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Chapter 3. Allelopathy: Principles, Procedures, Processes, and Promises for Biological Control

Chapter 3. Allelopathy: Principles, Procedures, Processes, and Promises for Biological Control

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M. Additional Comments

A. Additive Activities of Allelopathic Chemicals

B. Autotoxicity

C . Practical Considerations

D. Rhizosphere Ecology

E. Multifaceted Approach

E Statistical Analysis

X. Concluding Remarks


Allelopathy can be defined as chemical interactions between and among both plants

and microorganismsvia releases of biologically active chemical compounds into the environment. During the past three decades this scientific field has received growing

attention from soil scientists, microbiologists, ecologists, plant physiologists, biochemists, botanists, weed scientists, agronomists, and natural product chemists. Although a few studies are. acknowledged to have demonstrated probable allelopathy in

nature, many appeared limited to in vifro circumstances.This difficultyreflects the complexity of allelopathic interactions.Allelopathic effects are often modified by additional biotic and abiotic stress factors, uncertain meteorological events, or physical, chemical, and biological soil factors. all of which can influence the residence time,

persistence,concentration, and fate of allelopathic compounds in the environment. Special emphasis is given to an overview of the allelopathic activities of weed and crop

species, especially via crop residues in the agroecosystem. and to consideration of the

biotic and abiotic factors which influence the expression of allelopathy.A brief discussion of secondary metabolites with allelopathic activities and of the mechanisms of action of allelopathic compounds is also provided. The allelopathic potential of certain

weed and crop species can influence the growth and distribution of associated weed

species and the yield of desired plants, and allelopathy has been employed successfully in biocontrol programs focusing on control of problematic weeds and plant diseases.

Thus, it plays an important role in an agroecosystem and it is clear that a better understanding of allelopathy can help both in crop improvement and in developing more sustainable agriculture.

0 1999 Academic Press


The term allelopathy (allelon, to each other; pathos, to suffer) was coined by

German scientist Hans Molisch in 1937. Observations on allelopathy, however,

were recorded 2000 years ago (Putnam and Tang, 1986; Rice, 1984, 1995; Willis,

1997a,b), and modern scientists described the phenomenon in the 1920s. Massey

(1925) reported that black walnut (Juglans nigru) and butternut walnut (J. cinerea)

caused wilting and dying of alfalfa, tomato, and potato, and Davis (1928) associciated the toxicity of black walnut with synthetic juglone (5-hydroxy-a-



napthaquinone)and reported its toxic effects on alfalfa and tomato. It was not until 1974, however, that Elroy L. Rice’s English text focused attention on the phenomenon. Rice (1984) offered the definition of allelopathy as the effect(s) of one

plant (including microorganisms) on another plant(s) through the release of a

chemical compound(s) in the environment. The effects could be either inhibitory

or stimulatory, depending on the concentration of the compounds. The compounds

involved in allelopathic interference are often termed allelopathic compounds, allelochemicals, or phytotoxins. We prefer the terms allelopathic compound or allelopathic chemical rather than allelochemical or phytotoxin because the traditional use of the term allelopathy includes both inhibitory and stimulatory

activities. While traditional use must be respected, this traditional use of the pathy

suffix is awkwardly broad for most current scientific discussion. Also, currently

the term allelochemical is used in a wider context in the field of chemical ecology

in which it includes, but is not limited to, plant and microbial interactions. We identify the plant that releases allelopathic compounds as the producer, or the donor

plant, and the plant that is affected as the target, or afflicted, plant.

Many researchers (Willis, 1985; Putnam and Tang, 1986; Horsley, 1991; Inderjit and Dakshini, 1995a) have suggested protocols suitable for certain demonstration of allelopathy. These can be summarized as follows:

1. Consistent demonstration of quantitative effects on the growth of the target

species due to chemicals released in the donor plant extract, leachate, or exudatewith appropriate controls.

2. Isolation, purification, and characterization of allelopathic compounds, followed by assay of these chemicals against species that are associated with the

donor plant in natural systems.

3. Induction under field conditions of responses similar to those observed in the

laboratory by the addition to the substratum of the compounds identified from the

producer plant.

4. Verification of the in siru release of allelopathic chemicals from the donor

plant and of their bioactive concentration in the vicinity of the target plant in nature.

