Agriculture Reference
In-Depth Information
In response to this evolutionary trend, plants have developed progressively more active
cell responses, including the formation of specialised
transfer
cells induced in the host
tissue by the nematode to sustain its feeding for long periods of time (Wyss, 1981).
Once nematode populations have developed at a site where a particular host plant
has been cropped, they may survive as resistant cysts for periods up to 10 years, as in the
genus
Heterodera.
Population density generally increases steadily for five years and then
stabilises. If the abundance attained at that stage is detrimental to crop yields, the usual
recommendation is to cease growing the particular crop and leave the soil to fallow,
or to cultivate non-susceptible crops until the cysts are eliminated. Certain gall-forming
nematodes such as
Meloidogyne sp.
may significantly reduce crop production after only
two or three years.
Free-living micro- and mesofauna in the rhizosphere
Microfoodwebs comprising the microflora, a number of microflora predators and their
own next-level predators are well developed in the rhizosphere where they play specific
roles in mediating plant nutrient uptake (see Section IV.3.2.1.1). They comprise protists
(ciliates, amoebae and flagellates), a range of root-parasitic nematodes and Acari, or mites.
Protists and free living nematodes are significantly concentrated within the
rhizosphere. Rouatt
et al.
(1960) reported a two-fold increase of protozoan population
density in the wheat rhizosphere, and more evidence of such concentrations have been
provided by Darbyshire and Greaves (1967), Clarholm (1989), Foster and Dormaar
(1991) and others. Darbyshire and Greaves (1967) estimated populations of 3000 to
14,000 amoebae soil in the rhizosphere of
Lolium perenne,
with no evidence for
the selection of particular species in the rhizosphere as compared with root-free soil.
In a microcosm study, Clarholm (1985) noted a three-fold increase in Ciliates, a 2.4-fold
increase in flagellates and a 25-fold decrease in amoebae in the wheat rhizosphere as
compared with a soil with no plants. In a North American short-grass prairie, Ingham (1981)
reported that 20 to 40 % of populations of microbial feeding nematodes were concentrated
in the rhizosphere although this represents only 3 % of the soil in the upper horizons.
In a laboratory study, Parmelee
et al.
(1993) demonstrated a significant concentration
of nematodes (1.6 to 8.2-fold increase) and micro-arthropods (2.2-2.5-fold) in the rhizo-
sphere of coniferous seedlings grown in a mineral soil. Increased micro-arthropod
populations were only found where root density was high and no significant effect was
observed in the case of seedlings grown in an organic soil. In field studies in a humid
African savanna (Lamto, Côte d'Ivoire), nematodes and micro-arthropods were found
to be more populous (3.7 and 2.2-fold, respectively) in the soil below grass tussocks than
elsewhere (Malcevski, 1978; Athias, 1974).
It is not yet clear whether the concentrations of organisms noted in the rhizosphere
occur within specific microsites as proposed by Clarholm (1985) or are distributed
throughout the rhizosphere. Microscopic observations by Foster (1981), and Foster and
Dormaar (1991) offer support to the hypothesis that bacterial populations initially
develop in the root cap zone where mucilage is produced, and then as protists start to
feed on these bacteria, they move into the space freed by the bacterial consumption of
exudates.
