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Allelopathy in Agroecosystems:
An Overview
H. P. Singh
Daizy R. Batish
R. K. Kohli
SUMMARY. Allelopathy plays an important role in the agroecosys-
tems leading to a wide array of interactions between crop-crop, crop-
weed and tree-crops. Generally, these interactions are harmful to the
receiver plants but provide a selective benefit to the donor. Soil mi-
crobes play a key role in determining such interactions as they not only
alter the nature of allelopathic interactions but also modify the expres-
sion of allelochemicals. Soil sickness problem in the croplands could
also be attributed to the allelopathic property or even the autotoxicity.
The allelochemicals released largely by the plant residues that are left in
the fields after the harvest of crops add to the multifarious problems. If
properly managed, these residues could be used for controlling weeds
and pests. As is true for any chemical based response, allelopathic
interactions are also concentration specific. The promotory functions
that are inbuilt need to be worked out and exploited. Now a days
allelopathic interactions, in general, and the allelochemicals, in particu-
lar, are viewed as an important tool for sustainable weed and pest
management, and disease control. In this direction, a number of strate-
H. P. Singh is Research Associate, Daizy R. Batish is Reader, and R. K. Kohli is
Professor, Department of Botany, Panjab University, Chandigarh-160 014, India.
Address correspondence to: R. K. Kohli at the above address (E-mail: rkkohli45@
yahoo.com).
H. P. Singh is thankful to Council of Scientific and Industrial Research (CSIR),
India for financial assistance.
[Haworth co-indexing entry note]: ‘‘Allelopathy in Agroecosystems: An Overview.’’ Singh, H. P.,
Daizy R. Batish, and R. K. Kohli. Co-published simultaneously in Journal of Crop Production (Food
Products Press, an imprint of The Haworth Press, Inc.) Vol. 4, No. 2 (#8), 2001, pp. 1-41; and: Allelopathy
in Agroecosystems (ed: Ravinder K. Kohli, Harminder Pal Singh, and Daizy R. Batish) Food Products
Press, an imprint of The Haworth Press, Inc., 2001, pp. 1-41. Single or multiple copies of this article are
available for a fee from The Haworth Document Delivery Service [1-800-342-9678, 9:00 a.m. - 5:00 p.m.
(EST). E-mail address: getinfo@haworthpressinc. com].
E2001 by The Haworth Press, Inc. All rights reserved. 1
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ALLELOPATHY IN AGROECOSYSTEMS
2
gies like use of cover or smother or companion crops for weed manage-
ment, direct use of allelochemicals as natural pesticides, and even the
transfer of allelopathic traits/principles to modern day cultivars are
being adopted. The purified allelochemicals and/or their derivatives
and even the compounds synthesized on their chemistry can be used as
novel agrochemicals for sustainable management in an eco-friendly
manner. The present paper aims to discuss all the above mentioned
roles and aspects of allelopathy in the agroecosystems. [Article copies
available for a fee from The Haworth Document Delivery Service:
1-800-342-9678. E-mail address: <getinfo@haworthpressinc.com> Website:
<http://www.HaworthPress.com> E2001 by The Haworth Press, Inc. All rights
reserved.]
KEYWORDS. Allelopathy, sustainable agriculture, role of microbes,
soil environment, novel agrochemicals, eco-friendly herbicides, pest
management
INTRODUCTION
Plants produce an array of secondary metabolites which apart from play-
ing an important physiological function for the self, result in an orderly
interaction between them and consequential impact on the neighboring envi-
ronment. The chemical interactions are diverse and complex and provide the
donor plant a number of selective advantages (Reigosa, Sánchez-Moreiras,
and González, 1999). There are several historical references in the literature
that point to the inhibitory effects of one plant on the other through the
release of chemical substances in the environment (Theophrastus, 300 BC;
Pliny II, 1 AD; Culpeper, 1633; Young, 1804; de Candolle, 1832). Since all
such references were based on simple observations or were merely state-
ments/anecdotes that lacked scientific proofs and experimentation, these
failed to attract the attention of scientific community. Moreover, the lack of
coherence regarding the appropriate and meaningful terminology for chemi-
cal based interactions added to the confusion, as different workers used
different terms for the chemicals responsible for the interactions.
The term allelopathy was coined in 1937 by Hans Molisch choosing two
Greek words ‘Allelo’and‘Pathos’ literally meaning mutual sufferings. Based
on the Molisch’s concept, Rice (1984) defined Allelopathy as any direct or
indirect positive or negative effect of one plant on the other (including the
microbes) through the release of chemicals into the environment.
The progress in the field of allelopathy started rather slowly in the begin-
ning of Post-Molisch period until 1960’s when Muller and his co-workers did
some classic and pioneer work on the Californian Chaparral (Muller, Muller,
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Singh, Batish, and Kohli 3
and Haines, 1964; Muller, 1965, 1966; del Moral and Muller, 1969; Chou and
Muller, 1972). Later, Rice revolutionized this field by his excellent research
work and exhaustive writings. Now, allelopathy is a well-established interdis-
ciplinary science with multifaceted approach (Inderjit, Dakshini, and Foy,
1999). This is evident from the fact that International Allelopathy Society has
already organized two International Allelopathy Congresses--one in Spain
and the second in Canada. During the last two decades, the science of allelo-
pathy has attracted a number of scientists from the diverse fields and there
has been a spurt in the number of research papers, reports, articles, reviews
and books in this field. To mention a few--Grodzinsky (1970-1974, 1977);
Rice (1984, 1995); Thompson (1985); Einhellig (1985b); Chou and Waller
(1989); Kohli (1990, 1993, 1997); Rizvi and Rizvi (1992); Putnam and Tang
(1986); Waller (1987); Narwal and Tauro (1994); Inderjit, Dakshini, and
Einhellig (1995); Seigler (1996); Kohli, Batish, and Singh (1998a); Chou,
Waller, and Reinhardt (1999); Cutler and Cutler (1999a); Inderjit, Dakshini,
and Foy (1999); Macias et al. (1999a); and Narwal (1999a).
HISTORICAL BACKGROUND
In the historical perspective of allelopathy, the first statement related to it
came from none other than Theophrastus (300 BC), who observed that chick-
pea plants exert adverse effect on the other by exhausting the soil. Years later,
Pliny II (Plinius Secundus, 1 A.D.) made almost similar observations. Like-
wise, a few more observations and references were made by some other
scientists like Culpeper, Young and de Candolle. However, none of these was
based on experimentation (Rice, 1984). In the beginning of the 19th Century,
de Candolle--a well-known plant taxonomist generated some interest in this
field when he observed that root exudations of some plants are the cause of
Soil Sickness and that this can be overcome by the suitable crop rotation (de
Candolle, 1832). His observations were based on simple experiments. Later,
nevertheless, his hypothesis was rejected by a majority of the scientists. After
a lag period, in the beginning of the 20th century, some interest in this field
was again generated by the studies of Schreiner and his co-workers in USA
(Schreiner and Reed, 1907, 1908) and Pickering and his team in UK (Willis,
1997). To summarize, the history of allelopathy could be divided into 3
phases of its development:
i. de Candolle Phase: The period of late 18th and early 19th century, es-
pecially between 1785 and 1845,
ii. Pre-Molisch Phase: The period in the beginning of the 20th century
(from 1900-1920) known by the work of Pickering and Schreiner, and
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ALLELOPATHY IN AGROECOSYSTEMS
4
iii. Post-Molisch Phase: 1937 onwards which actually could progress
since 1960 (Willis, 1997).
