Biotic Factors - Agroecology: The Ecology of Sustainable Food Systems - page 156

D EMONSTRATING A LLELOPATHY

Allelopathic chemicals released by weeds can directly
influence crop seed germination and emergence, crop
growth and development, and the health of associated crop
symbionts in the soil. Recent research on weed allelopathy
has shown that many weeds use multiple mechanisms to
inhibit crop growth and development, and such knowledge
is an important component in developing alternative weed
management strategies (Qasem and Foy, 2001).
An example of an allelopathic weed is bitter grass
( Paspalum conjugatum ), an aggressive weed in annual
cropping systems in Tabasco, Mexico. Figure 11.4 illus-
trates the inhibitory effect of bitter grass when it is present
in a corn crop. As the dominance of the grass increases,
the stunting of the corn becomes more noticeable, reach-
ing a point where the corn is not even able to establish.
Water extracts made from the dry grass that has not
yet been leached by rains showed the ability to affect both
germination and early growth of corn seed. Local farmers
recognize the negative impacts of the grass on the soil,
referring to a heating effect that can cause the stunting or
yellowing of the crop. When researchers could find no
temperature differences in the field with thermometers,
allelopathy became suspect. Although the evidence is not
sufficient to rule out competitive interference from the
grass, the inhibitory effect exists even when farmers add
recommended levels of chemical fertilizers to the crop and
when rainfall is more than sufficient.
In a study in California, two common weeds — lambs-
quarters ( Chenopodium album ) and red root pigweed ( Ama-
ranthus retroflexus ) — were tested for allelopathic potential
against green beans ( Phaseolus vulgaris ). Both weed species
showed allelopathic potential in laboratory bioassays; in the
field it was found that bean plants grown with pigweed were
stunted but had normal numbers of nodules of symbiotic
Rhizobium bacteria, and that beans grown with lambsquar-
ters were both stunted and had greatly reduced numbers of
nodules (Espinosa, 1984). These results indicate that the
chemicals released by the two different weeds were impact-
ing the crop plants in different ways, with one affecting the
growth of the beans directly and the other inhibiting the
activity of N-fixing bacteria. Since the farm field was irri-
gated, had recently been fertilized, and crop spacing ensured
that adequate light reached the beans, removal interference
was probably minimal.
A weed species that has been studied in great detail
in order to demonstrate its allelopathic mechanisms is
quackgrass ( Agropyron repens ). The following findings
are described in a review by Putnam and Weston (1986):

In order for allelopathy to be fully implicated in an inter-
ference interaction, the following steps must be followed:

1. Determine the presence of a potential allelo-

pathic compound in the suspected plant and
plant part. A screening system that employs
some type of bioassay is a common procedure
for doing this test (Leather and Einhellig,
1986). A positive bioassay can only be used to
imply that there is a potentially allelopathically
active chemical present in the plant.

2. Show that the compounds are released from the

donor plant.

3. Determine that the compounds accumulate or

concentrate to toxic levels in the environment.

4. Show that uptake or absorption of the com-

pounds by the target organism takes place.

5. Demonstrate that inhibition (or stimulation) of

the target species takes place in the field.

6. Identify the chemical compounds and deter-

mine the actual physiological basis for the
response.

7. Finally, determine how the allelopathic com-

pound interacts with other factors in the envi-
ronment so as to either reduce or enhance its
effect. (Rarely does an allelopathic compound
kill another organism outright).

Under ideal situations, all of these steps could be carried
out before attempting to manage allelopathy in an agro-
ecosystem setting. But most of the time, such intensity of
research is not possible, and farmers are faced with the
need to make decisions on their farms every day. Astute
observation coupled with research results can make allel-
opathy one more tool for managing the farm environment
for the benefit of the crop.

A LLELOPATHIC E FFECTS OF W EEDS

Weeds are responsible for the loss of crop production all
over the world. The literature abounds with reports on the
“competitive effects” of weeds, but seldom is allelopathy
considered or even mentioned as one of the mechanisms
by which weeds impact crops. Whenever weeds and crops
are in the same planting together, many possible forms of
interference are going to be working together or in
sequence. Allelopathic potential has been suggested for a
large number of weed species (Putnam and Weston, 1986),
but we are just beginning to understand the mechanisms
of release of the potentially phytotoxic compounds into
the environment, how they are taken up by crops, how
they inhibit crops, and how the negative impacts of the
compounds can be ameliorated.

• Quackgrass inhibited several crop types (e.g.,

clover, alfalfa, and barley), and this inhibition
could not be explained by removal interference
(that is, competition).

• Laboratory and greenhouse bioassays demon-

strated the inhibitory potential of both quackgrass