3. Long-chain fatty acids and polyacetylenes.
4. Quinines (benzoquinone, anthraquinone, and complex quinines).
5. Phenolic.
6. Cinnamic acid and its derivatives.
7. Coumarins.
8. Flavonoids.
9. Tannins.
10. Steroids and terpenoids (sesquiterpene lactones, diterpenes, and triterpenoids).
2.1 Biosynthetic Pathways of Allelochemicals
Allelochemicals, such as polyphenols, include several classes of structurally different natural products and biogenetically originating from the shikimatephenylpropanoids-flavonoids pathways. In comparison with animals, plants produce
a vast spectrum of plant secondary metabolites because of their immobile nature
which made them impossible to get away from its enemies or competitors; thus, they
developed such a biochemical-based defense mechanism to protect them from their
predators. There are divergent principles to classify allelochemicals. Some of them
are based on the carbon skeleton structure and functional groups. Some
allelochemicals are classified based on the biogenetic origin of metabolites (Latif
et al. 2017). Based on the carbon skeleton and functional groups, allelochemicals can
be further divided into alkaloids, fatty acids, flavonoids, phenolic compounds,
quinones, and more common terpenoids (Latif et al. 2017). Quinones and phenolics
are the maximum frequently characterized classes of allelopathic phytotoxins
(Inderjit 1996). Quinones are oxidized phenols, and phenols are produced from the
redox reaction of quinones, and changes between these chemical states have physiological and biological connotations (Harborne 1989). The group of allelochemicals
and their ecological role were tabulated in Table 5.1.
Allelochemicals are commonly called “secondary metabolites” synthesized by
organisms, such as plants, animals, or microorganisms, and are not needed for basic
(primary) metabolism. Allelochemicals are produced by using three different biosynthetic pathways, viz., shikimic acid, acetate-malonate, and mevalonic acid, which
are the main precursors to biosynthesize the allelopathic compounds. So far, various
allelochemicals have been recognized and analyzed from a range of plant species.
These allelochemicals are chemically diverse, being represented by phenolic compounds (simple phenolics, flavonoids, coumarins, and quinones), terpenoids (monoterpenes, sesquiterpenes, diterpenes, triterpenes, and steroids), alkaloids and
nitrogen-containing chemicals (nonprotein amino acids, benzoxazinoids, cyanogenic glycosides), and many other chemical families. It is holding the researcher’s
attention that secondary allelochemicals are biosynthesized substantially from a few
primary metabolites: α-amino acids, acetyl-coenzyme A, mevalonic acid, and intermediates of the shikimic acid pathway (Hebert 1981). The biosynthetic pathways of
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4. Quinines (benzoquinone, anthraquinone, and complex quinines).
5. Phenolic.
6. Cinnamic acid and its derivatives.
7. Coumarins.
8. Flavonoids.
9. Tannins.
10. Steroids and terpenoids (sesquiterpene lactones, diterpenes, and triterpenoids).
2.1 Biosynthetic Pathways of Allelochemicals
Allelochemicals, such as polyphenols, include several classes of structurally different natural products and biogenetically originating from the shikimatephenylpropanoids-flavonoids pathways. In comparison with animals, plants produce
a vast spectrum of plant secondary metabolites because of their immobile nature
which made them impossible to get away from its enemies or competitors; thus, they
developed such a biochemical-based defense mechanism to protect them from their
predators. There are divergent principles to classify allelochemicals. Some of them
are based on the carbon skeleton structure and functional groups. Some
allelochemicals are classified based on the biogenetic origin of metabolites (Latif
et al. 2017). Based on the carbon skeleton and functional groups, allelochemicals can
be further divided into alkaloids, fatty acids, flavonoids, phenolic compounds,
quinones, and more common terpenoids (Latif et al. 2017). Quinones and phenolics
are the maximum frequently characterized classes of allelopathic phytotoxins
(Inderjit 1996). Quinones are oxidized phenols, and phenols are produced from the
redox reaction of quinones, and changes between these chemical states have physiological and biological connotations (Harborne 1989). The group of allelochemicals
and their ecological role were tabulated in Table 5.1.
Allelochemicals are commonly called “secondary metabolites” synthesized by
organisms, such as plants, animals, or microorganisms, and are not needed for basic
(primary) metabolism. Allelochemicals are produced by using three different biosynthetic pathways, viz., shikimic acid, acetate-malonate, and mevalonic acid, which
are the main precursors to biosynthesize the allelopathic compounds. So far, various
allelochemicals have been recognized and analyzed from a range of plant species.
These allelochemicals are chemically diverse, being represented by phenolic compounds (simple phenolics, flavonoids, coumarins, and quinones), terpenoids (monoterpenes, sesquiterpenes, diterpenes, triterpenes, and steroids), alkaloids and
nitrogen-containing chemicals (nonprotein amino acids, benzoxazinoids, cyanogenic glycosides), and many other chemical families. It is holding the researcher’s
attention that secondary allelochemicals are biosynthesized substantially from a few
primary metabolites: α-amino acids, acetyl-coenzyme A, mevalonic acid, and intermediates of the shikimic acid pathway (Hebert 1981). The biosynthetic pathways of
5 Allelochemicals as Natural Herbicides for Sustainable Agriculture to Promote. . .
97