However, as a result of better techniques and methodologies, collaborative
approach among the scientists and availability of suitable bioassays, this
science picked up the momentum during the last two decades or so.
ALLELOPATHY- PERSPECTIVES AND PROBLEMS
The field of allelopathy offers an exciting, inter-disciplinary, complex and
challenging studies and, therefore, invites the attention of the botanists, phyto-
chemists, microbiologist, soil scientists, agronomists, ecologists and foresters
from world over. The current focus of allelopathic research is to understand
various ecological problems in agriculture, forestry/plantations, wastelands,
aquatic and other ecosystems. The increase in the literature and experimental
improvements in the subject, apart from the better computational facilities
has provided this science a firm footing. In spite of all this, criticism from a
section of scientists like Harper (1977); Connell (1990); Begon, Harper, and
Townsend (1995); and Watkinson (1998) on the science can not be ignored as
such. The critics base their arguments on the following:
SLack of adequate field studies; as most of the studies reported are based
on laboratory work
SDifficulties in determining critical concentrations of allelochemicals in
soil
SInappropriate bioassay models
SDifficulty in establishing the role of microbes in litter decomposition,
release of allelochemicals and subsequent transformation
SProblems in determining the mechanism of action, particularly when
the chemical nature of allelochemicals is unknown.
In view of the above apprehensions and difficulties, the proofs required for
establishing an allelopathic phenomenon, than any other aspect of ecology, is
usually put to rigorous evaluation (Williamson, 1990). Perhaps for such rea-
sons, the traditional ecologists either prefer to omit the phenomenon or just
make a brief mention of it in their writings/books. In spite of the lack of
legitimate support, the probing of the science of allelopathy on one hand and
experimental handicaps that it suffers from, on the other, it has now become
an integral component of chemical ecology (Anaya, 1999). In terms of practi-
cal significance, the allelochemicals are now considered as an alternative to
synthetic herbicides and pesticides for controlling weeds and pests in the
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Singh, Batish, and Kohli 5
agroecosystems for the protection of the quality of environment (Macias,
1995; Macias et al., 1997; Anaya, 1999; Rizvi et al., 1999). Allelochemicals
are also used to manage the biotic resources (Anaya et al., 1995). Further, in
several instances the synthetic chemicals fail to provide desired results owing
to development of resistance among the pests. Under such circumstances,
allelochemicals, being the natural plant products, provide an excellent source
of novel agrochemicals for the sustainable management of pests and weeds in
the agroecosystems (Cutler, 1988; Macias, 1995; Duke et al., 1997; Kohli,
Batish, and Singh, 1998a,b; Cutler and Cutler, 1999a,b; Rizvi et al., 1999;
Duke et al., 2000).
ALLELOCHEMICALS- DIVERSITY, NATURE,
MODE OF RELEASE AND SIGNIFICANCE
The chemicals responsible for the phenomenon of allelopathy are general-
ly referred to as Allelochemicals or Allelochemics (Whittaker and Feeny,
1971). Since the inception of the term allelopathy, these chemicals have been
given different names by the different workers. To name a few, there are
terms like Antibiotic,Koline,Marasmin and Phytoncide based on the type of
donor and the recipient (Grümmer, 1955); Saproinhibitins and Phytoinhibi-
tins based on the type of donor (Fuerst and Putnam, 1983). These being
non-descriptive and non-specific nor have received much acceptability (Put-
nam and Weston, 1986). Of late, these chemicals have also been named as
Plant Eco-Chemicals (Mizutani, 1991, 1999). These phytochemicals are syn-
thesized in plants as secondary metabolites that appear to have no direct
function in the growth and development but serve as a defensive adaptation.
The significance of their synthesis seems to be an interaction between the
plant and its environment (Harborne, 1989; Berenbaum, 1995a,b; Seigler,
1996). Plant allelochemicals/phytotoxins are generally localized and seques-
tered in certain specialized organs which may be glandular or sub-epidermal
(Duke et al., 1999). The distribution of particular secondary metabolites, is
restricted within a group of taxonomically related species. For instance, ‘sali-
cin,’ a phenol glucoside is characteristic of the members of the family Salica-
ceae while sesquiterpene lactones are found in the members of family Astera-
ceae. Contrary to the earlier belief that secondary plant products are the
metabolic wastes and thus, functionless (Ellis, 1997), their ecological signifi-
cance is now being realized. They not only dispel harmful insects, pests and
pathogens, but also increase reproductive fitness of the plants (Taiz and
Zeiger, 1998). Perhaps, their presence represents an evolutionary change
through heritable mutations and natural selection. Depending upon the nature
of metabolite, there is, however, insufficient data that selection pressure has
favored plants with allelopathic phenomenon (Newman, 1978). Allelochemi-
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ALLELOPATHY IN AGROECOSYSTEMS
6
cals are also now being considered even for their role in biological life
support system for long duration space missions (Stutte, 1999).
The allelochemicals broadly classified as plant phenolics and terpenoids,
show a great chemical diversity and are involved in a number of metabolic
and ecological processes. The mode of release from the plant could be
through leachation, volatilization, root exudation and the death and decay of
the fallen plant parts either through biotic or abiotic means (Anaya, 1999).
Upon release, these are involved in a number of metabolic and physico-
chemical processes (Rice, 1984; Einhellig, 1985a; Waller, Feng, and Fujii,
1999; Mizutani, 1999). The toxicity of released allelochemicals in the envi-
ronment is a function of concentration, flux rates, age and metabolic stage of
the plant, prevailing climate and season, and environmental conditions (Rice,
1984; Daizy, 1990; Wyman-Simpson et al., 1991; Wardle, Nicholson, and
Rahman, 1993; Weidenhamer, 1996; Gallet and Pellissier, 1997; Nilsson,
Gallet, and Wallstedt, 1998). Besides, their production not only varies in
quantity and quality throughout the year (Devi, Pellissier, and Prasad, 1997)
but is also dependent upon the age and the cultivar of the donor plant (Argan-
dona, Niemeyer, and Corcuera, 1981; Wyman-Simpson et al., 1991; Hanson
et al., 1981; Burgos, Talbert, and Mattice, 1999). A density dependent effect
of allelochemicals has also been demonstrated wherein the effect has been
shown to be more at low plant densities compared to the high densities (Thijs,
Shann, and Weidenhamer, 1994). This provides a strong evidence for the
chemical interference (Weidenhamer, 1996). Under natural conditions, the
released allelochemicals act synergistically and exert inhibitory effect as
complex mixtures and therefore the inhibitory effects are observed at con-
centrations well below their individual inhibitory levels (Blum, 1995, 1996).
