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VIII. Roles for Allelopathy in Biocontrol Programs

VIII. Roles for Allelopathy in Biocontrol Programs

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ALLELOPATHY



191



son and Peterson, 1986; Forney et al., 1985; Leather, 1983; Einhellig and Leather,

1988). Many have considered the allelopathic suppression of weeds by various

cover crops-buckwheat (Eskelsen and Crabtree, 1995; Rice, 1995), black mustard (Bell and Muller, 1973; Rice, 1984; Jimhez-Osornio and Gliessman, 1987),

sorghum (Forney and Foy, 1985; Weston et al., 1989; Alsaadawi et al., 1986a),

wheat (Shilling et al., 1985), and rye (Barnes and Putnam, 1983; 1987; Yenish et

al., 1995).Residues of several cover crops, such as winter wheat, barley, oats, rye,

grain sorghum, and Sudan grass, have demonstrated an allelopathic potential to

suppress weeds (Barnes and Potnam, 1983; Einhellig and Leather, 1988; Rice,

1995; Weston, 1996; Miller, 1996; Teasdale, 1998). Cereal crops such as wheat,

maize, and rye have the potential to produce hydroxamic acids, which are important in Poaceae to different kinds of pests and diseases (Niemeyer, 1988; Niemeyer and Perez, 1995). Hydroxamic acids are released into the soils through root

exudation. Various workers suggest that detoxification of triazine herbicides is associated with hydroxamic acids (Niemeyer and Perez, 1995). Velvet bean (Mucunu pruriens var. utilis), a legume cultivated for green manure, has a beneficial impact on the yield of graminaceous crops and has the ability to smother noxious

weeds such as purple nutsedge and cogon grass (Fujii et al., 1992). Fujii and

coworkers screened 70 plant species for their ability to smother weeds. They found

that velvet bean has significant potential to smother noxious weeds. It was found

that L-DOPA (~-3,4-dihydroxyphenylalanine)

is mainly responsible for this allelopathic activity. Worsham and Blum (1992) reported that weeds, such as species

of amaranth (A. retrojexus, A. spinosus, and A. hybridus) and common lamb’squarter, can be controlled when planted into killed cover crops of rye and subterranean clover. Macharia and Peffley (1995) reported the inhibitory effects of Alliumjstulosum and A. cepa genotypes on seed germination and plant growth of

spiny amaranth (A. spinosus) and kochia (Kochiu scopariu).

The allelopathic effects of a cover crop may vary with different plant parts.

While separating the allelopathic effects of root and shoot residues of rye and

sorghum, Hoffman’s group (1996a)found that cover crop root residues suppressed

the growth of weeds but that the shoot residues of cover crops, in general, do not

suppress the growth of weeds. Hoffman et al. (1996b) also investigated the allelopathic interference of germinating seedling of different cover crops on seedling

growth of weeds (Table VIII), and found that while germinating sorghum suppressed the radicle growth of weeds, the germinating rye enhanced the weed radicle length.

Increasing soil erosion from cropland has stimulated interest in using surface

soil residues for the control of soil erosion. Tillage systems which leave crop

residues in the field are effective at reducing soil erosion (Phillips et al., 1980).

However, conservation tillage systems may have poorer crop stands and more uneven growth rates than conventional tillage systems (Griffith el ul., 1973). In cultivated fields, the growth reduction which accompanies surface mulcheshesidues



INDERJIT AND K. IRWIN KEATING



192



'hble WI

Gmwtb of 6-Day-OldWeeds Germinatedwith Cover Crop Species Compared

to "hat of Weeds Germinated Alone"

% of control



Weed species

Germinated with sorghum

Velvetleaf

Smooth pigweed

Large crabgrass

Green foxtail

Germinated with annual white sweet clover

Velvetleaf

Smooth pigweed

Large crabgrass

Green foxtail

Germinated with rye

Cheat

Shepherd's purse

Hairy vetch

Green foxtail

Germinated with crimson clover

Cheat

Shepherd's purse

Hairy vetch

Green foxtail



Radicle

length



Shoot

length



96

106

100

52*



67*

71*

93

74*



102

86*

99

101



86

110

85

96



77*

90



94

103



Germination



101



104



89



103



111

100

111

105



123*

94

128*

108*



121*

85

133

94



106

76

115

96



129*

78

109

108



135*

86

111

106



"Source: Hoffman, M. L., Weston, L. A,, Snyder, J. C., and Regnier. E. E.(1996b). Allelopathic

influence of germinating seeds and seedlings of cover crops on weed species. Weed Sci. 44,579-584.

