natural soils where fine particles can stick together to form much larger
crumbs, soil structure is the primary determinant.
A perhaps more important finding of Wallace’s was that movement by
nematodes is fastest not in water-saturated soil, but rather when soil water
content is near field capacity (approximately−0.05 bar = 50 cm water),
i.e. when soil pores have drained just to the extent achievable by gravity.
In fact, when movement was measured at various water potentials in a
series of soils with particle sizes ranging from fine to coarse, the optimal
water potential for movement was the same (−0.05 bar) regardless of
soil texture or moisture content. This effect was shown with infective
juveniles of bothHeterodera schachtiiand the much largerD. dipsaci
(Wallace, 1956, 1958a,b). Finally, in any soil at soil matric potentials drier
thanc.−0.5 bar, nematode movement essentially stopped.Osmotic pressure and salts
Nematodes have a large surface-to-volume ratio and a body covering
permeable to water, and require body turgor for normal movement. Thus,
osmoregulation is particularly important to behaviour. Nematodes within
soil touch the strongly hygroscopic, electrically charged surfaces of clay
particles, and nematodes within plant tissue feed on, perforate and often
find themselves wedged between plant cells with high internal turgor.
The internal osmotic pressures measured for plant, fresh water and soil
nematodes (Wright and Newall, 1976; Wright, 1998) are 50–100 mM NaCl
(equivalent to−2.2 to−4.2 bar), and thus several times greater than typical
ionic concentrations in soil water (0 to−0.5 bar). These nematode species
are generally able to regulate their body water contents well in hypo-
osmotic solutions, consistent with the low osmotic pressure of water in
soil (c. 22 mosmol kg−^1 ).
In glucose, mannitol and polyethylene glycol solutions, however,
vermiform stages of various plant-parasitic nematodes remain motile and
suffer no obvious volume loss at 110 mosmol kg−^1 (−2.5 bar) (Viglierchio
et al., 1969; Wyss, 1970; Castro and Thomason, 1973; Robinsonet al.,
1984a,b; Robinson and Carter, 1986). Wallace’s observation that migration
in soil ceases at about−0.5 bar matric potential, therefore, would indicate
that the primary factor limiting nematode movement in soil is physical
rather than physiological. Blake (1961) elegantly demonstrated this for
D. dipsaciby showing that the relationship between matric potential and
the speed of nematode movement in soil was the same whether soil was
wetted with water or a subtoxic solution of urea. On the other hand,
nematodes within living plant tissues may be exposed to water potentials
near−15 bar, and osmoregulation in these nematodes, which is virtually
unstudied, may be quite important.
Given the high permeability of nematodes to ions (Markset al., 1968;
Castro and Thomason, 1973; Robinson and Carter, 1986) and the profound
effects that unbalanced salts have on membrane-bound ion pumps, nervesHost Finding by Plant-parasitic Nematodes 93