6 Wood Discoloration

56 3 Physiology
1987, 1991). Translocation of nutrients is predominantly by water flow. Water
flow is generated by the uptake of nutrients, particularly carbohydrates, by
the mycelium such that the hyphae have a lower water potential than the
substrate. In consequence, water flows into the hyphae and the hydrostatic
pressure so generated drives a flow of solution towards the mycelial growth
front.Thevolumeflowisdissipatedatthegrowthfrontbytheincreasein
volume of the hyphae and the production of droplets at the hyphal apices.
The droplets have a lower osmotic potential than the hyphae or that of the
substrate from which the mycelium grows. This means that the water leaves
the cytoplasm ultrafiltered by the plasmalemma of many of the nutrients
in the translocation stream. Pressure-driven flow of solution has been studied
particularly in Serpula lacrymans(Jennings 1991). It must occur in a wide range
of fungi because droplets (guttation) are common among fungi. Guttation
often occurs in white-rot fungi, like during growth of Donkioporia expansa
in buildings and in the edible mushrooms Lentinula edodes and Pleurotus
ostreatus when the colonization phase of the substrate is completed and the
fungi start fruiting. In S. lacrymans, the droplets at the hyphal tips are slightly
acidic (pH 3–4), which was related to the ability of the fungus to colonize
alkaline substrates (Bech-Andersen 1987a).
The dry weight of fungal mycelium consists of about 5% of nitrogen (% N
of the Kjeldahl method ×4.4 corresponds to the protein content of fungi. Additional
nitrogen is included, e.g., in the chitin). Wood typically has a very low
nitrogen content. The average nitrogen for healthy hardwoods and softwoods
was 0.09% of the dry weight of wood and reached to about 0.2% N (Rayner and
Boddy 1988; Fengel and Wegener 1989; Reading et al. 2003) with an average
carbon to nitrogen ration of 500 to 600:1. Nitrogen content changes over the
wood cross section and is lower in wounded or decayed tissue. With regard to
lignocelluloses, it has to be considered, however, that the majority of carbon
is present as a cell wall component and thus enzymatically difficulty accessible,
while the nitrogen compounds are more easily degradable. Altogether
nitrogen, however, is a limiting factor. Fungi do not fix atmospheric nitrogen,
how this some bacteria are able to do. Instead, fungi use nitrogen rationally,
as nitrogen compounds are translocated to the growth front at the hyphal tips
due to different turgor pressure in the mycelium (Watkinson et al. 1981; Jennings
1987). Protein-rich woods, e.g., Pycnanthus angolensis, are colonized by
bacteria after felling and during the drying process, which leads to undesirable
discolorations (Chap. 5.2) (Bauch et al. 1985). For wood fungi, ammonium is
a suitable inorganic source of nitrogen in vitro, while nitrate is usually not
used. Organic nitrogen from amino acid mixtures in pepton or malt extract
results in good growth on agar.
There are several minerals in wood. The main inorganic components found
in wood ash are K, Ca, Mg, Na, Fe, silica, phosphate, chloride, and carbonate
(e.g., Fengel and Wegener 1989; also Wa˙zny and Wa˙zny 1964). By SEM-EDXA,
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