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V. Secondary Metabolites with Allelopathic Potential

V. Secondary Metabolites with Allelopathic Potential

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INDERJIT AND K. IRWIN KEATING

HO



Shikimic acid



-1



Some quinoner



Flavonolds

and + a n



lroflavonoidr



Fiavolanr



Phenylalanine



p-Hydroxycinnamio acid



+



3malonrte

thonu

and

Hydroxyrtilbsn-



\HO

pSlydroxydnr\.myl alcohol



A I--.-I



Phenyiproponu



Llgnans



UgniM



Figure 10 Biosynthetic origin of plant phenolics from shikimate and phenylalanine pathways

[reprinted from Harbome, J. B., 1989, General procedures and measurement of total phenolics, In

“Methods in Plant Biochemistry: Plant Phenolics” (J. B. Harborne, Ed.), Vol. I, pp. 1-28, by permission of the publisher Academic Press Limited, London].



from shikimate and phenylalanine pathways (Fig. 10). The major classes of phenolics are (i) simple phenols and benzoquinones (C,) and (ii) phenolic acids (C6C acetophenones and phenylacetic acids (C6-C2); hydroxycinnamic acids,

phenylpropanes, coumarins, isocoumarins, and chromones (C,-C,); naphthoquinones (C6-C4); xanthones (C,-C, -C6); stilbenes and anthraquinones (C6C,-C,); flavonoids and isoflavonoids (C6-C3-C6); lignans and neolignins [(C,C,),]; biflavonoids [(C,-C,-C,),]; lignins [(C,-C,),]; catechol melanins [(C,),];

and flavolans, i.e., condensed tannins [(C,-C,-C,),J (Harborne, 1989).

The allelopathic potential of simple phenols, benzoic and cinnamic acid derivatives, flavonoids, and tannins is well demonstrated in the literature (Rice, 1984,

1995; Indejit et al., 1995). Fisher (1979, p. 323) stated that, “Phenolics comprise

the largest group of secondary compounds in plants and are more often identified

as allelopathic agents than all other compounds put together.” Furthermore, phenolic compounds are water soluble and could easily be leached by rain, whereas

leaves are still attached to the plant or, thereafter, from leaf litter (Alsaadawi et

al., 1985). Water-soluble compounds are of even more ecological relevance in situations in which irrigation is frequent (Del Moral and Muller, 1970); however,

all water-soluble compounds are not always allelopathic in nature. Many highly



ALLELOPATHY



183



water-soluble compounds have low biological activity, whereas many slightly

water-soluble compounds have high biological activity (J. D. Weidenhamer, personal communication). From the standpoint of allelopathy, as long as the solubility exceeds concentrations required for biological activity, the compound

should be regarded as potentially able to exert allelopathic effects (Weidenhamer

et al., 1993).



B. PLANTTERPENOIDS

The different classes of terpenoids are mono (Clo), sesqui- (CIS),di- (C,,), tri(Gershenzon, 1994). There are several reviews on

the ecological, physiological, and biochemical aspects of terpenoids in the Journal of Chemical Ecology (Fischer et al., 1994; Gershenzon, 1994; Langenhein,

1994; Takabayashi et al., 1994; White, 1994). Certain terpenoids are produced

solely for defense purposes, i.e., in response to herbivory or to pathogen attack

(Takabayashi et al., 1994; Gershenzon, 1994). The enzymes responsible for induction of such terpenoids are not detected in healthy plants or in plants not subjected to herbivory but are known to occur from infected plants or plants under

herbivory stress (Gershenzon, 1994). For example, Gershenzon and Croteau

(199 1) reported that grand fir (Abies grandis) produces large amounts of monoterpenes after being wounded, and these monotepnoids serve as defense against bark

beetles and fungi.

Terpenoids are the second largest group (after phenolics) of secondary metabolites implicated in allelopathy.The allelopathhic potential of monoterpenoids (e.g.,

camphene, 1,&cineole, a-pinene, P-pinene, dipentene, a-phellandrene, pcymene, piquerol A, piquerol B, limonene, borneol, and pulegone) is well reported (Muller and Chou, 1972; Gant and Clesbsh, 1975; Nishimura et al., 1982;

Fischer, 1986; Weidenhamer ef al., 1993). Weidenhamer and coworkers (1993)

suggested that unsaturated solutions of monoterpenoids in a natural system may

possess significant allelopathic activities. Fischer (1986) discussed the allelopathic potential of several sesquiterpenoids, e.g., P-bisaabolene, P-caryophyllene,

bergaamotene, a-guayene, a-bulnosene. P-patchoutin, (E,E)farnesol, p-selinene,

vitrenal, phomenone, metabpodin B, and cinerenin. Fischer and coworkers (1 994)

demonstrated that allelopathy is a mechanism restricting the fire-prone grasses and

pines from invading scrub communities in Florida and implicated terpenoids

as probable allelopathic candidates. Finally, aqueous leachates of Conradim

canescens significantly inhibited the sandhill grasses, e.g., Schizachyrium scoparium, due to the presence of monoterpenes (1,8-cineole, carveol, carvone, a-terpineol, camphor, bomeol, myrtenal, and myrtenol) and some triterpenoids (ursolic

acid and betulin).