ALLELOPATHY AND SOIL ENVIRONMENT
Allelopathic interactions in soil environment may be a function of interact-
ing substances that may be neutral, promoters and/or even inhibitors (Blum et
al., 1993). The importance of the allelopathic interactions in natural and
managed systems greatly depend upon turn over rate of allelochemicals and
their interaction with clay, organic matter and other competitive sinks which
change physico-chemical and biotic characteristics of soil (Blum, 1995; Blum,
Shafer, and Lehman, 1999). A number of reports suggest that various soil
factors affect allelopathic compounds and hence the phenomenon (Rice,
1984; Dalton, Blum, and Weed, 1989; Cheng, 1995; Blum, 1996). Since the
soil particles adsorb the allelochemicals, their degree of sorption is greatly
influenced by the soil texture. Generally, the loose and sandy soils exhibit
more inhibitory effect than the heavy and loamy soil, as allelochemicals fail
to be sorbed on the former (Oleszek and Jurzysta, 1987). The soil factors
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Singh, Batish, and Kohli 7
which affect the allelopathic expression of a plant are soil pH, organic car-
bon, organic matter and available nitrogen (Blum, 1996). The sorption of the
allelochemicals is greatly influenced by slightly basic pH and it increases
with organic matter and multivalent cation content (Dalton, Blum and Weed,
1983). These characteristics, in turn, influence the uptake and immobilization
of inorganic ions and thus accumulation of nutrients (Facelli and Pickett,
1991; Blum, 1996). Low organic matter in soil induces those microbes which
are able to utilize phenolics as carbon source (Blum and Shafer, 1988). The
allelopathic activities are also influenced by the soil nutrients (Koeppe,
Southwick, and Bittell, 1976; Rice, 1984). Other abiotic factors which influ-
ence the allelopathic phenomenon or the accumulation of allelochemicals are
temperature (Fischer et al., 1994), irradiance (Bhowmik and Doll, 1983),
moisture deficit (Einhellig, 1989), etc. Allelochemicals, particularly the phe-
nolic acids, interfere with the mineral uptake (Einhellig, 1995) and under
stress conditions the amount of allelochemicals is substantially increased
(Hall, Blum, and Fites, 1982) and the allelopathic effects are observed at less
than optimal concentrations of allelochemicals (Einhellig, 1996). Allelochemi-
cals also influence the availability of the nutrients to plants. For instance,
some phenolic acids are known to bind with the minerals like iron, manga-
nese and aluminum, and thus increase the availability of phosphate which
otherwise forms complex with these metal ions (Appel, 1993).
One of the important aspects of the allelochemicals in the soil environment
can be the demonstration of their active concentration. Most of the bioassays
are based on single toxin effect and do not provide adequate information on
the concentration required for plant growth inhibition, since in soil toxins
occur in complex organic mixtures. Much depends upon their differential
utilization by the soil microbes and associated rate of this uptake, metabolic
conversion and creation of anaerobic conditions in the medium (Pue et al.,
1995). Magnitude of allelopathic effect is also governed by distribution of
allelochemicals and plant roots in the rhizosphere (Lehman, Blum, and Gerig,
1994).
ALLELOPATHIC INTERACTIONS IN AGROECOSYSTEMS
An agroecosystem, a man-made distinct unit in agricultural fields, is regu-
lated by ecological principles where all the biotic and abiotic factors play an
important role. Modern agriculture is an outcome of long process of domes-
tication, improvement of new techniques with the prime objective of increas-
ing crop production and requires a high degree of inputs and modern technol-
ogy (Anaya, 1999). This has led to spectacular increase in food production
and self-sufficiency in several countries. In spite of all this progress, scien-
tists world over are worried that this increase in food production has not been
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ALLELOPATHY IN AGROECOSYSTEMS
8
able to keep the desired pace with the unabated growth in population espe-
cially in the tropics and subtropics. Moreover, agriculture is a fragile resource
and is shrinking very fast. Increasing production on the existing land, there-
fore, seems the only alternative. In order to accomplish this target, sectoral
monocultural practice is being undertaken where any other biological compo-
nent (even if it is the normal seral stage of the succession) is considered
non-crop and hence removed to minimize competition for the crop plants. To
eliminate the non-crop biological components such as weeds, insects and
pathogens, the synthetic chemicals are being used indiscriminately to the
extent of even making the agroecosystems unsustainable. Their excessive use
has not only affected the human health and impaired the quality of environ-
ment but also, in some circumstances failed to provide expected protection
due to development of resistance against synthetic chemicals. For enhancing
crop productivity, new high yielding varieties are also being selected/evolved
and so are the pests, insects and pathogens. This has become an unending
process. The modern agroecosystems with monocultures of high yielding
crop varieties are characterized by the presence of residues of synthetic
agrochemicals (herbicides/fungicides/pesticides/fertilizers), less diversity and
resistant pests which makes it ecologically unsustainable. Sustainable agro-
ecosystems on the other hand are organic, regenerative, biodynamic and
resource conserving (Anaya, 1999). Hardly any crop rotation practice, cover
cropping, companion cropping or polyculture cropping practice, as was used
to be done in our traditional systems, is followed in the present agricultural
set up. Fast deteriorating agroecosystems have become a serious global prob-
lem and to improve them a holistic approach, though slow, needs to be
followed (Allen and van Dusen, 1988).
In the modern agriculture, allelopathy has a great potential in improving
crop productivity, genetic diversity, maintenance of ecosystem stability, nu-
trient cycling and nutrient conservation, weed control, disease management
and pest control (Jordan, 1993; Einhellig, 1996; Halbrendt, 1996; Swanton
and Murphy, 1996; Weston, 1996; Kohli, Batish, and Singh, 1998a; Anaya,
1999; Chou, 1999; Caldiz and Fernandez, 1999; Cutler, 1999a,b). Before
discussing the application part, it is important to understand the allelopathic
interactions among major interacting components of agroecosystems.
Allelopathic Weeds
Out of 30,000 plant species identified as weeds, 250 are really important
and about 80 are known to reduce crop yields (Sauerborn, 1999). They are
the part of dynamic agroecosystems created by man with the aim of produc-
ing useful plants. Weeds, which have been described as unwanted plants at a
given place, are integral component of agroecosystems and have co-evolved
with the crop plants. They have a number of physiological, agronomic and
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Singh, Batish, and Kohli 9
reproductive characteristics which make them successful compared to the
other plants (Cobb, 1992). In the agroecosystems, they compete with the crop
plants for resources, interfere in crop handling, reduce crop yield and deterio-
rate their quality and thus the financial loss. They are rather more harmful
than the insects and pathogens, as they are static, co-evolved with the crop
plants and often develop resistance to the herbicides. Nearly 12% of the total
loss of crop yields has been attributed to the weeds alone (Anaya, 1999). A
number of allelopathic weeds have been reported to affect crop plants in
agroecosystem (Putnam and Weston, 1986; Kohli, Batish, and Singh, 1998a).
Some of the important weeds exhibiting allelopathy and crop losses are Agro-
pyron repens (Lynch and Penn, 1980), Ageratum conyzoides (Kohli, 1997),
Amaranthus retroflexus (Bhowmik and Doll, 1983), Avena fatua (Schumach-
er, Thill, and Lee, 1983), Brassica napus (Mason-Sedun, Jessop, and Lovett,
1986; Choesin and Boerner, 1991), Chenopodium album (Qasem and Hill,
1989), Chenopodium murale (Qasem, 1993), Cyperus rotundus (William and
Warren, 1975), Datura stramonium (Levitt, Lovett, and Garlick, 1984), Echi-
nochloa crus-galli (Bhowmik and Doll, 1979), Lantana camara (Singh,
Tamma, and Nigg, 1989; Arora and Kohli, 1993), Parthenium hysterophorus
(Kohli and Batish, 1994), and Sicyos deppei (Cruz-Ortega et al., 1998;
Anaya, 1999).
Though weeds interfere with the crop plants and cause huge economic
losses, yet they are an integral component of the agricultural system (Altieri
and Liebman, 1988; Zimdahl, 1999). The herbicidal control of weeds is not
only a costly affair but also deteriorates the quality of soil, water, other life
support systems, human health and food. Therefore, their management in an
eco-friendly and low-input manner requires alternative management practic-
es which involve both short and long-term strategies and emphasize greatly
on prevention of weed emergence so as to minimize the competition with
crops (Buhler, 1996, 1999). Some of the proposed suggestions for weed
management in agroecosystems are:
SUnderstanding the biology, ecology and life cycle pattern of weeds in
order to devise the suitable means of their control.
