2000). These effects undoubtedly contributed to debates among early
investigators regarding the importance of CO 2 -induced changes in redox
potential, pH, carbonic acid, bicarbonate and carbonate in the soil.
In unbuffered, enzyme-free solutions at standard atmospheric pressure,
dissolved CO 2 reacts slowly with water to form carbonic acid, which in
turn dissociates (pK= 6.1) to yield bicarbonate and a proton, lowering the
pH. Buffers, enzymes, atmospheric pressure and temperature influence
the ratios of CO 2 ,H 2 CO 3 ,HCO 3 −,CO^23 −and H+present. The current con-
sensus is that CO 2 can attract many nematodes at subtoxic concentrations
and either dissolved CO 2 or carbonic acid is the nematode-attractive
component.
Repulsion by ammonia may modulate attraction to CO 2. The root-knot
nematodeM. incognitais repelled by ammonia and several nitrogenous
salts in vitro (Castro et al., 1990, 1991). When entomopathogenic
nematodes are released into the soil in large numbers, they accumulate
around roots and root invasion by plant-parasitic nematodes is suppres-
sed. Grewalet al. (1999) suggested that the plant-parasitic nematodes in
this case are repelled by ammonia released by the entomopathogenic
nematodes’ symbioticXenorhabdusbacteria.
Pline and Dusenbery (1987) analysed responses ofM. incognitaon
agar exposed to horizontal bilaminar flow of air from two parallel gas
jets emitting different concentrations of CO 2. This produced a different
atmosphere over each half of the agar plate. They found that the threshold
gradient varied with the ambient concentration, i.e. the nematodes
became more sensitive as the ambient concentration dropped and thus
could detect about the same relative change at any ambient concentration.
The threshold forM. incognita(Pline and Dusenbery, 1987) corresponded
to a relative change of about 3% cm−^1 , which was subsequently calculated
(Dusenbery, 1987) to allow detection of roots from at least 5 cm and
perhaps as far away as 500 cm. This effect is very important ecologically
because it shows that nematodes can detect gradients at far greater
distances from the source than would be possible with a fixed con-
centration differential threshold. As emphasized by Dusenbery, these
predictions contrast sharply with Prot’s (1980) conclusion that CO 2 only
attracts nematodes within 1 or 2 cm of the source.
Klingler (1963) positioned a capillary CO 2 delivery tube within a thin
layer of air between agar and a glass cover to attractD. dipsaci. The
minimum effective gradient in the air, as determined by gas chromato-
graphy, was approximately 1 mmol CO 2 mol−^1 cm−^1. In a more recent
study, gradients were established in cylinders of moist sand by enclosing
the cylinder in a plastic tube and equilibrating the two ends against
air masses containing controlled CO 2 concentrations. M. incognita,
R. reniformisandS. glaserimigrated up-gradient in response to a change
of 0.2% cm−^1 (2 mmol mol−^1 cm−^1 ) at a mean CO 2 concentration of 1.2%
(12 mmol mol−^1 ), which is a relative change of about 16% cm−^1 or 1%
per nematode body length (Robinson, 1995; Robinson and Jaffee, 1996).
For comparison, first-instar western maize root worms can distinguish96 A.F. Robinson