Introduction to Fungi, Third Edition

426 HYMENOASCOMYCETES: PEZIZALES (OPERCULATE DISCOMYCETES)
have shown that ascosporogenesis resembles
that found in most other Euascomycetes, but
instead of being delimited by the invagination of
an ascus vesicle the ascospores become delimited
individually by vesicles formed by material
derived from lomasomes or from invaginations
of the ascus plasma membrane (Berta & Fusconi,
1983).
At first, the developing ascocarp is attached
by hyphal connections to a host tree, i.e. it grows
symbiotically. Eventually it breaks free and
continues to develop independently during a
saprotrophic phase of growth. In T. melanosporum,
tufts of hyphae extend into the surrounding soil
from the pyramidal scales on the outside of
the peridium. Application of radioactive tracers
(e.g.
32 PO 4 ,
3 H 2 O and 14 C-labelled mannose) to
these hyphae is rapidly followed by the appearance
of radioactive material in the inner portions
of the gleba and especially in the fertile
veins (i.e. the dark regions of the ascocarp
interior; see Fig. 14.7a) at rates and in patterns
which could not be accounted for by simple diffusion
(Barry et al., 1994, 1995). This finding
and reports that fruit bodies of truffles may be
found a considerable distance away from living
tree roots suggest that mature ascocarps may
be autonomous and can obtain water and nutrients
directly from the soil, from decaying
roots and faecal deposits from the soil fauna
(Callot, 1999).
The species of Tuber which have been investigated
have a wide mycorrhizal host range,
including Angiosperms and some Gymnosperms.
The host ranges of four species of Tuber are
shown in Table 14.2.
14.7.3 Truffle collecting
The truffles of greatest commercial value are
Tuber melanosporum (the black truffle of Périgord)
and T. magnatum (the white truffle of Piedmont),
which can command prices of up to E2000 kg 1
in Continental Europe. Several other species
are also traded. The only common British truffle
which can be used for culinary purposes is
T. aestivum. Périgord and Piedmont truffles are
most abundant in Southern Europe (France,
Spain and Italy) within latitudes 40° and 50°
North in well-drained calcareous soils (Callot,
1999). Both are collected with the aid of pigs
and dogs trained to detect them by smell.
The Périgord truffle can also be detected by
the presence of a ‘burnt’ ring-like zone (brûlé)
Table14.2. Host species relationships of four species of Tuber. The main hosts are indicated by black symbols.
Marks in brackets indicate the formation of mycorrhiza with some, but not all members of the host genus
tested. Data summarized from Giovannetti et al. (1994).
Tubermagnatum T.melanosporum T.aestivumgroup T. albidum group
Alnus cordata
Carpinus betulus
Castanea sativa
Cistus (2 spp.)
Corylus avellana
Fagus sylvatica
Ostrya carpinifolia
Populus (2 spp.)
Quercus (6 spp.) ()
Salix (2 spp.)
Tilia (3 spp.)
Abies alba
Cedrus (2 spp.)
Pinus (7 spp.) () () ()