SUnderstanding the exact mode of their interference in the croplands,
i.e., whether density dependent or density independent, competitive or
allelopathic, or both.
SChanging cropping patterns like increased rotation, new tillage and im-
proved resource utilization (Wyse, 1992; Buhler, 1999).
SFinding alternate control methods like using competitive cultivars or al-
lelopathic crop plants or allelochemical/natural plant products for weed
management (Buhler, 1999).
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ALLELOPATHY IN AGROECOSYSTEMS
10
Allelopathic Crops
A number of crops have also been known to exhibit allelopathic property
on other crops growing in succession or simultaneously or may even exhibit
autotoxicity (Einhellig, 1985a; Putnam and Weston, 1986; Chou, 1999; Anaya,
1999). Among these, especially the cover crops may be exploited for the
purpose of weed management (Teasdale, 1996; Weston, 1996; Foley, 1999).
Crop autotoxicity provides an interesting dimension to allelopathy in agro-
ecosystems where a crop is inhibitory to the self, particularly in the succes-
sive years (Einhellig, 1985a; Singh, Batish, and Kohli, 1999a; Reigosa,
Sánchez-Moreiras, and González, 1999). For the common man and farmers,
the commonly encountered problem of Soil Sickness or Soil Fatigue is attrib-
uted to crop autotoxicity as it is not even rectified by the addition of fertilizers
(Yu, 1999a). The principal causes of crop autotoxicity include the deliberate
leaving of crop residues or old roots in soil that release phytotoxins which
may directly affect the succeeding crops, cause microbial imbalance, changes
in organic matter of soil, increase ion leakage, disturb nutrient uptake and
immobilization (Waksman, 1937; Katznelson, 1972; Kimber, 1973; Yu and
Matsui, 1997). Some of the highly worked out important crops exhibiting
crop autotoxicity include rice (Chou, 1995), wheat (Kimber, 1973), maize
(Yakle and Cruse, 1983, 1984), sugarcane (Chou, 1995), alfalfa (Miller,
1983, 1996), sunflower (Kohli, 1993) and a few others (Rice, 1995; Singh,
Batish, and Kohli, 1999a). Several vegetable crops like cucumber, carrot,
fennel, watermelon, eggplant, tomato and even pea are known to exhibit
autotoxicity (Singh, Batish, and Kohli, 1999a; Yu, 1999a). The problem of
crop autotoxicity is particularly acute in croplands where tillage is not prac-
ticed.
There are a number of instances where crops are known to affect even the
other crops growing in vicinity leading to serious implications to agricultural
practices like stubble mulch farming or reduced or no-tilled farming where
crop residues are left on the soil surface to avoid soil erosion, preserve
moisture, increase nutrient balance and a number of other benefits (Einhellig,
1985a; Rice, 1984, 1995; Chou, 1995, 1999). The residues of preceding crops
affect the performance of other crops through the release of allelochemicals
(Kimber, 1973; Guenzi and McCalla, 1966; Lodhi, Bilal, and Malik, 1987;
Thorne et al., 1990; Kohli, 1993). Kalburtji and Gagianas (1997) reported the
poor performance of cotton following the sugar beet crop due to allelochemi-
cals released from the residues of sugar beet. However, a number of water-
soluble phytotoxins/allelochemicals released by the residues accumulate in
the soil and the crop roots may have a chance encounter with these chemicals
leading to serious repercussions on the quality and quantity of crop yields
(Einhellig, 1985a). Due to this reason a number of traditional cropping prac-
tices like cover cropping, companion cropping, polyculture and green manur-
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Singh, Batish, and Kohli 11
ing, etc., need to be thoroughly revised. Some studies have shown that crop
residues, if properly managed, may even lead to enhancement in yield (Chou,
1999). Different approaches that need to be looked into are:
SUnderstanding the mechanism of crop-weed interference in agroeco-
systems.
SRevival of traditional practices that are organic, biodynamic and sus-
tainable (Anaya, 1999).
SShift in cropping pattern from mono- to poly-cultures and effectively
reviving the old practices of cover cropping, companion cropping and
crop rotation with new and scientific approach.
SImproving crops through genetic manipulations using conventional
breeding techniques and molecular genetic recombinant technology so
that they have ability to protect themselves from the pests (Foley,
1999).
Allelopathic Agroforestry Trees
Trees are fast becoming an integral part of the agriculture under various
intensive and extensive agroforestry programmes. Several studies show that
this practice increases productivity, improves soil quality, microclimate and
nutrient cycling, conserves soil, manages weeds and increases overall sus-
tainability, even though a number of negative interactions (including allelo-
pathic) have also been recognized (Ong, 1991; Burch and Parker, 1992;
Harris and Natrajan, 1987; Rao, Nair, and Ong, 1997). A number of tree
species like Acacia spp., Albizzia lebbeck,Eucalyptus spp., Grewia optiva,
Glircidia sepium,Leucaena leucocephala,Moringa oleifera,Populus del-
toides, are known to affect the performance of crops through the phenome-
non of allelopathy (Rice, 1984; Kohli, 1990; Malik and Sharma, 1990; Kohli,
Singh, and Verma, 1990; Ralhan, Singh, and Dhanda, 1992; Singh and Kohli,
1992; Rice, 1995; Singh, Batish, and Kohli, 1999b; Singh, Kohli, and Batish,
1998, 1999; Rizvi et al., 1999). In most of the instances, the litter from the
tree interferes with the growth and establishment of the adjoining crop plants.
Unfortunately, very little has been done to understand this phenomenon in
agroforestry systems, though it is an important factor in determining the
success of trees (Kuo, Chou, and Hu, 1983; Kohli, Batish, and Singh, 1998a;
Rizvi et al., 1999). Therefore, understanding allelopathic mechanism opera-
tive in agroforestry systems can enhance the crop productivity through con-
trol of weeds, nematodes, pathogens and insects. Allelochemicals from agro-
forestry trees can be used for controlling the weeds (Kohli, Batish, and Singh,
1998b). Agroforestry can, therefore, be manipulated to make agroecosystems
sustainable through proper management and/or mulching of the litter of the
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ALLELOPATHY IN AGROECOSYSTEMS
12
trees growing in agroecosystems to improve the soil quality, conserve mois-
ture and bring about the cooling effect.
Role of Microbes
Microbes have a profound effect on the allelopathic activity by altering
and/or transforming the amount of allelochemicals, particularly the phenolic
acids, in the soil depending upon the available carbon source and other
environmental factors (Pellissier, 1998; Blum, Shafer, and Lehman, 1999;
Pellissier and Souto, 1999). The microbes may metabolize the released phe-
nolic acids by addition or deletion of side groups, polymerization, production
of other organic molecules and/or incorporation of carbon from other phenol-
ic acids into microbial biomass (Martin and Haider, 1976; Haider, Martin,
and Rietz, 1977; Blum, 1998; Blum, Shafer, and Lehman, 1999). The trans-
formed or newly synthesized phenolics may differ in their phytotoxicity from
the original ones that entered the soil (Blum, 1998). Further, in the soil the
preferential utilization of carbon sources may also affect the plant-microbe-
soil system and the allelopathic phenomenon (Pue et al., 1995). Some of the
bacteria like Streptomyces sagononensis,S.hygroscopicus,Pseudomonas fluo-
rescencs and many others are allelopathic and inhibit the growth of the
nearby plant (Suslow and Schroth, 1982; Hoagland, 1990; Alstrom, 1991;
Heisey and Putnam, 1986; Barazani and Friedman, 1999). The allelochemi-
cals from microorganisms are generally non-specific and inhibit the growth
of several annual and perennial species (Cutler, 1988; Hoagland, 1990). They
may be effective at very low concentration, i.e., cycloheximide at 1 Ng/l
(Heisey et al., 1988) and have variable effect on the different cultivars (Al-
strom, 1991).
