Biotic Factors - Agroecology: The Ecology of Sustainable Food Systems - page 160

Squash has been implicated as being an especially
effective allelopathic crop (Fujiyoshi et al., 2002). Rain
leaches inhibitors out of the large, horizontally arranged
leaves, and once in the soil, these compounds can suppress
weeds. The shade that the leaves cast probably enhances
the effect, combining a removal interference with addition
interference. Bioassays show the allelopathic potential of
water extracts of intact leaves on a range of species, with
weeds often being inhibited to a greater extent than crop
plants (Table 11.4). When squash is added to an inter-
cropped agroecosystem such as corn and beans, it takes
on the important role of weed suppressor for the entire
mixture.
Other research has shown that older varieties of some
crops, especially the varieties most closely related to wild
stock, show the greatest allelopathic potential (Putnam and
Duke, 1974; Batish et al., 2001). Crop breeding may have
selected against allelopathic potential in exchange for
higher crop yields. Screening for allelopathic types in
germplasm collections of crops could lead to incorpora-
tion of greater allelopathic potential in current crop types
through conventional crop breeding or the use of more
recently developed genetic engineering technologies.
Considering the problems associated with currently
used weed control strategies — possible environmental
pollution, groundwater contamination, increased cost of
developing and testing new herbicides, increased herbi-
cide resistance by weeds, and the difficulties of registering
new herbicides — allelopathic potential in crops will
become a more attractive alternative. Connecting the
plant's allelopathic potential with an understanding of the
fate and activity of the phytotoxic compounds once they
leave the plant will make these alternatives most useful.

G ROWTH S TIMULATION

The emphasis in the foregoing discussion has been prima-
rily on the inhibitory or negative impacts of chemicals
added to the environment by plants. There are, however,
limited reports of plants releasing compounds into the
environment that have stimulatory effects on other plants
around them. Such stimulatory addition interferences can
be classified as allelopathy as well, since the term was
originally coined to include them along with inhibitory
effects.
In some cases, low concentrations of otherwise inhibi-
tory chemicals may actually have a stimulatory effect.
Bioassays for allelopathic potential often show increased
root elongation in newly germinated seeds when plant
extracts are at low concentrations. In other cases, plants
produce compounds with wholly stimulatory effects. For
example, a study reported in a review by Rice (1986)
found that a weed known as corn cockle ( Agrostemma
githago ) had an appreciable stimulatory effect on wheat
yields when grown in mixed stands as compared to wheat
grown alone. A stimulatory substance isolated from corn
cockle was named agrostemmin, and when applied sepa-
rately to wheat fields, was shown to increase wheat
yields in both fertilized and unfertilized areas. Rice also
reports on work where chopped alfalfa added to soil stim-
ulated the growth of tobacco, cucumber, and lettuce, and
a substance known as triacontanol was identified as the
stimulant. Even some substances isolated from weeds
have stimulatory effects at certain concentrations.
Researchers are challenged to demonstrate ways that
some of these effects can be practically incorporated into
cropping system management, but the potential certainly
exists once the full mechanisms of the interference are
worked out.

TABLE 11.4
Initial Root Elongation of the Germinating Seeds of
Two Weeds and Two Crops in Laboratory Bioassays
of Squash Leaf Extracts

THE IMPORTANCE OF INTERACTIONS
AMONG ORGANISMS

Organisms can have positive and negative influence on
each other depending on the nature of their interactions.
These interactions have dynamic and potentially important
impacts on the environment of agroecosystems. This chap-
ter proposes a model for the study and understanding of
such interactions that focuses on the mechanisms through
which one organism adds to or removes from its immedi-
ate environment some resource or material that can have
consequences for the other organisms living there.
As we will see in Section III, finding effective ways
of harnessing and managing the interactions among organ-
isms is at the very heart of developing more sustainable
practices in agriculture. The autecological perspectives on
these interactions developed in this chapter will be a nec-
essary basis for exploring their action and management at

Distilled
Water
Control a (%)

2.5%
Squash Leaf
Extract b (%)

5.0%
Squash Leaf
Extract b (%) Target Species

Avena fatua 100 61.0 40.1

Brassica kaber 100 48.2 30.7

Raphanus sativa 100 112.1 57.1

Hordeum secale 100 122.0 57.8

a Root elongation after 72 h at 25°C in distilled water defined as 100%
growth.

b Air-dried intact squash leaves were soaked in distilled water for 2 h
and the resulting solution filtered and used to irrigate seeds. Concen-
tration based on ratio of grams of squash leaf to grams of water.

Source: Data from Gliessman 1988a.