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V. Secondary Metabolites with Allelopathic Potential
INDERJIT AND K. IRWIN KEATING
and + a n
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
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).
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
c. OTHER CLASSES OF SECONDARYMETABOLITES &OWN
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
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
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-
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.
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.
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