On the other hand, allelochemicals may influence the growth of microbes
positively or negatively and, thereby indirectly interfere with the availability
of nutrients, particularly nitrogen and phosphorus, in the soil (Pellissier,
1993; Wardle and Nilsson, 1997; Anaya, 1999). Crop residues of several
crops like corn, barley, and potato in the continuous single crop cultivation
support several non-pathogenic microbial population which may considerably
harm the growth of plants (Bakker and Schippers, 1987; Schippers, Bakker,
and Bakker, 1987; Fredrickson, Elliot, Engibous, 1987; Turco et al., 1990;
Alstrom, 1992). Phenolic compounds released in soil from decomposing
residues may cause microbial imbalance (Chou, 1995). However, flooding
may eliminate phytotoxins leading to improved microbial balance and restor-
ing the yield (Wang, Kao, and Li, 1984).
In orchards microbes play an important role in replant problem besides the
autotoxicity (Singh, Kohli, and Batish, 1999a). Presence of Penicillium ex-
pansum in apple orchards facilitates the release of allelochemicals (Börner,
1963; Berestetsky, 1972). Vesicular Arbuscular Mycorhizae (VAM) also
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Singh, Batish, and Kohli 13
helps in changing rhizosphere microflora and help in increasing biomass of
apple seedling (Catska, 1994). In case of peach orchards, nematodes are
known to play an important role in releasing and hydrolyzing amygdalin--a
cynogenic compound causing autotoxicity and replant problem. In case of
Asparagus it has been shown that allelochemicals synergise with fungal
pathogens thereby markedly increasing the disease incidence (Peirce and
Colby, 1987).
ROLE OF ALLELOPATHY IN SUSTAINABLE AGRICULTURE
The modern agriculture is commercial and target oriented and hence gov-
erned by the market laws and not the sustainable ecological principles. Thus,
in order to maintain the sustainability of the agroecosystems it is essential to
determine the factor(s) disturbing it and adopt appropriate conservation strat-
egies. Integrated biological weed control in croplands follow two approaches--
the technological and ecological (Müller-Schärer, Scheepens, and Greaves,
2000). There is, therefore, an urgent need to explore the eco-friendly means
of alternative weed management for profitability and environmental sustain-
ability. There is also need to revive our traditional system practices of poly-
culture, crop rotation, cover crops and use of green manure. For the manage-
ment of the weeds, allelopathy particularly that of crops, can play an
important role, if suitably understood, managed and applied (Weston, 1996;
Anaya, 1999; Chou, 1999; Wu et al., 1999).
Use of Allelopathic Cover/Smother/Rotational/Companion Crops
Use of cover crops and smother crops has been an age-old practice. Cover
crops are usually grown during the fallow period to reduce soil erosion,
conserve moisture, improve nutrient status and manage weeds besides pro-
viding biomass (Gallandt, Liebman, and Huggins, 1999; Foley, 1999). Like-
wise, smother crops are grown in rotation or as catch crop and shade out the
weeds due to their quick growth and thick stands. Some of the noteworthy
cover, smother or companion or rotational crops are buckwheat (Fagopyrum
esculentum), foxtail millet (Setaria italica), rye (Secale cereale)andSor-
ghum spp., alfalfa (Medicago sativa), sunflower and cruciferous plants (Put-
nam, DeFrank, and Barnes, 1983; Leather, 1983, 1987; Oleszek, 1987;
Schreiber, 1992; Kohli, 1993; Foley, 1999; Narwal, 1999b; Vaughn, 1999;
Weston, Nimbal, and Jeandet, 1999). It has been seen that allelopathic cover
and smother crops can considerably reduce the weed species in the croplands
if suitably manipulated and may provide a non-herbicidal method of weed
control (Robbins, Crafts, and Raynor, 1942; Altieri and Doll, 1978; Barnes
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ALLELOPATHY IN AGROECOSYSTEMS
14
and Putnam, 1983, Liebel et al., 1992; Swanton and Murphy, 1996; Weston,
1996; Anaya, 1999; Chou, 1999). Mucuna pruriens var. utilis, commonly
known as velvetbean and used as green manure to smother weeds, has a
potential to suppress the weeds through the release of chemical substance
L-3,4-dihydroxy phenylalanine (L--DOPA) present in this legume (Fujii, Shi-
buya, and Yasuda, 1991; Fujii, 1999). They not only help in enhancing the
biomass but also possess the weed suppressing abilities. Genetic approaches
should, therefore, be explored for crop improvement and development of
molecular markers to aid in breeding cover crops so as to bring about weed
suppression (Foley, 1999). Putnam, DeFrank, and Barnes (1983) reported the
use of mulches and cover crops in order to achieve weed management natu-
rally. The residues of cover crops like rye, wheat, barley killed by contact
herbicides and desiccants reduced the density and biomass of broad-leaved
weeds (Barnes and Putnam, 1983). Later, from the rye and wheat mulch
phytotoxins like C-phenyl lactic acid and C-hydroxybutyric acid were identi-
fied (Shilling, Liebl, and Worsham, 1985). Rye residues release 2,4-dihy-
droxy-1,4 (2H)-benzoxasin-3-one (DIBOA) which along with its break down
product 2(3H)-benzoxazolinone (BOA) suppresses the growth of weeds
(Barnes and Putnam, 1987). Wheat straw has been found to be an excellent
mulch crop due to the presence of triterpenoids and other phenolic acids in
no-till farming (Gaspar, Neves, and Pereira, 1999).
Screening of Crops and Transfer of Allelopathic Traits
Using Modern Techniques
Several wild accessions of modern day crops plants are found to possess
allelopathic traits that impart them resistance against weeds and pests (Hoult
and Lovett, 1993). During the process of cultivation and selection of high
yielding varieties these traits were never preferred and hence gradually got
eliminated from the modern day plants. Since accessions are of different
origin and stage of improvement, this combined with other factors show that
allelopathic potential is a polygenic character and weakly correlated with the
yield and other characters (Olofsdotter, Navarez, and Moody, 1995). Howev-
er, they can be incorporated in the present day cultivars through several
modern techniques as a plant protection strategy for major food crops. Even
the different cultivars of the present day crop plants differ in their allelopathic
property and superior genotypes with allelopathic potential can be selected
for Integrated Weed Management (IWM) programmes (Wu et al., 1999).
Some of the crops which have been screened for allelopathic traits in the wild
accessions are Avena sp. (Fay and Duke, 1977), Cucumis sativa (Putnam and
Duke, 1978), Glycine max (Massantini, Coporali, and Zellini, 1977), Brassi-
ca spp. (Sarmah, Narwal, and Yadav, 1992; Narwal, 1999b) and Oryza sativa
(Olofsdotter, Navarez and Moody, 1995; Dilday et al., 1998; Olofsdotter et
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Singh, Batish, and Kohli 15
al., 1999). Dilday et al. (1998) used 16,000 rice accessions collected from
different countries out of which many exhibited allelopathic suppression of
ducksalad (Heteranthera limosa) and redstem (Ammenia coccinea). Even an
hybrid between allelopathic and non-allelopathic accessions through breed-
ing techniques revealed fewer weed species in the rice fields and the workers
found that allelopathic activity in rice is inherited quantitatively (Dilday et
al., 1998). There is thus a great potential for breeding of allelopathic high
yielding varieties (Olofsdotter, 1998; Olofsdotter et al., 1999).