Reproduced with permission from the Weed Science Society of America.

*The parameter measured differed from the control according to Fisher's LSD (p = 0.05) test.



is evident at early growth stages (Willis et al.. 1957). Residues of many crops have

been known to influence the growth and yield of crops (Rice, 1984; Yakle and

Cruse, 1983;Guenzi and McCalla, 1966; Patrick and Koch, 1958;Abdul-Baki and

Teasdale, 1997). The type of mulches should be carefully monitored in order to

avoid allelopathic growth suppression in the following years (e.g., sorghum

residues have been reported to be phytotoxic even to next year's crop; Worsham,

1991). Bewick and coworkers (1994) investigated the allelopathic interference of

celery root residues on the growth of certain weed species [bamyard grass, common purslane, green foxtail, large crabgrass, spiny amaranths, wild mustard, rice

flatsedge (Cyperus iria), and black nightshade (Solanum nigrum)] and crops [celery, carrot, lettuce, radish, and escarole (Cichorium endiva)].The relative sensi-



ALLELOPATHY



193



tivity of the different weed species to celery root residues was spiny amaranths >

barnyard grass > wild mustard > black nightshade > large crabgrass > green foxtail = rice flatsedge > common purslane. The relative sensitivity of the crop

species was radish escarole + lettuce + carrot + celery.

Downum and coworkers (1989) screened 115 species from 57 genera and eight

plant families for their phototoxic activities using standard antimicrobial bioassays. They found the presence of many phototoxic compounds in the family Asteraceae, particularly in the subtribe Pectidinae and the tribe Heliantheae. These compounds are generally reported from species evolved under high light conditions

and provided with efficient plant defense mechanisms.

Przepiorkowski and Gorski (1 994) carried out greenhouse and laboratory studies to determine the influence of rye on germination and growth of three triazineresistant weed species: barnyard grass, willow herb (Epilobiurn ciliaturn), and

horseweed (Conyza canadensis).They found that willow herb and horseweed germination was suppressed with aqueous extracts of rye shoot tissues and soil containing rye seeds. However,barnyard grass germination was not influenced. Growth

suppression was observed in both biotypes (resistant and susceptible)of three weed

species in soil containing rye roots. It has been shown that weed suppression due

to rye cover crop will last 4 weeks after killing of the cover crop (Barnes and Putnam, 1986, 1987).Yenish and coworkers (1995) reported that rye residues take 15

weeks to decompose to 50% of first-day level. Their studies show that the duration

of weed suppression due to rye cover crop is related more to the disappearance of

rye allelochemicals from rye residues than to the disappearance of rye residues.

Worsham (1991) discusses other beneficial aspects of cover crops, including (i)

conserving soil moisture, (ii) increasing soil organic matter, (iii) controlling wind

and water erosion, and (iv) maintaining soil fertility of recycled nutrients.

The beneficial effects of crop rotation, especially for cereals and legumes, have

been established (Rice, 1995; Weston, 1996).Rotational crops such as tall red fescue (Festuca arundinacea), creeping red fescue ( E rubra), asparagus, sorghum,

alfalfa, black mustard, and oats are used for weed suppression (Weston, 1996).

Leguminous crop plants may benefit cereals in crop rotation by (i) providing nitrogen compounds; (ii) improving soil physical properties; (iii) reducing soil erosion, and (iv) suppressing weeds, insects, and diseases (Sarobol and Anderson,

1992). Various crops, such as corn, soybean, wheat, cotton, sorghum, and barley,

have been reported to benefit from being preceded by different crops in the previous year (Crookston, 1984). However, to date the phenomenon of yield improvement in crop rotation is not well understood. Allelopathy may be involved in the

residual effects of crop rotation. Sarobol and Anderson (1992) reviewed the cornsoybean rotation and found that increased yield of corn following soybean, in comparison to yield of continuous corn, was due to the combination of the adverse

effects of corn on corn and beneficial effects of soybean on subsequent corn. Allelopathic effects of cruciferous crop plants in crop rotation have also been well

+



194



INDERJIT AND K. IRWIN KEATING



documented (Grodzinsky, 1992).The yield of cruciferous plants, when introduced

to crop rotation, was reported to be higher (17-20%) than that in monoculture

(Grodzinsky, 1992).

