FUNCTIONING OF THE SOIL SYSTEM - Soil Ecology - page 432

fungi requiring amino acids for maximal growth and lower proportions of those
requiring nutrients supplied in soil extracts (Curl and Truelove, 1986). Bacteria capable
of synthesising growth factors for plants and other micro-organisms, and also toxic
metabolites, have been reported to be more abundant in the rhizosphere than in the
non-rhizosphere soil. Nonetheless, Curl and Truelove noted that 'presently available
methodology precludes elucidation of the true balance between these groups in natural
environments'. Rhizosphere communities seem to be less diverse than those external
to it and two genera of bacteria and fungi respectively, Pseudomonas and Fusarium
have been reported several times to comprise up to 60-90 % of total rhizosphere
populations (Kreutzer, 1972; Vancura, 1980). Mavingui et al. (1992) noted a diminished
genetic diversity in Bacillus polymixa populations in the rhizoplane of wheat as
compared to rhizosphere and non-rhizosphere soil.

3.1.2.2 Symbiotic micro-organisms

Symbiotic associations between roots and micro-organisms involve mycorrhizal fungi,
the Rhizobial bacteria that form nodules on legume roots and the actinobacteria
( Frankia spp.) associated with the roots of the Casuarinales and a number of other plants.

Mycorrhizal fungi
Frank (1885) was the first to recognise mycorrhizae as associations of mixed plant
and fungal origin. These composite organs have three components: the fine roots of their
host plants, the fungal structures that interface with the host plant and an often extensive
network of external nutrient- and water-absorbing hyphae that explore the surrounding
soil environment. In the internal or arbuscular mycorrhizae, the first two components
are associated with the root hair regions of plant roots but do not occur on very young
or lignified root parts. In the external or ectomycorrhizae, the final order lateral roots
may be covered by a dense sheath of fungal hyphae. Mycorrhizal associations are
normally mutualistic: they enhance the acquisition of nutrients by the host plant and may
provide other benefits (as discussed in Section IV.3.2.1.2) while the fungi involved
receive energy in the form of organic carbon compounds.
Mycorrhizae are of two basic types: endomycorrhizae in which the fungus penetrates
the cell wall (but not the plasmalemma) and ectomycorrhizae where they do not.
Harley (1989) recognised seven different mycorrhizal types and these are listed in
Table IV.9 together with information on the host and fungal taxa involved in each and
other selected properties of these associations. However, the internal arbuscular
(or vesicular-arbuscular) and ectomycorrhizal types are the most widespread and clearly
the most important in terms of both agricultural and forestry production,
and in their influences on the natural communities within which they occur. It is these
two types that is given most prominence here.
The majority of plant species enter into mycorrhizal associations and this is considered
to be the primitive condition. Fungi similar to the arbuscular types are thought to have
originated 353-362 million years ago (Simon et al., 1993) and this is consistent with
the hypothesis that symbiotic fungi were instrumental in the colonisation of land by
ancient plants (Pirozynski and Dalpé, 1989).