These allelopathic crops have also been studied at the genetic and molecu-
lar levels. The genes responsible for synthesis of allelopathic chemical in
these plants can be transferred through DNA Recombinant Technology or/
and even through conventional breeding methods. The selected phenotypes
can be analyzed by genetic and molecular genetic techniques like progeny
analysis, Polymer Chain Reaction (PCR), Random Amplified Polymorphic
DNA (RADP), Restriction Fragment Length Polymorphism (RFLP), and
Near Isogenic Lines (NIL) (Williams et al., 1990; McCloskey and Holt,
1990), cloning genes (Lovett, 1994; Jansen, 1996) and also by preparing
genetic maps of the higher plants. In case of maize, 5 genes have been
identified for the biosynthesis of DIBOA (Frey et al., 1997). The genes
responsible for biosynthesis of protective chemicals thus are important tools
in agriculture. In an attempt to improve the crop yield, these got eliminated
from the crop plants during the centuries long domestication process, as the
phytochemicals for which they encoded were not palatable.
Allelochemicals as Novel Agrochemicals
The overuse of the synthetic chemicals for the control of pests is posing a
serious threat to the environment (Chou, 1995; Cutler and Cutler, 1999b;
Kohli et al., 1999). It is strongly realized that the natural plant products, being
biodegradable are eco-friendly and can be relied upon more to enhance the
crop productivity in a sustainable way. Allelochemicals--the natural plant
products from the higher plants and microbes can, therefore, prove to be ideal
agrochemicals (Cutler, 1988, 1999a; Putnam, 1988; Rice, 1995; Dayan et al.,
1999; Duke, Dayan, and Rimando, 1998; Duke et al., 1996a,b, 1997, 2000;
Hoagland, 1990, 1999; Kohli et al., 1999; Macias et al., 1997, 1999b). Three
categories of the agrochemicals and pharmaceuticals such as benzodiaze-
pines, phenoxy compounds and organo-phosphates with potential use as
fungicides, insecticides or growth regulators have been identified (Cutler and
Cutler, 1999b). They are normally broad-spectrum molecules that are bio-ef-
ficaceous, economical and environmentally safe. Besides, they have greater
shelf life, require less space, have wide range of storage condition and ease of
application. Even the changes in the structure of the natural plant product can
improve the bio-efficacy for the desired properties. In case of parthenin--a
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ALLELOPATHY IN AGROECOSYSTEMS
16
sesquiterpene lactone and a known allelochemical of Parthenium hystero-
phorus the changes in the structure made by various photo-chemical reac-
tions considerably altered the phytotoxicity of the chemical (Saxena et al.,
1991) and the analogues were found to have better growth regulatory effect
compared to parthenin itself (Batish et al., 1997).
Some of the allelochemicals from higher plants with known weed sup-
pressing ability are given below in the Table 1.
TABLE 1. Allelochemicals from higher plants with weed suppressing ability.
Allelochemical Plant Source Chemical Nature Reference
Ailanthone Ailanthus altissisima Quassinoid Heisey, 1996
Artemisinin Artemisia annua Sesquiterpene lactone Duke et al., 1987
Azadirachtin Azadirachta indica Sesquiterpene lactone Koul, Isman, and
Ketkar, 1990
Allyl- and Benzyl- Brassica spp. -- Wolf, Spencer,
Isothiocyanate and Kwolek,
(AITC, BITC) 1984; Boydston
and Hang, 1995;
Krishnan,
Holshouser, and
Nissen, 1998.
Caffeine Coffea arabica Alkaloid Rizvi, Mukerji,
and Mathur, 1980
1,4- and 1,8- Eucalyptus spp., Salvia Monoterpenes Kohli, Batish, and
Cineole spp., Singh, 1998b;
Romagni, Allen,
and Dayan, 2000
Cnicin Centaurea maculosa Sesquiterpene lactone Kelsey and
Locken, 1987
DIBOA and BOA Secale cereale Benzoxazinones Barnes and
Putnam, 1987
Juglone Juglans nigra Quinone Rietveld, 1983
Leptospermone Callistemon spp. Triketone Lee et al., 1997
L-DOPA Mucuna pruriens -- Fujii, 1999
Mimosine Leucaena leucocephala Non-protein amino acid Rizvi, Sinha, and
Rizvi, 1990
Parthenin Parthenium Sesquiterpene lactone Pandey, 1996;
hysterophorus Batish et al., 1997
Saponins Medicago sativa Saponins Waller, Jurzysta,
and Thorne, 1993
Sorgoleone Sorghum bicolor Quinoline Einhellig and
Souza, 1992
B-Terthienyl (B-T) Members of Asteraceae -- Lambert et al.,
1991
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Singh, Batish, and Kohli 17
Some of the natural compounds like cineole, benzoxazinones, quinolinic
acid and leptospermones from higher plants have been marketed under differ-
ent trade names by companies based in Germany, USA and Japan, e.g.,
Benzoxazinones and Quinolinic acid by BASF, Germany; Cineole as Cinme-
thylene by Shell (USA); Leptospermones as Triketones by Zeneca (Dayan et
al., 1999; Kohli, Batish, and Singh, 1998a). Though cinmethylene has at-
tracted the attention of scientists worldwide, yet there has been a little success
with this herbicide (Grayson et al., 1987).
Microbes (fungi, bacteria and actinomycetes) too provide an excellent
source of natural herbicide products. Some of the potent phytotoxins from
microbes with potential herbicidal properties are Anisomycin from Strepto-
myces spp., AAL-toxin from Alternaria alternata, Bialaphos from Strepto-
myces viridochromogenes and S. hygroscopicus, Coformycin from Actinomy-
cete, Cornexistin from Paecilomyces variotii, Cyperine from Ascochyta
cypericola and Phoma sorghina, (+)-epiepoformin from Scropulariopsis brump-
tii,()-dehydropyrenophorin from Drechslera avenae, gougerotin from
Streptomyces spp., Herbicidins A, B, E, F, G from S. saganonensis,Herbimy-
cins from S. hygroscopicus, Hydantocidin from Streptomyces hygroscopicus,
Phosphinothricin from S. hygroscopicus, Pironetin from Streptomyces spp.,
Rhizobitoxine from Bradyrhizobium japonicum and Pseudomonas andropo-
gonis, Tentoxin from Alternaria alternata f. sp. tenuis and other Alternaria
spp., Ustloxins A, B, C, D from Ustilaginoidea virens, and vulgamycin from
Streptomyces hygroscopicus (Owens et al., 1972; Sugawara and Strobel,
1986; Huang et al., 1989; Mitchell, 1989; Babczinski et al., 1991; Duke et al.,
1996a,b; Duke, Dayan, and Rimando, 1998; Cutler, 1999a; Hoagland, 1999).
