Agriculture Reference
In-Depth Information
of roots; however, most only fruit when symbiotically associated with plant roots
(Molina
et al.,
1992). In contrast to the arbuscular mycorrhizal fungi, ectomycorrhizal
fungi exhibit a considerable degree of variation in their host specificities. Some are
relatively narrow in their host range and are restricted to single genera of plants while
most have much broader host ranges (Molina
et al.,
1992). Multiple fungi may colonise
the roots of a single host plant.
The fungi that form arbuscular endomycorrhizal associations are obligate and
probably asexual symbionts belonging to the orders Glomales and Endogonales, tenta-
tively placed in the Zygomycota (Morton and Benny, 1990). More than 160 species
are distributed between eight genera in both orders (Walker and Trappe, 1993). Some
species are very widespread, none are host specific and most species fruit underground
or in the litter layers. As with the fungi forming ectomycorrhizal associations, multiple
colonisation of single host plants may occur.
The principal types of mycorrhizae.
As indicated above, the two principal types of
mycorrhizae are the external or sheathing mycorrhizae and the internal arbuscular types
although the ericoid and orchid mycorrhizae are important in particular habitats and in
certain plant taxa. The remaining minor types are considered in Smith and Read (1997)
(Figure IV.35).
i. Ectomycorrhizae are associations in which hyphae form a dense external hyphal
sheath or mantle over the fine lateral roots of their host plants. The hyphae also grow
inwards between the cells of the epidermis and outer cortex of the root to form the Hartig
network; while the hyphae do not penetrate the host cells, the latter become modified
on contact with the network (Smith and Read, 1997). Hyphae also grow outwards from
the sheath into the soil to form an external mycelial network. The individual hyphae of this
network are small and may penetrate micropores and micro-aggregates inaccessible to root
hairs. Such hyphae may ramify very extensively: values ranging from 300 to 8000 m
of
root have been reported across a number of studies (Smith and Read, 1997).
Many species form diffuse or anastomosing hyphal networks although more specific
structures are also known. Such external hyphal structures include hyphal mats and
the circular 'fairy ring' distributions of hyphae and fruiting bodies also found in sapro-
phytic species. The former comprise dominant, single-species aggregations of hyphae
that locally dominate the microbial biomass, often at the interface of the mineral soil
and the litter layer. They are known to secrete organic acids such as oxalate which act
as chelating agents and are active in mineral weathering and in solublising nutrient and
other elements. These mats are therefore of nutritional and pedological significance
(Griffiths
et al
., 1994).
Rhizomorphs are long-distance compound conducting structures composed of many
hyphae that may extend for considerable distances from the root cylinder. Their overall
diameter may exceed 100 Molina
et al.
(1992) report that
Pisolithus tinctorius
will
sometimes fruit on a road shoulder on the opposite side of the road from its host tree and
be joined to it only by rhizomorphs.