(C3J, and tetra terpenoids (C,)



INDERJIT AND K. IRWIN KEATING



184



c. OTHER CLASSES OF SECONDARYMETABOLITES &OWN

TO



POSSESS

ALLELOPATHIC

ACTMTIES



Alkaloids have received considerable attention for their allelopathic activities.

Alkaloids possess nitrogen in a heterocyclic ring or side chain and generally occur in plants as salts of organic acids (Wink, 1983; Rice, 1984; Levitt and Lovett,

1985; Waller, 1989). Lovett and coworkers (1987) reported the allelopathic potential of hyoscyamine and scopolamine from thornapple. Thereafter, Lovett

(1989) showed that the alkaloids gramine and hordenine, produced by grain barley, interfere allelopathically with seedling growth of white mustard. Many other

alkaloids (e.g., scopolamine, hyoscyamine, caffeine, theophylline, theobromine,

paraxantheine, colchicine, podophyllotoxin, and vinblastine) have been suggested to possess allelopathicactivities (Worsham, 1989;Wink and Twardowski, 1992;

Wink and Latz-Briining, 1995). Waller and Burstom (1969) reported that diterpenoid alkaloids, delcosine and ajacocnine, from Delphinium ajacis had allelopathic effects on cambium growth of the pea.

Stevens (1986a) discussed the allelopathic potential of polyacetylenes known

to possess allelopathic activities. He (1986b) reported polyacetylenes from Russian knapweed and demonstrated their allelopathic potential. Griimmer (1961) reported antimicrobial activities of agropyrene, a polyacetylene produced from

quackgrass. Also, the polyacetylene cis-dehydromatricaria ester from Solidago

altissmia and cis- and trans-matricaria and cis-lachnophyllum from Erigeron annuus have been reported to possess allelopathic activities (Rice, 1984). However, little information is available on the allelopathic potential of polyacetynes, and

it is important to demonstrate the allelopathic activities of the polyacetylenes in

nature.

Although the allelopathic potential of one class of secondary metabolites may

be demonstrated, the possible involvement of compounds from another class cannot automatically be ruled out. To date, the determination of such allelopathic activity has been serendipitous, focusing on a particular class of compounds depending on the amount of compound of a particular class detected, on its biological

activity, and on the personal research interests, expertise, and facilities available

in a given laboratory.



VI. MECHANISMS OF ACTION OF

ALLELOPATHIC CHEMICM,S

Allelopathy is often categorized under ecological chemistry/chemical ecology

or physiological ecology. In 1969, while discussing chemical interactions among

organisms, Hegnauer suggested the term ecological chemistry. Ecological chem-



ALLELOPATHY



185



istry involves using chemistry and biochemistry to explain ecologically significant

interactions among organisms (Towers er al., 1989), as distinguished from the

more general term physiological ecology, in which physiology is used to explain

ecological interactions. When we identify some plant to plant interference in nature, we first need to identify an ecological interplay, i.e., whether the observed

pattern is best explained by allelopathy, resource competition, microbial nutrient

immobilization,etc. Once we identify the problem, and demonstrate that allelopathy best explains the observed growth pattern, we need to study the physiological/

biochemical mechanisms of action of allelopathic chemicals. In this section, we

will discuss some of the important physiological/biochemicalmechanisms of action of allelopathic chemicals in allelopathy.

Various workers discussed the mechanisms of action of allelopathic chemicals

in allelopathy (Rice, 1984;’Muller,1986; Einhellig, 1986, 1995b; Waller, 1989).

We will discuss how allelopathic chemicals interfere with various physiological,

biochemical, and molecular processes of target plant species.



A. INTERFERENCE

WITH CELL

ELONGATION

Allelopathic chemicals play an important role in the regulation of plant cell

growth, and there are many reports on the interference of allelopathic chemicals

with cell elongation and cell division (Muller, 1965; Jankay and Muller, 1976;

Rice, 1984; Ortega et al., 1988).Many bioassays for allelopathhy employ seed germination, seedling lengths, or fresh seedling weight, to quantify allelopathic effects. Wink and Latz-Briining (1995) reported that many salts, amino acids, sugars, phenolic compounds, organic acids, terpenoids, and alkaloids influence the

hypocotyl elongation and root growth of garden cress (Lepidiurn sativurn).Aliotta and coworkers ( 1993)investigated the interference of several phenylpropanoids

and coumarins with germination and subsequentroot growth of radish. They found

that coumarins inhibited cell elongation of the differentiating zone of the root.

They also noted an apical shift of root hair differentiation to form tufts not observed in the control. Li and coworkers (1993) reported that juglone, at concentrations of lop4 and

M, inhibited cell elongation in the epicotyl sections of

etiolated bean (F! sativurn) seedlings.



B. INTERFERENCE

w r r PHOTOSYNTHESIS

~

Several studies have shown adverse effects of allelopathic compounds on photosynthesis (Rice, 1984).Einhellig’sgroup ( 1970) reported a significantreduction

in photosynthesis of tobacco plants when treated with lop3 and lop4M concentrations of scopoletin. Several workers reported a reduction in photosynthesis in



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