5 . Demonstration that the afflicted plant is sensitive to the allelopathic compound and that it has some means of contact with, or uptake of, the compound. Because target species may be indirectly affected by the nutrient imbalances, microbial ecology shift, and/or microbial nutrient immobilization due to addition of

allelopathic compounds in the substratum, this direct contact and sensitivity may

not be a valid criterion for all cases of allelopathy.

6. Exclusion of resource competition, herbivory, disease, or other biotic interferences as the basic cause of a pattern of growth inhibition.

The allelopathic effects of compoundscould be due to (i) direct release of chemical compounds from the donor plant; (ii) degraded or transformed products of re-



leased compounds resulting from abiotic and biotic soil or water influences; (iii)

effects of released compounds on physical, chemical, and biological soil or water

characteristics;or (iv) induction of release of biologically active compounds by a

third species. Various ecological and agroecological factors, such as soil and water properties (physical, chemical, and biological), climate, and agricultural practices, greatly influence allelopathy. This review discusses especially the allelopathic interferences of weeds, crop species, and crop residues in agroecosystems.

The importance of various biotic factors such as age, density, life cycle pattern,

and morphological characteristicsof donor plant in influencing allelopathy will be


Alper (1998) reported that long-chain sugars secreted by certain cyanobacteria

and bacteria helped bind soil into a black crust which protects these microorganisms from heat. While this is not an example of allelopathy, it illustrates the significance of compounds in shaping habitat for species. In India and elsewhere,

cyanobacterial inoculum is used to increase the N content of paddy soils. However, recent laboratory studies suggest that cyanobacterial inoculum, when mixed in

high amounts into soils, can adversely affect paddy growth (Inderjit and Dakshini, 1997). Keating (1987) has clearly shown that different species of cyanobacteria have quite distinct allelopathic potentials. In fact, it is most reasonable to consider a strain of the same species, isolated from a different locale, to be of uncertain

value when used to replace the original isolate in a system which might benefit by

N fixation. The difficulty here is not just that biologically active compounds might

be idiosyncratic products of a given isolate of the same species but also that the

taxonomy of the cyanobacteria, unlike that of the other prokaryotes, is based on

morphology and not biochemistry. This is a remnant of the past association of the

blue-green prokaryotic algae with the eukaryotic algae. The prokaryotic algae,

much like the other prokaryotes,do not exhibit sufficient morphological difference

to support taxonomic distinctions and many cyanobacterial taxons are poorly

placed in the phylogeny of prokaryotes. In addition to a discussion of allelopathy

among annual and perennial weed and crop plant species, the allelopathic potential of the cyanobacteria and its relevance to agriculture is also discussed.

Allelopathic effects can be stimulatory or inhibitory depending on the identity

of the active compound on the static and dynamic availability,persistence, and fate

of organics in the environment, and on the particular target species. While water

is a more efficient dispersing agent, soil especially has its own mechanisms of

detoxification. Processes such as sorption, degradation, retention, and transformation greatly influence the quantitative and qualitative availability of organic molecules. The importance of phenolic compounds and terpenoids in the inhibition of

nitrification has been questioned. For example, it has been reported that phenolic

compounds, such as caffeic and ferulic acids, myricetin, tannins, and tannin derivative compounds, can inhibit the oxidation of NH,+to NO; by Nitmsomonus

(Rice, 1984).However, some workers (McCarty and Bremner, 1986)disagree with



this viewpoint and report that terpenoids and phenolics enhance the immobilization of ammonium N by soil organisms rather than the inhibition of nitrification.

These controversial aspects will be explored to gain better insight.

The allelopathic potential of certain weeds and cover crops has been exploited

in biocontrol programs. Convincing evidence has been presented concerning ways

to exploit allelopathy both for crop improvement and for development of a more

sustainable agriculture, including weed control, cover crops, pest management

through crop rotation, nutrient enrichment, and residue management. Because of

an increased understanding of allelopathy, it is clear that allelopathy can help in

the progress toward a more sustainable agriculture worldwide.

