Integrated Environmental Biotechnology - Environmental Biotechnology: Theory and Application - page 254

different organisms often bringing great benefit to the plant in environments
where nutrients are somewhat deficient.
Microbes external to the plant
There are clearly two distinct areas of a higher plant which are inhabited by
different communities of microorganisms: above ground around and on the
surface of leaves, stems, seeds and flowers and below ground in zones of
increasing distance away from the root mass. These rhizospheres, or zones
around the roots, which are more accurately envisaged as a continuous gradient
of nutrients, are the result of plant metabolic activity constantly drawing from
the surrounding soil. Nutrients may also be transferred in the reverse direction,
that is to the soil, as is the case with aerating plants exemplified by Phragmites
used in reed bed systems described in Chapter 7. It appears that colonisation
of the rhizosphere by bacteria is stimulated by exudate from the plant. The
first phase is attraction to the plant roots, the second is a 'settlement' phase
during which bacteria grow to form colonies and the third is a 'residence'
phase when a balance is established between root mass and bacterial numbers
(Espinosa-Urgel, Kolter and Ramos, 2002). The microbes in the rhizosphere are,
to a large extent, dependent on the plants for a supply of many useable organic
substances. As plants die and decay, the components released by the degradative
bacteria return to the soil and the cycle begins again. Consequently, plants affect
the composition of the microbial community of the ground in which they grow,
especially in soils of low fertility. Not all the compounds released from plants
are stimulants of microbial growth, some plants may also produce inhibitors. The
microbes themselves have an effect on the plant growth characteristics. Some
release into the soil, gibberellins and cytokinins, both of which are plant growth
factors, and may also affect the flow of organic compounds, termed exudate,
from the plant into the rhizosphere. The rate at which exudate is transferred
to the soil is affected by many parameters; the presence of surrounding soil
bacteria as mentioned above, the reduction of plant mass by harvesting from
above ground level, and environmental changes, for example variations in light
or temperature. Both bacteria and fungi contribute to the microbial population
in the rhizosphere. Associations of fungi with roots of vascular plants, called
mycorrhizae, are quite common and may in some cases be very beneficial to
the plant. They may be external, ectomycorrhizal, or internal, endomycorrhizal.
Ectomycorrhizal associations more commonly occur in temperate regions and
often in beech, oak, birch and coniferous trees. Their association involves a
limited penetration of the root cortex by the fungi growing as a covering around
the tip of the root. They aid the growth of the plants as a result of their mycelia
reaching far out into the surrounding soil, thus assisting the plant in nutrient
uptake. This quality has received commercial attention. The effect on plant
growth and subsequent predation by insect larvae, of some species of fungus, for
example Pisolithus tinctorius , has received particular attention (Rieske, 2001).