Introduction to Fungi, Third Edition

MORCHELLA (MORCHELLACEAE)
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surrounding a tree with roots with mycorrhizal
connections, in which associated herbaceous
plants are wilting or dead. This is partly due
to deleterious volatile metabolites from the
Tuber mycelium (Pacioni, 1991) and possibly
also to parasitic attack by the mycelium on
roots of herbs. The Périgord truffle is associated
in the wild with the roots of oaks (Quercus spp.)
in France. Truffles are cultivated there in
plantations (truffières) of appropriate species
of oak or on hazel (Corylus avellana). Clonal
material of suitable hazel cultivars may be
used to ensure greater yield and uniformity of
cropping (Mamoun & Olivier, 1996). The seedling
roots of potential hosts are inoculated by
dipping them in a suspension of ascospores, or
seedlings can be naturally infected by growing
them close to mature mycorrhizal trees where
infection occurs by mycelial contact. Despite
this, truffle yields have fallen continuously, with
about 1000 tons harvested annually in France
around the year 1900 but less than one-tenth
of that yield collected 100 years later (Hall et al.,
2003). A method to cultivate truffles on a large
scale under axenic conditions would be a
marvellous achievement, but this is not yet
in sight.
The literature on truffles, stimulated by
their gourmet and high commercial value, is
enormous. Guides to identification have been
provided by Gross (1987), Pegler et al. (1993),
Riousset et al. (2001), and a computer-based
interactive key by Zambonelli et al. (2000). More
general accounts of truffle biology and cultivation
have been written by Delmas (1978), Hall
et al. (1994) and Callot (1999).
The ‘desert truffles’, Terfezia spp., are not
closely related to Tuber but may instead have
an affinity with the Pezizaceae (Percudani et al.,
1999). Terfezia occurs as a mycorrhizal associate
of shrubs in arid regions of Southern Europe
and the Middle East, where it is consumed as
food and traded on markets. The association
of Terfezia with the roots of shrubs such as
Helianthemum almeriense can greatly improve the
ability of the plant to withstand drought stress
and may play an important role in mediterranean
ecosystems (Morte et al., 2000).
14.8 Morchella (Morchellaceae)
The fruit bodies of Morchella spp., the true morels,
are among the most popular and highly prized
edible fungi (Plate 6g). They appear for a few
weeks in spring in cold-temperate regions soon
after snow-melt as the soil becomes warmer and
drier, but they are not confined to such areas.
Morchella spp. have two ecological strategies; as
saprotrophic ruderals, fruiting for a relatively
short period (a few years) on disturbed or burnt
ground, or in mycorrhizal association with tree
roots, fruiting over a longer period. The validity
of the alternative lifestyles has been confirmed
by comparative analyses of the isotopes 15 N and
13 C, showing some populations to be saprotrophs
whilst others are mycorrhizal (Hobbie et al.,
2001). Opinions on taxonomy vary, with some
authors recognizing about 50 species and others
as few as 3 5 species showing wide phenotypic
variation. Three broad groups of species
have been distinguished, i.e. the half-free morel
(M. semilibera), the black morels (M. elata, M. conica
and M. angusticeps), and the common or yellow
morels (M. esculenta, M. crassipes and M. deliciosa).
Molecular analysis indicates that the black
morels and yellow morels are separate taxonomic
groups (Bunyard et al., 1995; Gessner,
1995). The ascoma of a Morchella consists of
a hollow stipe and a fertile cap thrown into
shallow cup-like depressions or alveoli. The
alveoli are lined by asci and paraphyses but
the ridges or ribs which separate the alveoli are
sterile, containing only paraphyses (Janex-Favre
et al., 1998).
The cylindrical, unitunicate, operculate asci
contain eight unicellular ascospores. Karyogamy
occurs prior to ascus formation, but croziers
are apparently absent. Following meiosis in
the ascus, there are four successive mitoses so
that the ascospores are multinucleate (Volk &
Leonard, 1990). The tips of the asci are phototropic
and are directed towards the opening
of the alveolus. The ascospores are often discharged
simultaneously by puffing, generating
air currents which carry clouds of spores well
away from the fruit body (Buller, 1934). They