The objective of this review is to discuss (i) the allelopathic potential of certain

noxious weeds, crops, and their residues; (ii) the concerns of some ecologists regarding the way in which allelopathic research is done; (iii) the significance of

well-replicated field studies; (iv) what characteristics of donor plants favor their

allelopathic potential; (v) how stress (herbicide, disease, moisture, nutrient, and

light), site, climatic (temperature, growing seasons, etc.), habitat, and physical,

chemical, and biological environmental factors influence expression of allelopathy; (vi) how the allelopathicpotential of certain weeds and cover crops can be exploited in biocontrol programs; (vii) the significance of additive effects of allelopathic compounds and other organics at low concentrations;and (viii) the practical

difficulties in studying allelopathy that must be considered.


Ecologists often express concerns regarding the conclusive demonstration of allelopathy in natural system (Harper, 1975; Keeley et al., 1985; Connell, 1990;

Williamson, 1990; Thijs et al., 1994; Inderjit and Del Moral, 1997; Inderjit and

Dakshini, 1998a).According to Harper (1977, p. 372), “It is an extraordinarily difficult task to design an experiment that conclusively tests the toxin hypothesis of

plant interaction.” Lewis (1986) commented that many laboratory bioassays have

been conducted without much consideration of the evolutionary context of the organism, and it is difficult to conclude through laboratory bioassays alone that allelopathy is a main force influencing the competition of species in natural systems.

Griimmer (1961) reported on several phenolic compounds, including 4-hydroxybenzoic and vanillic acids from roots and rhizomes of couch grass (Agropyron

repens). He states p.221, “It is difficult to believe that an effect specific to couch

grass should depend on substances so common in the plant kingdom.” However,

many workers conclusively and convincingly demonstrated allelopathy in terrestrial and aquatic ecosystems (Rice, 1964,1968,1971; Muller, 1965,1966; Aubert,

197I ; Del Moral and Cates, 1971 ; Del Moral et al., 1978; Keating, 1977, 1978).



Wardle and coworkers (1996) investigated the allelopathic potential of six grasses-cocksfoot (Ducryfis gfornerata), phalaris (Phafaris aquatica), prairie grass

(Brornus wifdenowii),perennial ryegrass (Lolium perenne), tall fascue (Festuca

arundinacea), and Yorkshire fog (Hofcus lanata)-and four legumes-lucerne

(Medicago sativa), red clover (Trifoliurnpratense), subterranean clover (T subterraneurn), and white clover (T. repens). The test species used was Carduus nutans. Wardle and coworkers (1996) concluded that the allelopathic effects of the

10 grassland forage species on C. nufans in field plots were significantly correlated with the results of bioassays of these 10 species.

Like allelopathy, competition is difficult to demonstrate. Nonetheless, probably

reflecting the long use and interpretation of the term “competition,” this term continues to be accepted and used. It appears that the bar for proof of allelopathy in

nature is set higher than that for proof of competition because, although it is true

that not many studies on competition have investigated the possibility of an allelopathic component, studies of allelopathy are expected to eliminate all possibility of competition.This represents a need to include proof of the negative, which

is not a usual scientific practice. It also suggests that allelopathy and competition

for specific resources are mutually exclusive. This is faulty logic since natural selection would favor any and all of the traits which provide selective advantage to

an organism and that would ensure that multiple competitive tactics would be simultaneously in use. To date, we have found no published work which both eliminates allelopathy and could truly be said to demonstrate competition, and we do

not anticipate such material in the future. Some ecologists choose not to invoke allelopathy as a mechanism explaining plant interference unless there is specific evidence to support it and all other mechanisms have been eliminated. The most desired evidence involves isolation, identification, and characterizationof a bioactive

compound@)which can be shown to induce the allelopathy. Other desired evidence includes the demonstration that the allelopathic effects on other plants are a

primary function of the bioactive compound(s).The need for the primary function

must be challenged. An evolutionary explanation for the maintenance, in the genetic pharmacopoeia of a plant, of a compound which functions as an allelopathic substance would not require an exclusive, primary, or even significant allelopathically generated advantage for the producing plant. In terms of natural

selection, selection for continued maintenance of the compound would require

only a competitively advantageous use and not an allelochemical use. The allelopathic activity could be serendipitous.The allelopathic events may favor the producer but need not do so. Keating (1987) suggests the term “secondary allelochemistry” to categorize the set of circumstances she repeatedly observed during

her Linsley Lake study. In that aquatic community several bloom-dominant algal

forms (types of cyanobacteria) leave behind bioactive materials when they are no

longer able to maintain numerical dominance of the community. These materials

commonly favor the succeeding dominant form providing some of the complex of

advantageous factors which select it over all other co-occurring species. In a 5-



year study, no contradictory events occurred, i.e., no producer left bioactive substances behind that negatively affected a successor. The positive allelopathy was

readily demonstrated in vitro via cultures established in lake waters freshly collected before and after a bloom and was similarly demonstrated with cell-free filtrates of pure (some axenic) cultures of the producing and affected cyanobacteria

