Agriculture Reference
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(i) Aggregates are disrupted by the penetration of mucilage and organo-mineral
linkages are established between clay platelets and the gel matrix and also with organic
particles detached from the mucilage sheath (Breisch et al., 1975; Morel et al., 1991).
Such linkages occur in the absence of microbial processes and are important features
in the development of rhizosphere soil and soil-particle aggregation (Lespinat et al.
1975; Guckert, 1985; Gouzou et al., 1993).
(ii) Micro-organisms encountered by the growing root tip become enclosed in
the mucilage and those unable to lyse this material remain inactive. At the margins of the
mucilaginous slime, bacteria are attracted by chemotactic stimuli (Mandimba et al., 1986).
However, this response varies with the plant species, bacterial species and strain, thus
demonstrating a degree of specificity in the establishment of plant-bacteria associations
(Guckert, 1985). Fungal spores have been shown to germinate at distances up to 10 mm
from the root (Stanghellini and Hancock, 1971; Short and Lacy, 1974). Mucilage therefore
acts as a selective filter for microbial communities, and may protect the root from massive
colonisation by micro-organisms, particularly those of pathogenic habits (Samtsevich,
1971; Harris and Hartley, 1976).
(iii) The micro-organisms responding to the chemotactic stimuli start to lyse the mucilage
and soon establish microcolonies within it. The generation times of these bacteria may
be shortened to as little as 7 % of those measured in non-rhizosphere soil, although large
differences occur between microbial species (Bowen and Rovira, 1976). The presence of
these bacteria stimulates root exudation which may be twice as intense as that in a sterile
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