6 Wood Discoloration

62 3 Physiology
of the water vapor pressure in the substrate (p) and the pressure of pure water
(p0)(aw=p/p 0 ) (Siau 1984; Rayner and Boddy 1988; Reiß 1997; Table 3.5).
The minimum water activity (Table 3.5) is for most bacteria with 0.98 aw
higher than for many molds, which grow still at 0.80 aw. The minimum for
growth of wood-decay Basidiomycetes on agar is 0.97 aw. Xerotolerant and
xerophilic molds like some Aspergillus species still grow at 0.62 aw. Those
fungi grow in solutions of sodium chloride around 5–6 M (Jennings and Lysek
1999) and tolerate an 80% saccharose solution (Schlegel 1992; Reiß 1997),
generating the appropriate osmotic pressure within their protoplasm e.g., by
the synthesis of glycerol. Below 0.6 aw usually no microbial growth occurs.
The situation of high salt concentrations (sodium chloride) applies also
to marine fungi. Various “lower fungi”, Deuteromycetes, Ascomycetes, and
a few Basidiomycetes colonize wood in the sea (Kohlmeyer 1959; Volkmann-
Kohlmeyer and Kohlmeyer 1993). As in marine fungi vacuoles constitute no
more than about 20% of the volume of the protoplasm, there is no preferential
accumulation of sodium chloride in the vacuoles. Marine fungi synthesize glycerol
and other polyols (mannitol, arabitol) which contribute to their osmotic
potential (Jennings and Lysek 1999).
For growth and wood degradation by fungi, particularly at low water contents,
the water potential (MPa) is the most important factor for water availability.
It is defined as free energy of water in a system relative to pure water,
and because in the relevant range all values are negative, it can be defined
as that negative pressure (“subpressure”), which is necessary to extract water
from the substrate (Griffin 1977). The water potential is affected by different
factors (Siau 1984; Jennings 1991). These are particularly the size and form of
the boundary surfaces both between water and firm matrix and between water
and air (matrix potential), and the osmotic potential due to the occurrence
of solved substances. The influence of the water potential on growth of wood
fungi was first examined with simple substrates, like agar plates in Petri dishes,
in controlled air humidity (Bavendamm and Reichelt 1938). The observed values
of mycelial growth still at −14.5 MPa (aw 0.9), however, were later classified
as too low. Instead, as lower limit about −4 MPa were determined (Griffin 1977;
Griffith and Boddy 1991; Table 3.5). Serpula lacrymans did not grow on agar
below −0.6 MPa (Clarke et al. 1980).
Due to the occurrence of pores of different size (porosity of wood: Kollmann
1987), the special significance of the matrix potential becomes obvious with
increasing drying of wood tissue. In water-saturated wood, all cavities are
filled, and a neglectably small pressure difference is sufficient for dehydration.
With progressive drying, increasingly smaller openings become free from
water (Table 3.5). Large openings in wood tissue with radii over 5µm like
all cell lumens are free from water, if the matrix potential amounts to less
than about −0.03 MPa. Between −0.03 and −14.5 MPa, pores from 5–0.01µm
radius become empty (pits, boreholes by microhyphae). Below about −14 MPa,
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