Water Relations

Water is essential as a solvent for metabolic processes, for transport of metabolites, enzymes and organelles and, via turgor pressure, a vital skeletal ingredient and the driving force behind skeletal growth. Within solid organic resources, the availability of water to the decomposer organism is affected by two main forces, matric potential—exerted by the substratum, and osmotic potential due to dissolved salts (Boddy, 1983; Magan, 1997). Matric potential is a result of forces associated with the interfaces between water and the solid matrix, and osmotic potential is a result of the presence of solutes within the water. An advantage of using water potential as a measure of water availability is that the osmotic, matric and turgor components of the total water availability can be quantified. The water potential of a resource such as wood is an important and highly variable characteristic, often determining the growth of the organisms which decay it (Luard, 1982).

Wood-inhabiting fungi vary considerably in their abilities to grow at low-water content. Boddy (1983) studied the water relations of 12 temperate species of wood-inhabiting basidiomycetes and found that all grew very slowly or not at all on malt agar at —4.4 MPa water potential. Others, working mainly on temperate basidiomycetes, have reported similar results (Tresner and Hayes, 1971; Clarke et al., 1980; Eamus and Jennings, 1986). However, few if any of these studies have compared the tolerance/sensitivity of temperate or tropical basiodiomycetes to matric and solute stress.

In general, most studies examining the effect of ionic solutes (e.g. NaCl, KCl), non-ionic solutes (e.g. glycerol) and matric water stress (e.g. PEG 8000) on bas-idiomycetes have shown a decrease in growth rate with an increase in imposition of water stress (Fig. 2). Growth of most basidiomycetes, including A. bisporus, Pleurotus and Trametes species, are more sensitive to a reduction in matric than solute potential (Magan et al., 1995; Beecher et al., 2000; Lee et al., 2000).

Growth of most basiodiomycetes ceased completely or was extremely slow between —4 and —7 MPa osmotic potential. For example, growth of Rhizoctonia solani ceased at —5 MPa (Dube et al., 1971) and that of the dry rot species Serpula lacrymans ceased between —3 and —6 MPa on agar (Clarke et al., 1980). Tresner and Hayes (1971) found that 94 out of 107 basidiomycetes ceased growth at —3 MPa while growth of 12 temperate wood-decay basidiomycetes ceased between —4.4 and —7.1 MPa when KCl was used as a solute (Boddy, 1983).

Temperature has interactive effects with water potential on basidiomycete growth (Fig. 3; Mswaka and Magan, 1999); there was a significant decrease in matric-stress tolerance at marginal temperatures for growth. However, sometimes growth rates may not be a good indicator of ecological competence. I. stereoides grew significantly slower than other species, but had a wider optimum matric and solute potential growth range and lower minima than many other basidiomycetes examined (Mswaka and Magan, 1999). Although the natural habitat for this fungus is moist evergreen forest, it is found exclusively on attached dead branches and stems of its angiosperm host, which are usually desiccated because of their exposed position. Thus, tolerance of low-water stress is an ecologically important adaptation by the fungus to its niche. Again, an ascomycete, Xylaria hypoxylon, has been shown to have the ability to maintain low-water potentials in the wood that it occupies (Boddy, 1986), thereby preventing the more active decomposers such as T. versicolor from replacing it. It is possible that I. stereoides, itself a slow growing fungus, may be using the same

(a)

(b) Water potential

Figure 2 Comparison of effect of (a) solute potential (modified ionically with KCl or non-ionically with glycerol) and (b) matric potential (modified with PEG 8000) on relative growth rates of (a) T. socotrana and (b) Lenzites elegans.

(b) Water potential

Figure 2 Comparison of effect of (a) solute potential (modified ionically with KCl or non-ionically with glycerol) and (b) matric potential (modified with PEG 8000) on relative growth rates of (a) T. socotrana and (b) Lenzites elegans.

ecological strategy as that used by X. hypoxylon. A small reduction in solute/ matric potential resulted in a stimulation of the growth of I. stereoides indicating that this fungus is inhibited by very high-water availability in a resource.

Species isolated from hot and dry regions might be expected to have a better tolerance of extreme fluctuations in water stress. Many tropical species such as T. cingulata and T. modesta react to low-water stress by the formation of chlamydospores, an adaptation which favours their survival under desiccating conditions. These chlamydospores probably germinate at the onset of the wet season, allowing the fungus to continue to decompose its resource. Similarly, Hyphodontia paradoxa, which is unable to grow at low-water potentials but also produces chlamydospores (Boddy, 1983), was still viable after 81 days at greater than —100 MPa, but non-viable after 343 days (Miller and Meyer, 1934). However, the limiting water content for wood decay is reported to lie below 30%

T. cervina

T. cingulata

Lenzites elegans

T. pocas

T. socotrana

T. versicolor

P. decipiens

Irpex sp.

T. cervina

T. cingulata

Lenzites elegans

T. pocas

T. socotrana

T. versicolor

P. decipiens

Irpex sp.

Matric potential (-MPa)

Figure 3 Effect of temperature on the matric potential growth range of some tropical wood-decay fungi on 3% MEA. Source: adapted from Mswaka and Magan (1999).

Matric potential (-MPa)

Figure 3 Effect of temperature on the matric potential growth range of some tropical wood-decay fungi on 3% MEA. Source: adapted from Mswaka and Magan (1999).

(percentage oven-dry weight) in undecayed wood, which corresponds to water potentials in the region —4 MPa (Boddy and Rayner, 1988). At matric potentials below this, water is only present in the transient micropores which are too small to allow hyphae or enzyme molecules to enter and effectively decompose the wood (Griffin, 1981). Such information is critical for understanding the ecological role of saprotrophic species in colonisation and decay of wood.

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