(Fig. 1). Since the active material could be concentrated via ultrafilters, and nutritional limitations were excluded by complex nutrient supplementation,the demonstrations of allelopathy were generally accepted. Although no competitive advantage could accrue to the producing organism by virtue of enhancing the growth of

its successor, the genetic capacity to produce the bioactive compounds was maintained in the genome of the producer. Thus, some other, nonallelopathic advantage

was required to maintain the selective value of this production, and this positive

allelopathy could not be the “primary” function of the bioactive material.

Demonstrating allelopathy in natural systems is difficult because different

mechanisms of interference (resource competition, allelopathy, microbial nutrient

immobilization, etc.) can not be separated under field conditions.Also, allelopathic

compounds are interwoven with environmental stresses, and separating allelopathy and other mechanisms of interference such as resource competition is not realistic in nature (Inderjit and Del Moral, 1997). n o - w a y analysis of plant interference (i.e., studying competition and allelopathy simultaneously)is important to

the generation of more ecologically relevant data (Inderjit and Del Moral, 1997;

Inderjit, 1998). It is important to take into account that any adverse effect on the

growth and distribution of a plant could be due to organic molecules leached into

the soil by donor plants and passed to afflicted plants. Also, it could be due to effects of organic molecules on microbial ecology, microbial nutrient immobiliza-








Figure 1 Allelochemical activity of nine cell-free filtrates of cyanobacteria tested against diatoms

(D), cyanobacteria (C) chlorophytes ( G ) , and motile forms (M)(identical, autoclaved, cell-free filtrates-the combination of heat, pressure, and pH associated with autoclaving eliminates the activity);

light gray. growth promoted; dark gray, growth inhibited; dotted, neutral effect on growth.



tion, or soil nutrient availability. Furthermore, organic and inorganic soil components may modify the expression of allelopathy (Blum et al., 1992).In no case can

we prove unequivocally that allelopathy is the only factor responsible for the observed pattern-only that allelopathy offers the most reasonable explanation.

There are many problems with bioassays employed to demonstrate allelopathy

(Inderjit and Dakshini, 1995a; Inderjit, 1996; Maestrini and Bonin, 1981). In general, broad criteria to demonstrate allelopathy include (i) identificationof an allelopathic donor plant with reduced growth of other plants in its vicinity; (ii) the capacity of a donor plant to produce bioactive chemical compounds and to release

them into the environment; (iii) isolation, identification, and characterization of biologically active chemical compounds; and (iv) observation of the effects of isolated chemical compounds and their mixtures on seed germination andor growth of

certain plant species. However, there are many other aspects of this phenomenon

which need to be considered. For example, during phytochemical analysis, attention is often paid only to biologically active compounds with “appropriate” concentrations, and compounds with low concentrationremain neglected. Furthermore,

bioassays are often performed only with individual compounds, and insufficientattention is paid to the roles of compounds in mixtures of allelopathic chemicals.

The significance of soil texture, microorganisms, and associated species in relation to laboratory bioassays is discussed by Inderjit and Dakshini (1995a). It is

important to study at what concentration, and in what form, a chemical is available to the target species and to consider how the qualitative and quantitative concentration aspects of a given compound are influenced by habitat, by physical,

chemical, and biological soil factors, by climatic factors, and by many other characteristics of the habitat. Finally, it is not always true that a chemical is present in

the environment when allelopathic symptoms are observed (Cheng, 1989). It is

also likely that by the time allelopathic symptoms are observed, the allelopathic

compound has undergone degradation/transformation to other more or less active

compounds. These aspects are discussed in the following sections.



