plant surface microbiology.pdf

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Galdino Andrade
phosphate compounds. The nitrifying, sulphur oxidants and sulphur- reducing
bacteria can also solubilise insoluble phosphate salts and produce H 2S
under anaerobic conditions. Many microorganisms and plants can produce
organic acids by acting as solubilizing agents and quelants and releasing
orthophosphate in the soil solution (Sylvia et al. 1998). Soluble phosphate in
the soil solution can be absorbed and transported to the plant by arbuscular
mycorrhizal (AM) fungus mycelia. The interaction between the phosphate
solubilisers and the mycorrhizae can stimulate mycorrhizal colonisation
and/or plant growth by increasing the phosphorus levels (Fig. 6).
The arbuscular mycorrhizal fungi are symbiotic fungi of plant roots. This
symbiosis is present in almost all plants in the most different ecosystems
(Hayman 1982). The symbiotic relationship between plant roots and mycorrhizal
fungi improves plant mineral nutrient acquisition from the soil, especially
immobile elements such as P, Zn and Cu, but also more mobile ions such
as S, Ca, K, Fe, Mg, Mn, Cl, Br and N (Tinker 1984). In soils where such elements
may be deficient or less available, mycorrhizal fungi increase efficiency
of mineral uptake, resulting in increased plant growth (Linderman 1988).
The mycorrhizal complex (AM fungi and root) changes the nutritional and
physicochemical conditions of the rhizosphere, and has a large negative or
positive impact on the functional microorganism groups. This effect depends
on the cycle to which the functional group belongs. However, in spite of the
importance of the mycorrhizae, these groups should not be assessed in isolation.
7 Dynamics of the Rhizosphere Functional Groups
of Microrganisms
The interaction of specific biological systems, in a ecosystem or microcosm,
depends on the interplay of three general factors – environment, biological
community structure (biodiversity), and biological activity (function). The
role of diversity, particularly of microorganisms, and the relationship
between microbial diversity and function is largely unknown (Griffiths et al.
1997). As can be seen, each functional group can interact with different biogeochemical
soil cycles and the environmental impact caused by an agent can
be determined by the changes observed in the populations, as a determined
environmental condition can affect the microbial activity without affecting
the community biodiversity (Griffiths et al. 1997). The dynamic behaviour of
perturbed communities is a branch of general ecology closely related to the
study of natural and artificial disturbances in microbial habitats. Another
important factor is the relationship between resistance and resilience, whose
combined effects determine the ecosystem stability. Resistance is the inherent
capacity of the system to hold disturbance, whereas resilience is the capacity
to recover after disturbance.