Agriculture Reference
In-Depth Information
1978; Yeates, 1981; Bilgrami et al., 1985). Although these substances have not been
clearly identified, they are known to be water-soluble and dialysable (Blake, 1962).
The close relationship between the abundance of some nematode species and plant
distributions clearly demonstrates their attraction to the rhizosphere (Nusbaum and
Ferris, 1973; Rossi et al ., 1996) (Figure III.31).
Parasitic nematodes preferentially attack roots in the elongation zone, young
lateral apices and through tissue injuries inflicted by other agencies (Wyss, 1981).
Some nematodes are ectoparasites and feed by inserting their stylets into the root while
others are endoparasitic and most or all of their body may be located within the root.
Plants react in three different ways to nematode attacks: (i) by production of a local
swelling and 'stubby root' symptom and the immediate suppression of root growth;
(ii) by cell necrosis at sites where wounds and lesions have occurred; and (iii) through
the formation of root galls resulting from cell hypertrophy.
Evolution in nematodes has tended to result in a sedentary life-style, progressing from
ecto- to endoparasitism, with an increasing emphasis on female hypertrophy, enhanced
reproductive capacity and improved mechanisms of egg protection
(Figure IV.36).
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