Although hundreds of such compounds have been patented, yet only two, viz.,
bialaphos and phosphinothricin have been successfully commercialized (Duke
et al., 1996b; Dayan et al., 1999). Both have antibiotic and herbicidal prop-
erties and even the transgenic plants resistant to these chemicals have been
prepared (Hoagland, 1999). A metabolite, 6-pentyl-2H-pyrane-2-one isolated
from Trichoderma species, is reported to have antifungal activity and based
on this a natural and soft fungicide has been developed (Parker, Hill, and
Cutler, 1999). Recently, a compound, trans-4-aminoproline identified from a
fungus Ascochyta caulina, has been tested to be a promising mycoherbicide
(Evidente et al., 2000).
Though the commercialized products of natural products represent a small
fraction, yet they offer novel herbicide target sites (Duke et al., 1997, 2000).
The use of such products can restore the beneficial balance of natural envi-
ronment that is often lost in the agroecosystems (Hill et al., 1999). Some of
the marketing strategies for the natural plant products need careful revision
(Devine, Duke, and Fedtke, 1993). Further, attention to the following may
also prove to be useful for their future exploitation in agroecosystems.
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ALLELOPATHY IN AGROECOSYSTEMS
18
SCarefully monitored patent search.
SProtocols on synthesizing compounds for large-scale production should
be developed.
SUse of modern genetic and molecular techniques such as cell culture,
genetic engineering and biotechnology should be considered.
SStructural modifications of the compound that can improve bio-efficacy
and other properties should be considered.
SPlant derived chemicals could be used as template for commercial pes-
ticides.
SCompounds must be thoroughly tested for toxicological and environ-
mental properties.
Although natural products are normally considered ideal compounds, yet
they may have some limitations too (Duke, Dayan, and Rimando, 1998;
Dayan et al., 1999). These are:
STheir structures are normally too complex and therefore, the process of
simplification may lead to loss of activity.
SIn some cases, they too have short environment life which fails to pro-
vide the desired effects, because for a compound to act some persis-
tence is required.
SThe purification process of natural plant products is slow and cumber-
some.
SThe molecular sites of action of these compounds may not be known.
SThe allelochemicals may not be always safe. Sometimes they may
cause mammalian toxicity.
Allelopathy for Pathogen and Disease Management
World wide plant diseases are estimated to cause about 20% yield reduc-
tions of major food and cash crops (Oerke et al., 1994). The problems of
pollution and hazardous effects of synthetic chemicals on the non-target
plants have led scientists to find alternate methods such as use of natural plant
products and their analogues (Cutler, 1988; Al-Abed, Qasem and Abu-Blan,
1993; Qasem and Abu-Blan, 1995, 1996; Amadioha, 1998; Cutler, 1999b;
Cutler and Cutler, 1999a). The phenomenon of allelopathy which involves
natural plant products offers one of the best biological means of disease
control. Further, the disease resistance and defense mechanism is quicker in
plants than the inter-specific interactions that take a lot of time. Natural
products from the higher plants and microorganisms are an excellent source
of antifungal agents. A number of studies have reported the use of pure or
crude forms of allelochemicals, besides the role played by allelopathic rota-
tional and companion crops for the control of plant diseases (Rice, 1995;
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Singh, Batish, and Kohli 19
Rizvi et al., 1999). Allelopathic crops can also greatly reduce the incidence of
soil-borne diseases through crop residues releasing inhibitory substances (Yu,
1999b). Some allelochemicals also play an important role in plant defense by
acting as signal transducers (Gaspar, Neves, and Pereira, 1999).
DIMBOA and DIBOA commonly found in the cereals play an important
role against the harmful predators such as insects, fungi and even bacteria
(Kutchan, 1997). The allelochemicals from the weeds also inhibit the growth
and development of many plant pathogenic fungi and bacterial species (Qa-
sem, 1996 a,b). Under some circumstances, even enhancement of pathogen
susceptibility by allelochemicals has also been reported (Lynch and Penn,
1980). Several species of Brassica and even mustard oil are reported to
inhibit the growth and development of a large number of soil-borne phytopa-
thogenic fungi, and this affect is attributed to the presence of glucosinolates
which degrade enzymatically to isothiocyanates (Wilcoxon and McCallan,
1935; Walker, Morell, and Foster, 1937; Pryor, Walker, and Stahmann, 1940;
Papavizas and Lewis, 1971; Angus et al., 1994; Kirkegaard, Wong, and
Desmarchelier, 1996; Olivier et al., 1999; Vaughn, 1999). Saponins from
some higher plants like roots of alfalfa (Medicago sativa), unripe berries of
Phytolacca dodecandra, fruits of Swartzia medagascariensis have fungicidal
properties against a number of phytopathogenic fungi (Oleszek, 1999; Oles-
zek, Hoagland, and Zablotowicz, 1999; Waller, 1999). A large number of
flavonoids have been reported to have fungicidal property against a number
of seed borne fungi (Weidenborner et al., 1990, 1992; Weidenborner and Jha,
1993; Dixon, 1999).
Some of the higher plants with the potential to suppress plant pathogens
are listed in Table 2.
A number of allelopathic compounds from microbes like Polyangium
spp., Paecilomyces variotii,Streptomyces albospinus and Fusicoccum spp.
have a fungicidal potential against a number of phytopathogens (Cutler,
1999a). Several bacterial secondary metabolites like Phenazines and Phloro-
glucinols active in suppression of root diseases are known. These can be
transferred to plants through DNA recombinant technology to produce trans-
genic plants which will have natural protection against pathogens and this
method is environmentally safe (Fujimoto, Weller, and Thomashow, 1995;
Weller and Thomashow, 1999). Neem (Azadirachta indica), an Indian tree,
possesses a number of active principles like azadirachtin with potential to kill
pathogens (Ghewande, 1989). Its seed-cake, seed and fruit extract, seed ker-
nel powder and seed oil have been reported to control a wide spectrum of
fungal pathogens (Gunasekaran et al., 1986; Jeyarajan et al., 1987; Srivastava
et al., 1997). The biological activity of neem against pathogens is attributed
to the presence of sulphurous compounds in its seed oil. Moreover, neem
products also act as deterrent to pathogen carrying insects, thereby decreas-
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ALLELOPATHY IN AGROECOSYSTEMS
20
TABLE 2. Allelochemicals from higher plants with ability to suppress plant
pathogens.
Source Compound Plant Pathogens Reference
Allium tuberosum -- Pseudomonas solanacearum Yu, 1999
A. sativum Ajoene Phytophthora drechsleri f. Singh et al., 1992
sp. cajana
Artemisia borealis Polyacetylenes Cladosporium cucumerinum Wang et al., 1990
A. nelagrica Essential oils Pythium aphanidermatum Kishore and
Dwivedi, 1991
Brassica nigra Allyisothiocyanate Helminthosporium solani Olivier et al., 1999
Verticillium dahliae
Callistemon Essential oils Pythium aphanidermatum Kishore and
lanceolatus Dwivedi, 1991
Coffea arabica 1,3,7- Drechslera maydis Rizvi et al., 1980
trimethylxanthine
Eucalyptus spp. Volatile oils Sclerotium rolfsii Singh and
Dwivedi, 1990
E. rostrata Essential oil, Sclerotium cepivorum Salama et al.,
leaf powder 1988
Ferula assafoetida Asafoetida Rhizoctonia solani, Chaurasia and
Sclerotinia sclerotiorum Ram, 1992
Juniperus Essential oil Pythium aphanidermatum Kishore and
communis Dwivedi, 1991
Medicago sativa Medicagenic acid Trichoderma viride Oleszek et al.,
3-O-glucoside 1990
(Root saponins)
Nicotiana tabacum B-andC-duvatriene Peronospora tabacina Menetrez et al.,
diols 1990
Phellodendron Berberine Botrytis cinerea,Alternaria Park and Choi,
amurense alternata and some bacteria 1999
Pinus spp. Essential oils Pythium aphanidermatum Kishore and
Dwivedi, 1991
P. taeda Essential oils Pythium aphanidermatum Kishore and
Dwivedi, 1991
Ranunculus Aqueous extracts Fusarium oxysporium f. sp. Qasem, 1999
arvensis lycopersici,
Helminthosporium sativa
Salvia splendens Root extracts Bacteria Qureshi, Ahmed,
and Kapadia,1989
Tagetes erecta Essential oils Pythium aphanidermatum Kishore and
Dwivedi, 1991
Zingiber officinale Gingerenone A Pyricularia oryzae Endo, Kanno, and
Oshima, 1990
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Singh, Batish, and Kohli 21
ing the disease incidence (Saxena, Khan, and Bajet, 1985). Greenhouse stud-
ies have shown that water and ethanol extracts from the leaves and oil ex-
tracts from the seeds of Azadirachta indica significantly reduced the radial
growth of Pyricularia oryzae causing blast disease of rice under in vitro
conditions. This effect was comparable to commercial fungicide cabendazim
at the concentration of 0.1% used in vivo (Amadioha, 2000).
