and gas transport in unsaturated soil profiles can be assumed to occur
only through the gas phase. Gradients of volatile substances dissolved in
the soil water surrounding nematodes, like those of respiratory gases, can
also result not from diffusion through the soil water, but rather through
equilibrium between the air and aqueous phase within the soil interstices,
with movement of volatile molecules significant distances through the
soil occurring primarily through the air phase.
The consensus today is that most responses of nematodes to plants are
to general cues and lead to similar rates of root penetration in good and
poor hosts. Thus, general stimuli are probably commonly used, because
the host cannot easily avoid generating them. Gradients of CO 2 and salts
probably play the most important roles, but many other factors affect
movement. Holistic comparative studies in natural soil profiles are still
badly needed to sort out the relative importance of various stimuli to
different nematode species.
Some plant species repel or fail to attract nematodes. Nematode
responses, however, often do not correlate well with host specificity.
Roots of cucumber plants carrying theBi(bitter) locus for triterpenoid
curcurbitacins, for example, are reported to attract root-knot nematodes
less than roots of other cucumber plants (Kaplan and Keen, 1980), but,
none the less, plants with the Bi gene can become highly infected
and form extensive galls or swellings in response to nematode feeding.
The resistant grassAegilops variabiliswas less attractive to juveniles of
Meloidogyne naasithan susceptible barley (Hordeum vulgare) and wheat
(Triticum aestivum) (Balhadere and Evans, 1994), but susceptible and
resistant barley cultivars were similarly attractive. Griffin (1969) observed
that a nematode-susceptible lucerne genotype attracted two very different
nematodes,D. dipsaciandM. incognita, less than a resistant genotype
did, but did not show this effect for additional resistant and susceptible
cultivars. Roots of two very different plants, cabbage and carrot, perhaps
not surprisingly, attractedHirschmanniella oryzaemore than roots of
a third very different plant, onion (Bilgramiet al., 1985). Repellents
may cause some of these differences. In a search for both volatile and
non-volatile attractants ofM. incognitato host roots, the only attractive
chemical found was CO 2 , although the presence of complex unidentified
repellent chemicals was demonstrated (Diez and Dusenbery, 1989b;
McCallum and Dusenbery, 1992).
In some cases, a high degree of host specificity seems irrefutable.
Potato root exudate increases the motility of the obligate potato parasite,
G. rostochiensis, in sand (Clarke and Hennessy, 1984) and triggers
electrophysiological activity in the anterior end of the nematode (Rolfe
et al., 2000). Lee and Evans (1973) observed a strong positive correlation
between nematode fecundity and the attractiveness of 16 genotypes of
rice forAphelenchoides besseyi.The foliar nematode,D. phyllobius, has a
restricted host range, limited to foliage of certainSolanumspp. (Robinson
et al., 1978), and the nematode is strongly attracted to some unknownHost Finding by Plant-parasitic Nematodes 99