industry) may exhibit increased spore contents. The
lifespan of spores in free air is affected by temperature, air humidity, and sun
exposure. As unpigmented spores are sensitive to UV light, pigmented spores
predominate in the air. Exogenously dormant spores only germinate when the
environmental conditions (nutrients, temperature, pH value) become favorable.
Endogenously dormant spores fail to germinate even under favorable
conditions, which is due to factors within the spore such as nutrient impermeability
or the presence of endogenous inhibitors. Dormancy within these
spores is broken by ageing when nutrients begin to enter or the inhibitors leach
out (Robson 1999).
Priortotheemergenceofoneormoregermtubes,sporesundergoaprocess
of swelling during which they increase in diameter due to the uptake of water.
The metabolic activity, production of protein, DNA and RNA all increase.
The percentage of germinating spores depends on fungal species, spore age,
temperature, available moisture, and substrate. In Serpula lacrymans, only 30%
of sampled spores germinated in vitro (Hegarty et al. 1987). For the conidia of
Heterobasidion annosum, the thermal cardinal points were 0 ◦ C minimum, between
12 and 28 ◦ C optimum and 34 ◦ C maximum (Courtois 1972). Depending
on the species, the duration of the germination ability reaches from a few days
or weeks, like in Stereum species, to several years in Chaetomium globosum,
and can reach up to about 20 years in S. lacrymans (Grosser et al. 2003).
Germination of spores of wood fungi is favored by high air humidity,
warmth, and pH values of 4–6. In Serpula lacrymans, citricacid(Hegarty
et al. 1987) and vitamin B1 (Czaja and Pommer 1959) stimulated germination.
Heartwood compounds may inhibit.
2.3
Sexuality
The wood-inhabiting Ascomycetes and Basidiomycetes are either homothallic
or heterothallic (Ryvarden and Gilbertson 1993).
Homothallic fungi are self-fertile, that is no second mating type is required
for sexual reproduction. Fertilization takes place at the same mycelium. Many
Ascomycetes and about 10% of the Basidiomycetes belong to this type.
Heterothallism includes both bipolar and tetrapolar fungi. In bipolar (unifactorial)
species, incompatibility is controlled by a series of multiple alleles at
one locus. Any dikaryon has two alleles that segregate at meiosis so that half
www.taq.ir