Many weeds are considered troublesome in cropping systems (Zimdahl, 1993;

Aldrich and Kremer, 1997; Holm et al., 1997), and approximately 250 weed

species are known to be problematic in agriculture (Worsham, 1989). Allelopathy

has been suggested as a likely mechanism of interference in many weed species

(Rice, 1984, 1995; h t n a m and Weston, 1986; Waller, 1987; Inderjit et al., 1995)

(Table I). Many studies have been conducted under laboratory conditions to eval-



lsble I

Common Agroecosystem Weeds with Potential AUelopathic Activitiw

Weed species

Common name

Abutilon theophrasti

Agropyron r e p e d

Amaranthus dubius

Amaranthus palmeri




Palmer amaranth

Amaranthus retrofiexus

Ambrosia artemisiifolia

Ambrosia cumanensis

Ambrosia psilostachya

Ambrosia trifida

Antennaria microphylla

Argemone maxicana

Artemisia annw

Artemisia vulgaris

Asclepias syriaca

Avena f a t w

Berteroa incana

Bidens bipinnata

Bidens pilosa

Brachiaria mutica

Brassica nigra

Bromus japonicus

Calluna vulgaris

Camelina alyssum

Camelina sativa

Celosia argentea

Cenchrus pauciJorus

Centaurea diffusa

Centaurea maculosa

Centaurea repens

Centaurea solstitialis

Chenopodium album



Chromolaena odorata

Cirsium arvense

Cirsium discolor

Citrullis colocynthis

Citrullis lanatus

Convolvulus sepium

Cynodon dactylon

Cyperus breifolius

Cyperus esculentus

Cyperus kyllingia

Redroot pigweed

Common ragweed

Western ragweed

Giant ragweed

Small everlasting

Annual wormwood


Common milkweed

Wild oat

Hoary alyssum



Black mustard

Japanese brome


Flax weed

Large-seed falseflax


Field sandbur

Diffuse knapweed

Spotted knapweed

Russian knapweed

Yellow star thistle

Common lamb’s-quarter

Siam weed

Canada thistle

Tall thistle

Hedge bindweed

Bermuda grass


Yellow nutsedge



Dekker et al. (1983)

Weston and Putnam (1985,1986)

Altieri and Doll (1978)

Bradow and Connick (1987),

Menges (1988)

Bhowmik and Doll (1983)

Jackson and Willemsen (1976)

Anaya and De.1 Amo (1978)

Neil1 and Rice (1971)

Le Tourneau et al. (1956)

Manners and Galitz (1986)

Sharma and Nathawat (1987)

Lydon et al. (1997)

Mann and Barnes (1945)

Rasmussen and Einhellig (1975)

Schumacher et al. (1983)

Bhowmik and Doll (1979)

Meissner et al. (1986)

Stevens and Tang (1985)

Chou (1989)

Muller (1969)

Rice (1 964)

Jalal and Read (1983)

Grummer and Beyer (1960)

Lovett and Duffield (1981)

Pandya (1975)

Rice (1964)

Muir and Majak (1983)

Locken and Kelsey (1987)

Fletcher and Renney (1963)


Qasem and Hill (1989)

Jimhez-Osornio et al. (1996)

Sahid and Sugau (1993)

Stachon and Zimdahl(l980)

Le Tourneau el al. (1956)

Bhandari and Sen (1971)

Bhandari and Sen (1972)

@inn (1974)

Meissner er al. (1989)

Komai and Tang (1989)

Tames er al. (1973)

Komai and Tang (1989)




Table I-Continued


Weed species

Cyperus rotundus

Digera altemifolia

Digitaria sanguinalis

Echinochloa crus-galli

Echinops echinatus

Eleusine indica

Eragrostis pweoides

Erica australis

Erica scoparia

Eupatorium adenophorum

Euphorbia corollata

Euphorbia esula

Euphorbia granulaf a

Euphorbia prostrata

Euphorbia supina

Galium aparine

Gomphmna decumbens

Helenium amanun

Helianthus annuus

Helianthus mollis

Hemarthria altissima

Holcus lanatus

Imperata cylindrica

Iva xanthifolia

Kochia scoparia

Lactuca scariola

kersia hexundra

Lepidium virginicum

Leptochloa filifonnis

Lolium multiflonun

Lychnis alba

Matricaria inodora

Nepera cataria

Oenothera biennis

Oryza perennis

Panicum dichotorn#lonun

Parthenium hystemphorus

Common name

Purple nutsedge

Large crabgrass

Barnyard grass




Flowering spurge

Leafy spurge


Prostrate spurge


Bitter sneezeweed


Bigalta limpograss

Yorkshire fog

Cogon grass



Prickly lettuce


Plantago purshii

Pluchea lanceolata

Virginia pepperweed

Red sprangletop

Italian ryegrass

White cockle



Evening primrose

Wild rice

Fall panicum



Wooly plantain


Polygonum aviculare

Polygonum orientale

Prostrate knotweed



Tang et al. (1995)