The important question is whether we can achieve near 100% weed suppression

by using cover crops, crop residues, or rotational crops. While investigating weed

suppression by hairy vetch, Teasdale (1988) found that other methods of weed control are still needed. Worsham (1991) suggested that most of the many herbicides,

particularly postemergence herbicides, are still needed because allelopathic suppression is generally adequate for the first few weeks only. Worsham suggested that

herbicides are needed to kill cover crops since, if not killed, cover crops would be

likely to compete with planted crops for essential resources. He provided some approached to avoid using herbicides for killing cover crops: (i) mowing and sweeping to mechanically kill grain cover crops and legumes, respectively; (ii) planting

crops such as soybean and grain sorghum after cover crops mature naturally; and

(iii) killing cover crops such as sorghum and spring oat, which can be planted for 1

year and killed during the winter, resulting in dead mulch for next spring.

Indejit and Olofsdotter (1998) discussed the allelopathic potential of rice.

Olofsdotter and Navarez (1996) investigated the allelopathic potential of 111 rice

cultivars against problematic weeds. They reported that 10 cultivars had allelopathic effects on the growth of barnyard grass and argued the need for introducing

weed suppressing ability into rice. Dilday’s group (1991) evaluated 10,000rice accessions for allelopathic activities of ducksalad (Heteranthea limosa). They reported that 3.5% of accessions possess allelopathic activities. Dilday and coworkers (1998) reported that 412 rice accessions were allelopathic to ducksalad, 145 to

red stem (Ammannia coccinea), and 16 to both species. Hassan and coworkers

(1997) reported that 30 rice accessions had allelopathic activities against barnyard

grass, 15 against Cyperus difformis, and 5 against both species. Olofsdotter and

Navarez (1996) reported that 1 rice cultivar (Taichung native I ) had allelopathic

activities against the growth of barnyard grass, Trianthemmu portulacastrium, H.

limosa, and A. coccinea. Mattice and coworkers (1998) identified the phenolic

compounds 4-hydroxybenzoic, 4-hydroxyhydrocinnamic, and 3,4-dihydroxyhydrocinnamic acids in water from allelopathic rice cultivars. None of these compounds were detected in water from nonallelopathicrice cultivars. Olofsdotter and

coworkers (1997) stressed the need for formulating breeding strategies to exploit

allelopathic rice cultivars in biocontrol programs.



B. ALLELOPATHIC

CHEMICALS

AS NATURAL

HERBICIDES

Herbicidescontinue to be a key component in most integrated weed management

systems. Nevertheless, extensive use of synthetic herbicides poses serious threats

to both the environment and public health (Macias, 1995).From both public health

and environmental perspectives there is great incentive to discover biologically ac-



ALLELOPATHY



195



tive natural products from higher plants that are as good as or better than synthetic

agrochemicals and that are likely to be much safer. The development of natural

products as herbicides, fungicides, and pesticides and their role in biocontrol of

plant disease promise to reduce environmental and health hazards (Rice, 1995).

Furthermore, in comparison to long-persistence, nontarget toxicity, polluting, carcinogenic, and mutagenic activities of synthetic agrochemicals, natural plant products are biodegradable, somewhat specific, and likely to be recycled through nature

(Epstein et al., 1967; Matsunaka and Kwatsuka, 1975; Duke, 1988).

