Geoscience Reference
In-Depth Information
will release other mucilaginous material. The mixture of plant and microbial mucilages
is called mucigel. Mucigel has been shown to link clay particles, thereby increasing the
cohesion of soil materials and leading to the formation of microaggregates.
The population density of microbes in the rhizosphere , the soil that is directly inlu-
enced by root activity, may be 10-200 times greater than that in the bulk soil. This
increased concentration of soil microorganisms can be either beneicial or detrimental
to the plant depending on the species that dominate. Microbial activity may increase
availability of nutrients by mineralization of organic forms or by increasing the solubil-
ity of mineral forms. For instance, there are phosphorus-solubilizing fungi ( Penicillium
balaji ), some of which have been exploited commercially (Ehlers and Goss, 2003 ).
The most important microbes for phosphorus uptake are mycorrhiza fungi that form
associations with plant roots. The majority of plants establish such an association with
certain types of soil fungi; this association is known as a mycorrhiza . Micorrhizas are
generally mutualistic. Carbohydrate is passed from the plant to the fungus, and in return
the fungus facilitates increased nutrient uptake, particularly of phosphorus, from the soil
to the plant. Mycorrhizal fungi can also increase the availability of zinc to the root.
Interest in these symbioses has increased dramatically in recent years because of
their potential beneit in agriculture, forestry and re-vegetation of damaged ecosystems.
Some plants cannot become established or grow normally without an appropriate fun-
gal partner. Even when plants can survive without mycorrhizas, those with 'fungus-
roots' grow better on infertile soils and areas needing re-vegetation. The mechanism
by which this occurs is a combination of increased surface area for adsorption from the
soil solution and inward translocation of phosphorus from beyond zones of depletion
around the roots. Without micorrhizas, the depletion zone of phosphorus around a root
is ca. 1 mm, but micorrhizas can extend for 100 mm, thereby greatly increasing the zone
from which phosphorus is adsorbed. In addition to the larger volume of soil explored,
the kinetics of nutrient uptake may also be enhanced (Ehlers and Goss, 2003 ).
Micorrhizal fungi have also been shown to transport nitrogenous compounds
between plants, but the most effective microbes for providing higher plants with
nitrogen are rhizobia. These form symbioses with leguminous plants. The rhizobia
enter the host plant through the root hair and travel into the cortex via an infection
thread. The bacterium undergoes a transformation to produce many bacteriodes, and
the growth and division of infected root cells leads to the formation of root nodules.
For example, under ideal conditions, the symbiosis between soybean and rhizobia can
ix about 500 kg N ha -1 y -1 .
6.2.2 Structure of the Root Tip
Root tips , the growing ends of individual root branches, have a striking appearance.
They look fresh and white, rather like the colour of blanched asparagus. At some
distance from the tip, the colour of the branch changes to light brown. This is the
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