Though a large number of plants and microorganisms with antifungal
activity have been tested, yet only a few fungicides based on their chemistry
have been marketed. Two such compounds are Pyrrolnitrin from Pseudomo-
nas pyrrocinia and Strobilurin A from Strobilurus tenacellus (Gullino, Ler-
oux, and Smith, 2000).
Allelopathy for Insect and Nematode Control
Allelopathy can also be exploited for the control of harmful insects
(Anaya, 1999). Being non-motile, plants adopt either structural changes or
biochemical strategies and the presence of large amount of secondary prod-
ucts that deter insects. These may be flavonoids, saponins, terpenoids, alka-
loids, phenolics and limonoids. Over 2000 plant species have been reported
to possess insecticidal activity (Devakumar and Parmar, 1993). However,
only a few have been commercialized as yet. In addition to such chemicals, a
number of other synthetic insecticides based on their chemistry have been
synthesized (Khambay et al., 1997a, b). Allelochemicals deter insects by
acting as metabolic poisons or as feeding deterrents (Brattsten, 1986). A
number of flavonols, flavones and their derivatives are reported to deter a
wide spectrum of insects (Berhow and Vaughn, 1999). 2-phenylethylisothio-
cyanate (PEITC) isolated from turnip roots has been reported to be a natural
insecticide (Lichtenstein, Morgan, and Mueller, 1964). Rice extracts have
been reported to be active against brown planthopper Nilaparvata lugens
(Zhang et al., 1999). Saponins, one of the important allelochemicals, have not
shown much promising results regarding their insecticidal activity. Though
they have been tested against a number of alfalfa pests, but they lack species
specificity (Oleszek, 1999; Oleszek, Hoagland, and Zablotowicz, 1999).
Azadirachtin, a tetra triterpenoid from neem (Azadirachta indica), is known
to be effective against nearly 300 insect pests (Devakumar and Parmar,
1993). It acts as a phage and oviposition deterrent, repellent, anti-feedant,
growth retardant, etc. (Schumutterer, 1995). Remarkably, it is relatively non-
toxic to warm-blooded animals including human beings.
Nematodes possess thick wall that is resistant to chemicals and attack all
kinds of plants either as endo-parasites or as ecto-parasites on the roots.
These are even more harmful when they form associations with the patho-
gens. To control them is really a tedious job. Though, they have a number of
natural enemies, yet effective control measures are required to check their
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ALLELOPATHY IN AGROECOSYSTEMS
22
growth. Allelochemicals from the plants have the potential to control them.
Some of the important ones include Azadirachta indica (Alam and Khan,
1975), Coffea species (Tronocorn et al., 1986), Leucaena leucocephala (Jain
and Hassan, 1985) and Mucuna pruriens (Reddy et al., 1986). Green manure
of rapeseed (Brassica napus), extracts from black mustard (Brassica nigra)
and glucosinolates (mainly AITC) have been reported to be effective against
a number of nematodes like Meloidogyne chitwoodi,Caenorhabditis elegans,
Heterodera schachtii,H. rostochiensis (Mojtahedi et al., 1991, 1993; Lazzeri,
Tacconi, and Palmieri, 1993; Vaughan, 1999).
CONCLUSION AND FUTURE DIRECTIONS
The science of allelopathy exhibit complexities of biochemical interac-
tions among plants including microbes plays an important role in agroecosys-
tems where more often a decline in crop productivity, problem of soil sickness
and depletion of biodiversity are noticed. The allelochemicals responsible for
bringing about biochemical interactions are products of secondary metabo-
lism and exhibit diversity in chemical nature and functions as a group. The
phenomenon of allelopathy in agroecosystems can be exploited or manipu-
lated for enhancing crop production, if its mechanism is well understood.
Some of the major approaches by which the allelopathy in agroecosystems
can be utilized are:
SRevival of traditional cropping pattern with new and scientific ap-
proaches, viz., use of allelopathic cover crops, smother crops, compan-
ion crops and crop rotation.
SChanging the cropping sequence and cultural practices such as crops
with divergent life cycle, and selective allelopathic potential in rotation-
al sequence to check the build-up of weeds in the fields.
SSelecting crop cultivars with greater weed-suppressing/allelopathic ac-
tivity to inhibit the pests--weeds, pathogens, diseases, and nematodes.
SIdentification and transfer of allelopathic genes from wild relatives to
modern day crop cultivars using traditional plant breeding practice and
modern recombinant DNA technology.
SEmploying metabolic engineering strategies for quantitative and quali-
tative alteration of plants ability to produce bioactive natural products/
allelochemicals.
The allelochemicals from crops as well as from other plants can be utilized
for the purpose of managing weeds, pathogens, diseases, insect and nema-
todes under Integrated Pest and Weed Management Programmes (IPM, IWM)
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Singh, Batish, and Kohli 23
in sustainable agriculture. Such natural products/chemicals have short envi-
ronmental life, low environmental impact and mammalian toxicity, and are
more site specific and target oriented. Screening and testing of such alle-
lochemicals/phytochemicals is, therefore, a useful area of research. Even the
chemical compounds can be synthesized based on their novel chemistry.
Biotechnology can play an important role in the use of allelochemicals and
the phenomenon of allelopathy for sustainable enhancement of crop produc-
tion. For this, several genetical and molecular-genetical approaches like
RAPD, RFLP, etc., should therefore be followed to improve the allelopathic
crop plants so as to get the desired results of managing weeds and pests as
well as enhancing crop yields. Very little molecular and genetic evidences
regarding the evolutionary origin of metabolic pathways leading to the syn-
thesis of secondary metabolites are available. Proliferation of cloned genes
for natural product biosynthetic enzymes and systematic sequencing of Ara-
bidopsis genome may, however, provide an insight into several aspects of
secondary metabolism (Facchini, 1999).
Microorganisms which are efficient competitors and inhibitors of soil
pathogens should be promoted rather than killing them. Induced resistance is
an important tool in protecting crops from disease. Attempts should made to
induce natural resistance in crop plants by using agrochemicals and microbes
antagonistic to plant pathogens. Efforts should be made to explore the alle-
lochemicals whose mode of action involves the regulation of physiological
and biochemical aspects of host-pathogen interactions, and not killing the
pathogen (Hoagland, 1999).
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