Allelopathic chemicals show selectivity (Weston, 1996). Some allelochemicals

have already been investigated as possible inclusions in alternative weed management strategies (Macias, 1995). Biologically active natural products (allelochemicals) isolated from higher plants and microbes are now being employed as

herbicides and fungicides and in the biocontrol of plant diseases (Rice, 1985; Einhellig, 1984). Many weeds have become resistant to important herbicides classes

such as s-triazines (Gressel, 1985) and dinitroanilines (Mudge et al., 1984). It is

difficult to establish a direct relationship between the structuralcomplexity and the

activity of a compound (Macias, 1995).Tahara and coworkers (1994) reported that

mucondialdehyde (trans-2,trans-4-hexadienedial),an antifungal compound from

leaves of common lamb’s-quarter, was induced in response to cupric chloride

stress. Table IX lists the allelopathic chemicals from different classes of compounds with known potential use as natural herbicides. Macias ( 1995) suggested

that the normal range of concentrationstested for allelopathic chemical is between

and lo-’ M. According to him, good candidates for natural herbicide should

have activity between lop5 and lo-’ M. Many phenolic compounds, alkaloids,

and quinones, however, have an activity range of 10-2-10-5 M and thus are poor

candidates for natural herbicides (Macias, 1995).Heisey (1996) isolated a quassinoid compound, ailanthone, from root bark of tree-of-heaven (Ailanthus altissima). He reported the pre- and postemergence herbicidal activity of this compound

in greenhouse trials.

The previous discussion suggests the potential of several allelopathic compounds as natural herbicides. Because demonstrating potential herbicidal activity

for a particular compound under laboratory or controlled conditions may not guarantee its success in natural systems, more research is needed to test the suitability

of these potential herbicides under field conditions.



M.ADDITIONAL COMMENTS

A. ADDITIVEACTIVITIES OF ALLELOPATHIC

CHEMICALS

Plant species may respond differently to mixtures of two or more compounds

and this may not be predictable from their growth responses to individual allelo-



Table IX



Selected AUelopathic Cornponds with Potential Use as Natural Herbicides"

Allelopathic compound



~~



L-p-Hydroxybutyric acid ( 1

Ethyl propionate (1)



-



Target species



Activity range (ppb)



~~



Ethyl 2-methylbutyrate (1)



8.9



trans-DME ( 2 )

cis-Dihydro-ME ( 2 )

Arachidic, khenic, and

myristic acids (3)

2(3H)-benzoxazolinone(4)



103

5 x 103-5 x 104

5 x 103



2,2-Oxo-l.l-azoknzene (4)



5 x 104



Caffeine (4)

pHydroxyknzoic acid (5)



105-4 x 105

(6.9 x 104

inactive



Vanillic acid (5)



e . 6 x 104

inactive



p-Coumaric acid (6)



a . 2 x 104

inactive



X



102-4.4 X 103



105



m

\o



of activityb



~



Chenopodiumalbum, Amaranthus retrofkxus

Allium cepa, Lycopersicon esculentum

Daucus carota

Allium cepa

Daucus camta, Lycopersicon esculentum

Echinochloa crus-galli

Oryur sativa

Cynodon dactylon



Growth (-)

Growth met.)

Growth (-)

Growth (-)

Growth (Ret.)



Lepidium sativm, Cucwnis sativus, Phaseolus vulgaris

Echinochloa crus-galli

Echinochloa crus-galli,Lepidium sativm, Cucumis sativus,

Phaseolus vulgaris

Lactuca sativa

Lactuca sativa, Deschampsiajlexuosa. Chamaenerion angustifolium

Chamaenerion angustifolium

Lolium mulh~onun

Saccharum oficinarum, Lactuca sativa. Lolium multiporum

Senecio sylvaticus

Scrophularia nodosa

Deschampsiajkxuosa, Chamaenerion angustifolium

Senecio sylvaticus, Scrophularia nodosa

Lactuca sativa, Lolium multiporum

Raphanus sativum, Chamaenerionangustifolium,

Deschampsiajlexuosa

Scrophularia nodosa

Senecio sylvaticus



Growth (-)

NA

Growth (-)



Growth (-)

Growth (-)

Germination (-)



Growth (-)

Growth (-)

Germination (-)

Growth (-)

Growth (-)

Growth (+, -)

NA

Germination (-)

NA

Growth (-)

Growth (-)and

germination (-)

NA

Growth (+, -)



