have been studied most thoroughly in this respect. Their
dormancy cannot be explained in terms of a general
permeability barrier, because they are permeable to
radiolabeled oxygen, glucose, and water. Instead, their
dormancy is linked to an inability to use their major
storage reserve, trehalose, which is not metabolized
during dormancy, but is metabolized immediately
after activation. The enzyme trehalase, which cleaves
trehalose to glucose, is found to be associated with the
walls of the dormant spores, separated from its substrate.
Activation somehow causes the enzyme to enter the cell,
as one of the earliest detectable events in germination.
Constitutive dormancy of some other spores has
been linked to endogenous inhibitors. For example,
uredospores of the cereal rust fungus Puccinia
graminis contain methyl-cis-ferulate, and those of
bean rust, Uromyces phaseoli, contain methyl-cis-3,4-
dimethoxycinnamate. Prolonged washing of spores
can remove these inhibitors, and this perhaps occurs
when the spores are bathed in a water film on a plant
surface. At first sight it seems surprising that a spore
adapted for dispersal should have an endogenous
inhibitor. However, this might prevent the spores from
germinating in a sporing pustule (they do not require
exogenous nutrients for germination) and ensure that
they germinate only after they have been dispersed.
Ecological aspects of constitutive dormancy
The behavior of constitutively dormant spores often has
clear ecological relevance. For example, a characteristic
assemblage of fungi occurs on the dung of herbivorous
animals (Fig. 10.8). Their spores are ingested with the
herbage, are activated during passage through the gut,
and are then deposited in the dung where they ger-
minate to initiate a new phase of growth. The spores
of many of these coprophilous(dung-loving) fungi
can be activated in the laboratory by treatment at
37°C in acidic conditions, simulating the gut environ-
ment. Examples include the ascospores of Sordariaand
Ascobolus, the sporangiospores of several Zygomycota,
and basidiospores of the toadstool-producing fungi,
Coprinusand Bolbitius.
Heating to 60°C activates the spores of many ther-
mophilic fungi of composts (Chapter 11). It also
activates the ascospores of pyrophilous(fire-loving)
fungi such asNeurospora tetraspermawhich grows on
burnt ground or charred plant remains. Most of these
fungi are saprotrophs of little economic importance, but
one of them, Rhizina undulata(Ascomycota), causes the
“group dying” disease of coniferous trees in Britain and
elsewhere. It infects trees replanted into clear-felled
forests, and the foci of infection correspond to the sites
where the trash from the felled trees was stacked and
burned. The ascospores are heat-activated around or
beneath the fires, then the fungus grows as a saprotroph
on the stumps and dead roots and produces mycelial
cords that infect the newly planted trees. Once the cause
of this disease had been recognized, the problem was
easily solved by abandoning the practice of burning.
However this is not possible in regions where lightning-
induced fires are a periodic, natural occurrence. Some
of the plants in these areas have become adapted to
fire – their seeds remain dormant for years until they
are heat-activated. Some of the mycorrhizal fungi are
similarly adapted, an example being the ascospores of
the mycorrhizal Muciturbospp. in Australian eucalypt
forests.
Basidiospores of the common cultivated mushroom,
Agaricus bisporus, and of several mycorrhizal fungi
(see below) show constitutive dormancy, but these
spores will often germinate when placed next to grow-
ing colonies of the same fungus. For example, spores
of A. bisporusgerminate in the presence of isovaleric
acid and isoamyl alcohol, which may be released from
the “parent” hyphae. The significance of this behavior
could be to increase the gene pool of the colony.Dormancy and germination triggers in mycorrhizal
successions
Many forest trees of temperate and boreal regions
(e.g. pine, oak, beech, chestnut, etc.) form mycorrhizal
associations with Basidiomycota or Ascomycota
(Chapter 13). These mycorrhizal fungi produce a
sheath around the root tips and extend into the soil
as hyphae or mycelial cords (see Fig. 7.10). Several
experimental studies have shown that a succession of
mycorrhizal fungi occurs on young tree seedlings
that are planted in forest nurseries or in previously tree-
less sites. Some of these fungi establish mycorrhizas
rapidly on tree seedlings from airborne spores. Classic
examples of these “early (pioneer) colonizers” are
Laccariaand Hebelomaspp. Other mycorrhizal fungi
are slow to establish in new sites, but colonize after sev-
eral years and eventually become dominant on the root
systems. A classic example is the fly agaric (Amanita
muscaria) on birch or pine trees. Figure 13.6 shows
examples of some of these fungi.
These successional patterns have been studied in
experimental field plots where the toadstools of myc-
orrhizal fungi appear above ground in autumn (Fig. 10.3)
and where the range of different mycorrhizal fungi
on the roots can be identified by features of the mycor-
rhizas themselves. When cores of soil were taken at
different distances from the trees (Fig. 10.4) mycorrhizas
of Hebelomaspp. predominated near the periphery of
the tree root systems, mycorrhizas of Lactariusspp.
predominated near the mid-zone of the root systems,
and mycorrhizas of Leccinum(a polypore) and an
unidentified mycorrhizal type predominated closest
to the tree trunks. Thus, there was clear evidence of a