1.12 mmol mol−^1 from 0.99 mmol mol−^1 , i.e. a 12% relative difference in
concentration (Bernklau and Bjostad, 1998a,b). In the Robinson (1995)
study, effective release rates for attractingM. incognitaandR. reniformis
to a point source in sand ranged from 6 to 35μl min−^1 , with an optimal
flow of 15μl min−^1. Intervals of 40 and 29 h, respectively, were required
to achieve maximal attraction of the two species from a distance
of 52 mm. Enough gas to achieve the same level of attraction was
theoretically achievable with a germinating sunflower seed.Movement towards plants
Host finding by plant-parasitic nematodes is examined in over 100
published studies. Contemporary general reviews include Perry (1996,
1997) and Perry and Aumann (1998). All root-parasitic and most foliar-
parasitic nematodes have one or more infective stages that occur in the
soil. These stages typically must find roots or find stems emerging from
the soil to ascend foliage, in order to complete the life cycle.
Since roots respire, extract water, differentially take up salts and
release various organic compounds, they can significantly modify local
soil chemistry. As roots are approached, soil moisture decreases, Na+
increases, K+andNO 3 −decrease, O 2 decreases, CO 2 increases and amino
acids, sugars and secondary metabolites increase. Very near roots, tem-
perature increases slightly. Soil gases (ammonia, ethylene, methane),
pH, redox potential and electrical potential may increase or decrease.
Since the diffusivity of gases in the soil decreases by several orders
of magnitude as soil interstices fill with water (Campbell, 1985), soil
moisture content can markedly influence gradients of volatiles. Release
and diffusion of behaviourally active chemicals are influenced by temper-
ature (Pervez and Bilgrami, 2000).
Although Steiner (1925) postulated that nematodes are attracted to
roots, the well-known nematode accumulations near roots on agar plates
(Linford, 1939; Wieser, 1955, 1956; Widdowsonet al., 1958) were still
thought by some researchers in the early 1960s to result only from chance
encounters and physical trapping (Kühn, 1959; Sandstedtet al., 1961;
Sandstedt and Schuster, 1962). Numerous studies demonstrated directed
movement towards roots in soil, towards living but not dead roots on agar
plates and towards soil without roots but in which roots had been grown
(Prot and VanGundy, 1981).
By the mid-1960s, contradictory results were obtained regarding
nematode responses under controlled conditions to most substances
known to occur as gradients around roots (Klingler, 1965; Prot, 1980).
Although some studies showed that secondary metabolites correlated
with host specificity, most evidence pointed to general attractants. The
only consistent directions of movement were towards CO 2 and the wet
end of a soil-moisture gradient (Wallace, 1960). Movement towards
moisture vertically would help nematodes avoid desiccation butHost Finding by Plant-parasitic Nematodes 97