Ferulic acid (6)



9.6 x 104



Xanthotoxin (7)



<104 inactive



Senecio sylvaticus

Amtatica hierochuntica

Lwtuca sativa



Bergapten (7)






Lactuca sativa



Xanthotoxin (7)



<1.3 X 104



Lactuca sativa



Tujone (8)

Carvone (8)

Camphor (8)



3.3 x 103

8 X 103

2.7 x 104



Lactuca sativa

Lwtuca sativa, Leptochloa dubia, Schizochyrium scopariwn

Lactuca sativa

kptochloa dubia

kptochloa dubia, Schizochyrium scoparium



1,8-Cineol(8)



2 x 106



Schizochyrium scoparium



Emodin (9)

Physcion (9)



104-105

104-105



Juglone (9)



<9 x lo3 inactive



Guayulin A (10)



3.5 x 1 6



Heliannuol(10)



2.6 X 102-2.6 X l@



Lactuca sativa, Amaranthus viridis, Phleum pratense

Lactuca sativa, Amaranthus viridis

Phleum pratense

Alnus glutinosa

Rudbeckia h i m , Lactuca sativa

Amaranthw palmeri, A. retrofixus

Daucus carota

Lactuca sativa

Lactuca sativa



Lepidium sativum, Lolium multiporum, Chumaenerion angustifolium

Scrophulariaa nodosa

Deschampiaafelxuosa



Hordeum vulgare



Growth (-)

Germination (-)

Growth (-) and

germination (-)

Growth (+, -)

Growth (-)

Growth (-) and

germination (Growth (-) and

germination (Growth (-) and

germination (Germination (-)

Germination (-)

Germination (-)

NA

Growth (-) and

germination (-)

Growth (-) and

germination (-)

Growth (-)

Growth (+, -)

Growth (- )

Growth (-)

Germination (-)

Germination (-)

NA

Germination (+)

Growth (-) and

gemination (+)

NA

continues



'LgbleIX-Continued

Allelopathic compound



Activity range (ppb)



Target species



Strigol ( 10)

SoulangianolideA (11)

Melampomagnolide A (11)

Annuolide A-E (11)



0.3



11,13-Dihydroburrodin(11)

Parthenolide (11)



2.5

0.25-2.5 X 103



a-Santonin (1 1)

Hirsutin (12)



0.25-2.5 X lo4

2.2 x 103-4.4 x 104



Striga lutea

Lacruca sativa

Luctuca sativa

Lactuca sativa

Hordeum vulgam

Striga Iutea

StTiga lwea

Lactuca sativa

Phaseolus vulgaris

Loctuca sativa



Camelinin ( 12)



5 X lo4-2.6 X 1 P



Lactuca sativa



Betulinaldehyde (13)



0.5-5 x 104



h t u c a sativa

Honieum vulgare



Messagenin (13)



0.5-5 x 104



Soyasapogenol(l3)



10



Lactuca sativa

Lepidium sativum

Hordeum vulgare

Triticum aestivum

Agalinis purpura



0.25-2.5 x 104

0.25-2.5 X lo4

0.25-2.5 X 103



of activiv

Germination (+)

Germination (+)

Germination (+)

Germination (-)

NA

Germination (+)

Germination (+)

Growth (+I



Growth (+)

Growth (-) and

germination (-)

Growth (-) and

germination (-)

Germination (+)

Growth (+)and

germination (+)

Germination (+)

Growth (+)

Germination (+)

NA

Germination (+)



"Modified after Macias (1995).Copyright 0 1995 American Chemical Society.

bResults: +, stimulation; -, inhibition; Ret., retardation, NA, not active; +, -, stimulation or inhibition depending on the concentration.

'Chemical classes: 1, simple acids and esters; 2, polyacetylenes; 3, long-chain fatty acids; 4, alkaloids; 5, benzoic acid derivatives; 6, cinnamic acid derivatives; 7, coumarins; 8, monoterpenese; 9,quinones; 10, sesquiterpenes; 11, sesquiterpene lactones; 12, sulfured compounds; 13, triterpenes.



