Environmental Engineering Reference
In-Depth Information
Biologically, the primary goal of all life forms is to stay
alive. Organisms must feed themselves, find water, and
avoid predation; if successful, they can pass on their genetic
material to their offspring. Plants are exposed to sunlight,
and the process of photosynthesis provides food, but the
control of water supply and avoidance of predation have
led to various adaptive processes by which to achieve
these goals. From root depth to defensive chemicals, plants
are anything but passive sessile organisms at the mercy of
nature. Allelopathy is perhaps the best example of this.
chemicals not for use as a source of energy but solely to
keep the plant or parts of a plant from being destroyed by
herbivores. The manufacture of such protective compounds
has evolved because the loss of leaves to predation does not
provide the plant an advantage, and plants lose fewer leaves
if they can produce foul-tasting or toxic compounds. These
few examples indicate how complex “sessile” plants really
are, and how they can take simple substrates, such as CO
2
and water, and synthesize incredibly complex and toxic
compounds for both defensive and offensive chemicals.
Plants appear to be not only immobile but seemingly
unprotected and, therefore, easy prey. Long before the man-
ufacture and application of pesticides, however, plants
became naturally toxic to other plants and potential
herbivores by producing their own defensive chemicals. It
has long been known that plants can manufacture
compounds that are harmless to themselves but toxic to
other organisms. On the simplest, single-cell level, penicillin
is produced by a mold to ward off attacks by invading
harmful microbes. Perhaps closer to individual experience,
dermal contact with poison ivy can result in a localized
allergic reaction. The causative agent is a volatile organic
compound (VOC) in the leaf that contains allergens
designed to decrease successful herbivory.
The ability for multicellular organisms, such as plants, to
control their local environment by reducing competition
through a kind of low-grade chemical warfare is called
allelopathy—it literally means “mutual suffering.” Evidence
exists from Theophrastus and his writings during 300
BC
about the negative effect that chickpeas had on the growth
of other weeds. Pliny the Elder also commented that chick-
pea and barley made the cultivated land inhospitable to other
crops. Allelopathy can be considered as an extension of
simple physical competition for resources (Whittaker and
Feeny 1971). Trees growing close together have root com-
petition to acquire limited resources, although this seems to
be less true in groves of aspen or bamboo clones, where
evenly spaced plants may result from resource optimization.
A selective advantage is provided a plant to gain resources if
it can essentially contaminate the surrounding soil by the
release of allelopathic chemicals.
Allelopathic compounds can be produced by a wide range
of plant parts. The leaves, or needles, of pine trees, for
example, release organic acids upon decomposition around
the root zone at sufficient concentrations to decrease the soil
pH and render it inhospitable for plant seeds, other than pine,
to germinate (Fig.
11.9
). Other allelopathic compounds are
produced by plant shoots and leaves and enter the soil zone
either by washout from precipitation or after these parts have
fallen to the ground and release their chemicals or are
volatilized from the plant leaves. Moreover, some of the
most toxic poisons across both the animal and plant world
are synthesized by fungi associated with tree roots.
11.2.1 Allelopathy and Plant-Chemical Warfare
Upon first glance, most plants seem to be fairly passive and
defenseless and unable to control their environment, because
plants are relatively fixed in place. This notion of passivity,
however, is far from the truth. Plants are living creatures that
respond to or alter a variety of things in their immediate
environment, usually to their competitive advantage.
Certain plants have been successful because of deterrents
to predation that have evolved over time (Fig.
11.8
). For
example, the presence of visually attractive flowers is an
adaptation to sexual reproduction that uses insects and birds
for gene transfer. On the other hand, plants synthesize
Fig. 11.8
As shown by this honey locust (
Gleditsia triacanthos
),
thorns are an obvious indication that plants are not passive when it
comes to defensive approaches to stay alive. In this case, the develop-
ment of thorns may have occurred during the Cretaceous Period for
protection against damage by dinosaurs (Photograph by author).
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