Ashraf and Sen (1980)

Parenti and Rice ( 1969)

Bhowmik and Doll (1979),

Li et al. ( 1992b)

Jha and Sen (1981)

Altieri and Doll (1978)

Hussain et al. (1984)

Carballeira (1980)

Ballester er al. (1977)

Baruah et al. (1994)

Rice (1964)

Manners (1987)

Hussain (1980)

Alsaadawi et al. (1990)

Rice (1969)

Komai er al. (1983)

Solomon and Bhandari (1981)

Smith (1989)

Spring and Hager (1982).

Leather (1987)

Anderson ef al. (1978)

Tang and Young (1982)

Wardle et al. (1992)

Indejit and Dakshini (1991a)

Le Tourneau et al. (1956)

Karachi and Pieper (1987)

Rice (1964)

Chou et al. (1984)

Bieber and Hoveland (1968)

Altieri and Doll (1978)

Naqvi and Muller (1975)

Bhowmik and Doll (1979)

Mann and Barnes (1945)

Le Tourneau et al. (1956)

Bieber and Hoveland (1968)

Chouetal. (1991)

Bhowmik and Doll (1979)

Patil and Hedge (1988).

Megharaj er al. (1987)

Rice (1964)

Indejit (1998), Indejit and

Dakshini (1994a,b, 1996a,b)

h a a d a w i and Rice (1982)

Datta and Chatterjee (1978)




Table I- Continued

Weed species

Common name

Polygonum pensylvanicum

Polygonum sachalinese

Portulaca oleracea

Polypogon monspeliensis

Proboscidea louisianica

Rorippa sylvestris

Rorippa indica

Rwnex crispus

Saccharum spontaneum

Salsola kali

Salvadora oleoides

Salvia syriaca

Sasa cernua


Setaria glauca

Setaria viridis

Solanum suranense

Solidago sp.

Sorghum halepense

Stellaria media

Pennsylvania smartweed

Sakhalim knotweed

Common purslane

Unicorn (devil’s claw)


Yellow fieldcress


Wild cane

Russian thistle

Syrian sage


Giant foxtail

Yellow foxtail

Green foxtail




Stevia eupatoria

Striga densgora

Tagetes minuta


Tephmsia purpurea

Trianthema portulacastrum

Urgenia indica

Xanthium pensylvanicum

Xanthium strumarium

Kempton’s weed

Wild marigold

Common cocklebur



Le Toumeau et al. (1956)

Inoue et al. (1992)

Le Tourneau et al. (1956)

Inderjit and Dakshini (1995b)

Mercer et al. (1987),

Rime et al. (1990)

Mizutani and Yamane (1991)

Yamane et al. (1992a)

Einhellig and Rasmussen (1973)

Amritphale and Mall (1978)

Lodhi (1979)

Mohnot and SON (1976)

Abu-haileh and Qasem (1986)

Li et al. (199%)

Gilmore (1985)

Bhowmik and Doll (1983)

Rice (1964)

Sharma and Sen (1971)

Le Tourneau et al. (1956)

Abdul-Wahab and Rice (1967)

Mann and Barnes (1950).

Inderjit and Dakshini (1998)

Lovett ( 1982)

Zuberi et al. (1989)

Meissner et al. (1986)

Altieri and Doll (1978)

Sundaramoorthy and Sen (1990)

Sethi and Mohnot (1988)

Khare ( 1980)

Rice ( 1964)

Inam et al. (1987)

Note. From A. R. F’utnam and L. A. Weston (1986). Adverse impacts of allelopathy in agricultural

systems, In ‘The Science of Allelopathy” (A. R. Pumam and C. S. Tang, Eds.), pp. 43-56. Copyright

Q 1986 by John Wiley & Sons, Inc.; adapted with permission of John Wiley & Sons, Inc.