ALLELOPATHY



199



pathic compounds. In natural systems, allelopathic growth responses are the result

of either additive or partial antagonistic activities of allelopathic chemicals (Einhellig et al., 1982; Williams and Hoagland, 1982; Einhellig, 1995a; Blum, 1996;

Inderjit, 1996). Einhellig (1989) reported that a 50 pA4 mixture of p-hydroxybenzoic, protocatechuic, vanillic, gentisic, gallic, caffeic, p-coumaric, syringic, ferulic, and o-methoxybenzoic acids and 500 pA4 concentrations of individual compounds were equally inhibitory to the growth of velvetleaf. Blum (1996) suggested

that allelopathic activities are due to mixtures of allelopathic compounds (e.g.,

phenolic acids) and other organic compounds, and that the concentration of each

compound in a mixture might be significantly less than the concentration of individual compounds required to induce an allelopathic effect. Many recent studies

suggest the significance of a mixture of allelopathicchemicals in predicting growth

responses (Inderjit et al., 1997; VBronneau et al., 1997).

The literature deals with numerous mixture models, in which additivity of allelochemical or herbicide effects and additivity of doses are confused. Traditionally, some mixture research is based on empirical studies at some preset dose rates

in factorial designs and sometimes analyzed with polynomial regressions. In factorial designs with mixtures of, for example, biologically active compounds, the

interaction is based on the effects of the allelochemicals or herbicides and merely

tells us whether an effect of a allelochemical or herbicide remains unchanged in

mixture with another allelochemical or herbicide. Interaction will inevitably occur if the dose range is wide enough because at very low and very high doses the

responses approach the upper and lower limits of the dose-response curve. Consequently, such interactions are of little biological relevance but have been extensively used to claim antagonism and/or synergism (Nash, 1981). A more general

way to describe the joint action of allelochemical or herbicide mixtures is to use

the response curves of the allelochemicalsor herbicides applied alone and in mixtures and incorporate various joint action reference models, for example, the additive dose model and the multiplicativesurvival model outlined by Morse (1978).

Unfortunately, very few investigations have explicitly defined the mixture model

used (Streibig, 1992; Streibig et al., 1998).



B. AUTOTOXICITY

Autotoxic effects have often been discussed in allelopathic research. Autotoxicity, however, technically differs from allelopathy because (i) autotoxicity means

self-toxicity, and allelopathiceffects refer to the effects of one plant on another (including microorganisms) through the release of chemical compounds into the environment, and (ii) allelopathic effects can be both stimulatory and inhibitory,

whereas autotoxic effects are only inhibitory effects. While allelopathy is generally considered as a mechanism of interference, allelopathic compounds are also



200



INDERJIT AND K. IRWIN KEATING



considered as defense compounds. It is not certain why an organism would produce autotoxic compounds. The ecological role of autotoxic compounds may be

an open question, but unanticipated trace element deficiencies must be ruled out

before autotoxicity is assumed. It has been shown that some grassland forms grow

better when grown with leachates from their own species (Newman and Miller,

1977). These effects of beneficial growth response to self-produced allelopathic

chemicals cannot be considered a result of autotoxicity.

Lodhi (1979) reported autotoxic activities of phytotoxins from kochia. He concluded that a drastic reduction in growth of kochia during its second year of revegetation was due to its autotoxic properties. Autotoxicity in coffee (Cofleu urubicu), due to an alkaloid caffeine and theophylline, is reported by Waller (1989).

Autotoxicity has been well documented in asparagus plants (Friedman and Waller,

1985; Young and Chou, 1985; Hegde and Miller, 1990; Friedman, 1995).Asparagus replant problems and reduction in yield of asparagus for old plantations have

been reported (Klein and Miller, 1980; Kehr et al., 1983; Young, 1986). In a discussion of asparagus replant problems in tropical Taiwan, Young (1986) suggests

an interval period of 2 or 3 years between asparagus plantings. Asparagus is native to seacoast, riverside, and semidesert areas from southern Europe to southern

Russia. These areas have well-drained sandy and sandy-loam soils (Young, 1986).

