Agriculture Reference
In-Depth Information
micro-organisms including nitrogen-fixing bacteria, mycorrhizal fungi and actinobacteria
such as
Frankia
spp.
In terms of function, living roots 'fuel' microbial activity through the production of
readily-assimilable substances (exudates) and exfoliated cortical cells. These are released
into the immediate environment of the roots in a process known as rhizodeposition; such
materials promote microbial activity at the root surface through priming effects.
3.1.1
SPATIAL EXTENSION
3 1.1.1
Longitudinal characterisation of the rhizosphere
Interactions between roots and micro-organisms are confined to microsites which differ
in the nature of the organic materials produced and in their microfloral and microfaunal
communities. The following five main zones are located distally to proximally along the
growing root (Balandreau and Knowles, 1978; Rovira
et al
., 1979; Coleman
et al.,
1984a; Guckert, 1985): (i) root cap and meristem zone; (ii) elongation zone; (iii) root
hair zone; (iv) mature region; and (v) root litter (Figure III.25).
The root tip may be considered as the distal point of the rhizosphere. As described
by Rovira
et al.
(1979), it is a 1 to 2 mm zone of intense cell division which comprises
a meristematic zone protected by a root cap. Energy is mainly provided as root exudates,
i.e.,
polysaccharides of high molecular weight often comprising chains of galactose
and galacturonic acid, bearing blocks of neutral sugars (glucose, galactose, arabinose
and fucose) (Balandreau and Knowles, 1978) secreted by the root cap, together with low-
molecular weight exudates and sloughed cells from the short-lived tissues of the root cap.
Mucilage was formerly considered as a lubricant that would facilitate penetration and
the establishment of a close contact between the soil and the growing root. This region
is poorly colonised by micro-organisms. Root-cap cells have short life spans, estimated
at one day for maize (Clowes, 1976). These cells actively secrete mucilage from their
Golgi apparatus and, each day, approximately 10,000 cells from the root cap of a single
maize root tip produce mucilage before being sloughed off. Estimated secretion rates are
as high as 300 to 1250 and 3 to12.5 (Samtsevich, 1971).
The root elongation zone occurs just behind the root cap and is a microsite several
millimetres long where mucilage is secreted together with low molecular-weight exudates.
Intense bacterial growth occurs in this zone resulting in catabolism of the exudates and
the sloughed cells of the root cap and epidermis. Some bacteria may produce polysac-
charide slimes which mix with the mucilage to form mucigel. The microfauna is also
abundant with large populations of protists and nematodes; the mesofauna is also
concentrated in that zone (Section IV.3.1.2.3).
Root hairs occur in a zone commencing some 10 mm behind the root cap. Low
molecular-weight exudates are secreted in this zone and it is also the site of maximum
nutrient absorption by the root. The zone of maturation found just proximal to the zone
of root hairs is suberised and no longer produces exudates. Arbuscular mycorrhizal
fungi colonise roots in this zone.
Secondary roots grow through ruptures in the primary root; the increased exudation
associated with such microsites substantially stimulates microbial activity. Further, sec-
