Biotic Factors - Agroecology: The Ecology of Sustainable Food Systems - page 155

2001). Allelopathy thus serves as an excellent example
of how a research focus on the mechanisms of interfer-
ence can have important applications in agroecology.
Because allelopathy has such potential importance in
agroecological research and for sustainability, the
remainder of this chapter will be devoted to exploring
it in greater detail.
There are many possible allelopathic effects of weed
and crop species that need to be taken into account in
agroecosystem management. The production and release

of phytotoxic chemicals can originate from crops or
weeds, and they can play very important roles in crop
selection, weed management, crop rotations, the use of
cover crops, and intercropping design. Many examples
of such interactions have appeared in the international
publication Allelopathy Journal.
Our purpose in this section is to gain more insight into
the actual mechanisms of allelopathic interactions. The
implications and applications of these interactions will be
more fully explored in Chapter 13.

THE HISTORY OF THE STUDY OF ALLELOPATHY

The effects of allelopathy have been observed since the times of the Greeks and Romans, when Theophrastus
suggested that the “odors” of cabbage caused vine plants to “wilt and retreat” (Willis, 1985). Japanese sources
dating back to at least the 1600s independently document what we now know to be allelopathic interactions, and
such knowledge may have developed earlier and independently in other areas.
In Europe, scientific observations of allelopathic plant interactions were not made until the 17th century, when
De Candolle published an influential work describing his observations of the excretion of droplets of some sort
from the roots of Lolium temulentum . De Candolle believed that plants used their roots as excretory organs and that
these excretions contained chemicals that stayed in the soil and affected subsequent plant growth. His theory fell
out of favor, however, when Justus Von Liebig developed the theory of mineral nutrition, and the focus on plant
interactions shifted to nutrient depletion and competition.
It was not until the late 19th century that careful experiments in the U.S. and U.K. scientifically demonstrated
that allelopathy was an important plant interaction. In England, certain grasses were found to negatively impact the
growth of nearby trees, and the research indicated that the effects could not have been due to soil nutrient depletion.
In fact, leachates of soil from pots planted with the grasses impacted the trees as much as the grass itself. In the
U.S., Schreiner and his associates published a series of papers between 1907 and 1911 documenting the “exhaustion”
of soils planted continually in one crop and the extraction of the chemicals responsible for the exhaustion. This
was the first time researchers demonstrated the ability of plant chemicals to inhibit germination and seedling growth
of a plant species.
During the 1920s, some important work focused on the black walnut. Cook documented the tree's ability to
inhibit nearby plants, and Massey found that an extract of walnut bark in water caused tomato plants to wilt.
In 1937, the term allelopathy was coined by Molisch to describe any biochemical interaction between plants
and microorganisms, positive or negative. Soon afterward, studies by Benedict, Bonner and Galston, Evenari, and
McCalla and Duley again documented chemotrophic plant effects, and the term allelopathy came into common
usage for the first time (Willis, 1985).
Muller introduced the concept of interference in 1969 as a way of explaining both competition and allelopathy
in a single theory. Ecologists began to realize that competitive or allelopathic effects may work in tandem in any
given system, and that allelopathic interactions can be particularly important in multiple cropping systems (Rice,
1984). More recently, recognition of the importance of allelopathy in agriculture has lead to research on ways
phytotoxins can be involved in such practices as weed control, cover-cropping, soil biofumigation, and even pest
management (Gliessman, 2002a).
The difficulty of demonstrating how allelopathy actually works in the field has kept ecologists from attributing
a significant role to chemical interference in overall vegetation process. But recent work by Bais et al. (2003) has
firmly placed allelopathy back on center stage. They meticulously documented the displacement of native plant
species by the Eurasian spotted knapweed ( Centaurea maculosa ) in the western U.S., and the role that allelopathy
plays in the process. They identified the phytotoxin that this economically destructive plant invader produces, showed
how it is released from the roots, and characterized the mechanisms that trigger the death of susceptible native plant
neighbors. Such research clearly demonstrates how allelopathy must be reckoned with in plant species interactions
(Fitter, 2003), both in natural ecosystems and agroecosystems.