The better growth of asparagus in its native soils could be attributed to (i) better

leaching of chemicals in sandy soils and (ii) better aeration for degradation of

chemicals leached down by the asparagus plants. Miller (1983) reported that significantly lower seedling populations and second-year yield of asparagus cannot

be improved by eliminating N and P deficiencies; however, unrecognized trace nutrient limitations may be involved. Read and Jensen (1989) reported that certain

water-soluble compounds released from decomposing residues of asparagus may

have autotoxic effects. While investigating the autotoxic potential of mesquite

(Prosopisjulijbru),Warrag (1995) found that aqueous extracts of mesquite foliage

had autotoxic effects on its seed germination and early growth. However, it is important that the autotoxic potential of mesquite be tested under more natural conditions involving soils in bioassay experiments.

Waller (1989) suggested that autotoxic compounds are present in the outer part

of seeds and diaspores of certain plants. Such seeds can germinate only after these

compounds are washed away with rainwater or metabolized by microorganisms.

He applied the term “natural protectants” to those compounds which are difficult

to leach out from seeds and which have inhibitory effects when applied exogenously. Friedman and coworkers (1983) reported that an autotoxic compound, 8methoxypsorlaen, is stored in the outer dead layer of fruit of bishop’s weed (Arnrni

rnujus). This compound can only be released if the outer shell is broken; thus, the

embryo remains unaffected.

Aert’s group (1991a,b) reported that mixtures of alkaloids, cinchoncine, dihydrocinchonine, and quinamine (2:7:1) are strongly inhibitory to the seed germina-



ALLELOPATHY



201



tion of Cinchona ledgerianu seeds. The main alkaloids synthesized during germination were cinchonine and its dihydro derivatives, and minor amounts of one indole alkaloid quinamine were synthesized. These authors suggested that balancing and compartmentalization of alkaloids was probably the mechanism for

avoiding autotoxicity.

Production of nonprotein amino acids is a well-reported mechanism for avoiding autotoxic effects (McKey, 1979). Many nonprotein amino acids are toxic to

plants but do not show autotoxic activities (Friedman, 1995). Fowden and Lea

(1979) reported that proline is stored in shoots of Convulluriu mujulis and has no

toxic effects on the producer. However, it resulted in toxocity when applied to

mung bean (Phaselous uureus). Friedman (1995) explained the mechanism which

producers use to avoid autotoxicity due to nonprotein amino acids. The activation

of the amino acids by aminoacyl-tRNAsynthetaseoccurs prior to mRNA and chain

initiation and termination factors determine the nature of the protein. This results

in activation of analogs instead of common protein amino acids. However, in

plants that produce nonprotein amino acids, the aminoacyl-tRNA synthetase differentiates between the analog and protein amino acid. These mechanisms can explain the method of avoiding autotoxicity from chemicals present inside the plant

cell. Research is needed to investigate how plants avoid autotoxicity from chemicals present in their rhizospheres. Williamson (1990) discussed different mechanisms by which plants might avoid autotoxicity: (i) Allelopathic compounds are

produced after their removal from the donor plant; (ii) microorganisms present in

the rhizosphere produce allelopathic compounds; and (iii) less toxic compounds,

produced by the donor plant, may be degraded into more toxic compounds.



C. PRACTICAL

CONSIDERATIONS

Laboratory bioassays have certain limitations; however, they are an important

integral part of allelopathic research (Leather and Einhellig, 1986; Inderjit and

Dakshini, 1995a). It is true that laboratory experimental conditions exactly simulating those in the field represents an impossible goal, but one can and should avoid

steps which widen the gap between laboratory bioassays and field interactions.

Laboratory study can best be applied in situations which allow close examination

of carefully isolated components of the complex natural system. Often, to assess

allelopathic potential, preliminary bioassays are performed with leachate or extracts of allelopathic plants or artificial soil or natural soil is amended with debris

(Rice, 1984, 1995). The mere presence of allelopathic compounds in plant parts

does not demonstrate allelopathy (Heisey, 1990). Fisher (1979, p. 327) states that

“it seems unlikely that the allelochemicalsthat may be extracted from plant material are actually those that reach the host plant, yet all our information on allelopathic compounds is derived from extracts that have never been exposed to soil.”



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VIII. Roles for Allelopathy in Biocontrol Programs

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