“Many of the weed species have been tested for their allelpathic potential using leachate or extract

bioassays and do not demonstrate allelopathy conclusively.

bNew Weed Science Society of America approved name is Elytrigia repens.

uate the effects of weed species either (i) directly through aqueous or organic

leachates or extracts of donor plants (Bhandari and Sen, 1971; Friedman and

Horowitz, 1971; Sharma and Sen, 1971; Srivastva and Das, 1974; Chou and

Young, 1975; Turner and Quarterman, 1975; Datta and Chattejee, 1978; Sugha,

1978; Hussain, 1980; Murthy and Zakharia, 1980; Dam and Chakrabarti, 1982;



Biswas and Chakraborti, 1984; Manners and Galitz, 1986; Konar and Kushari,

1989;Perez and Ormeno-Nunez, 1991) or (ii) indirectly through extraction of the

leachate after addition to the sand, agar, or soil supporting growth of donor plants

(Mullerand Muller, 1964;Selleck, 1972;Lodhi and Nickell, 1973;Colton and Einhellig, 1980;Bhowmik and Doll, 1983;Alsaadawi et al., 1990;Anaya et al., 1990;

Inderjit and Dakshini, 1991a, 1992a, 1994a,b;Sahidand Sugau, 1993).Many studies on weed allelopathy (Table I) were conducted using an aqueous extract or

leachate and examined only seedling growth of afflicted plants as test growth parameters. It may be that allelopathy would not exist in many weed species if allelopathic studies are repeated under more natural conditions (Putnam, 1985).

There is a need for well-replicated and repeated field studies. Nevertheless, there

are some interesting studies which convincingly prove allelopathy. Weston and

Putnam (1985, 1986) demonstrated quackgrass (Elytrigia repens, formerly A.

repens) allelopathy in nature. They reported that living and herbicide (glyphosate,

N-(phosphonomethy1)glycine)-killed quackgrass significantly inhibited nodulation of snap bean (Phuseolus vulgaris). Figure 2 shows the allelopathic inhibition

of quackgrass residues on nodulation of snap bean. Figure 3 shows that snap bean

roots lack hairs when treated with quackgrass shoot extracts. Since the untreated

snap bean roots have root hairs, quackgrass may have eliminated the physiological sites (i.e., root hairs) for Rhizobium infection. Weidenhamerand Romeo (1989)

reported that soils infested with Polygonella myriphylla significantly suppressed

the seed germination and growth of Bahia grass (Paspalum notatum). Figure 4

shows patches of P. myriophylla, bordering a citrus field dominated by bahiagrass,

which convincingly suggested the operation of allelopathy in nature. Many studies have detected potential allelopathic chemicals from soil infested with allelopathic plants in nature (Levitt and Lovett, 1984; Lovett and Potts, 1987; Oleszek

and Jurzysta, 1987; Li et al., 1992a; Inderjit and Dakshini, 1991b, 1992b; Rice,

1995).Lovett and Potts (1987) showed the rapid release into soils of two alkaloids,

scopolamine and hyoscyamine, by Datura stramonium seeds. Levitt and Lovett

(1984)suggested that the fate and activity of scopolamineand hyoscyaminein natural soil depend on both physical and biological factors. Observations (Oleszek

and Jurzysta, 1987) on the fate of medicagenic acid glycoside released in the soil

by alfalfa root, as well as data (Li et al., 1992a)on the presence ofp-coumaric, ferulic, vanillic, andp-hydroxybenzoicacids in the rhizosphere soil of the weed Sasa

cernua, suggest the operation of allelopathic interference in the field.

Table I lists weed species that have been investigated for their allelopathic potential. Many of these weed species were tested for their allelopathic interference

using leachate or extract bioassays, and allelopathy is not convincingly proven.

Holm (1969) lists the 10 worst weeds: Bermuda grass (Cynodon dactylon), purple

nutsedge (Cyperus rotundus), barnyard grass (Echinochloa crus-galli),jungle rice

(Echinochloa colona), goosegrass (Eleusine indica), water hyacinth (Eichhomia

crassipes), cogon grass (Imperata cylindrica), lantana (Lantana camara), john-

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Chapter 3. Allelopathy: Principles, Procedures, Processes, and Promises for Biological Control

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