6 Wood Discoloration

3.6 Light and Force of Gravity 75
sterile“darkfruitbodies”onminetimber.Serpula lacrymans fruits in buildings
in twilight. In the laboratory, the daily light-dark cycles are suitable.
The Deuteromycetes Aspergillus niger and Paecilomyces variotii develop
conidia both with light and in the dark, likewise the ascomycete Chaetomium
globosum forms fertile cleistothecia. In other Ascomycetes, conidia formation
is induced by light, while in darkness ascospores develop (Reiß 1997). LightdarkcyclesleadtoarhythmicchangeofgrowthandreproductionofPenicillium
species and other Deuteromycetes. When the hyphae are irradiated,
their growth rate is reduced to differentiation into conidia. Concentric rings
develop on agar plates from the inoculum in periodically repeated distances
(Schwantes 1996; Reiß 1997; Jennings and Lysek 1999).
Some fungi can grow permanently on sites exposed to light, e.g., fungi
growing on plant surfaces (leaves, phylloplane). Typical phylloplane fungi
are Alternaria, Aureobasidium and Cladosporium species (Jennings and Lysek
1999). Some of them are potential parasites, but also effect blue stain of timber
as saprobionts.
UV light, particularly 254 nm, has a lethal and mutagenic effect. Nucleic
acids are damaged by UV-B of 260 nm by the photochemical induction of cyclobutan
dimers, which prevents the correct transcription and reduplication
of DNA (Panten et al. 1996). That is mycelium and colorless spores and bacteria
can be damaged by sunlight. Microbial pigmentation, particularly black
(conidia of Aspergillus niger) and yellow (e.g., bacterium Micrococcus luteus),
is interpreted as a protection against the irradiance. UV is thus used in microbiological
and molecular laboratories to reduce the amount of bacteria and
fungi in the air, on laboratory surfaces and devices.
Fungi may also use the direction from which the light is coming to orientate
themselves (Jennings and Lysek 1999). During the primordium growth
of Basidiomycetes, the stipe grows towards the light source. In the Pilobolus
species (Mucoraceae), there is a ring of yellow-orange carotenoids in the sporangiophore
below the subsporangial bladder, which is shaded by the spore
mass in the sporangium. If the light received by the ring is not at a minimum,
the sporangial stalk bends until it is, which gives the direction in which
the sporangium will be shot off, up to a distance of 40 cm (Jennings and Lysek
1999). The force of gravity takes effect immediately when the developing
pileus shades the tip of the stipe (Schwantes 1996). This ensures that the pores
and lamellae in the growing hymenium orientate to the earth’s center (positive
gravitropism, Nultsch 2001) that is, the mature basidiospores can sink
tothesoil.Aknownexampleofpositivegravitropismmayoccurinthefruit
body of Fomes fomentarius. The perennial, bracket fruit bodies are located
at the stem of beech trees. When the white-rotten tree is wind-thrown, the
fungus lives for a certain time as saprobiont in the laying stem. The new hymenia
developing on the “old” fruit body orientate with a 90-degree change
of direction again towards the earth’s center. If there is by chance a further
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