Allelopathy is a biological phenomenon where organisms produce biochemicals, known as allelochemicals, that impact the growth and reproduction of other organisms, with both positive and negative effects. This process is significant in plant communities and can be utilized in weed management, as specific allelochemicals can inhibit the growth of harmful plants. Additionally, research supports the development of new herbicides from naturally occurring allelochemicals to combat resistance in weeds.

1. Allelopathy
Msc-2, Paper 3, Unit 3
Ms. Ankita A. More
Patkar-Varde college, Mumbai
Ms. Ankita A. More

2. Allelopathy is a biological phenomenon by which an organism produces one or more
biochemicals that influence the growth, survival, and reproduction of other organisms.
These biochemicals are known as allelochemicals and can have beneficial (positive
allelopathy) or detrimental (negative allelopathy) effects on the target organisms.
Allelochemicals are a subset of secondary metabolites, which are not required for
metabolism (i.e. growth, development and reproduction) of the allelopathic organism.
The term allelopathy or Teletoxy was introduced by Molisch (1937).
Allelopathic substance was first detected by Davis (1928) in black walnut tree (Juglans
nigra) whose foliar leachate containing Juglone was found to damage germination and
seedling growth of crops beneath the tree.
Ms. Ankita A. More

3. Allelopathy is characteristic of certain plants, algae, bacteria, coral, and fungi.
Allelopathic interactions are an important factor in determining species distribution and
abundance within plant communities.
Allelochemicals are found to be released to environment in appreciable quantities via root
exudates, leaf leachates, roots and other degrading plant residues, which include a wide
range of phenolic acids such as benzoic and cinnamic acids, alkaloids, terpenoids and
others.
These compounds are known to modify growth, development of plants, including
germination and early seedling growth.
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4. Ms. Ankita A. More

5. Allelochemicals are released in the form of:
Vapour (released from plants as vapour): Some weeds release volatile compounds from
their leaves. Plants belonging to labiateae, compositeae yield volatile substances.
Leachates (from the foliage): From Eucalyptus allelo chemicals are leached out as
water toxins from the above ground parts by the action of rain, dew or fog.
Exudates from roots: Metabolites are released from Cirsium arvense roots in
surrounding rhizosphere.
Decomposition products of dead plant tissues and warn out tissues
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6. Allelopathy in weed management
For instance Dry dodder powder has been found to inhibit the growth of water hyacinth and
eventually kill the weed.
Likewise carrot gross powder found to detrimental to other aquatic weeds.
The presence of marigold (Tagetes erecta) plants exerted adverse allelopathic effect on
Parthenium spp. growth.
The weed sena (Cassia sp) show suppressive effect on Parthenium.
The Eucalyptus tree leaf leachates have been shown to suppress the growth of nut sedge and
bermuda grass.
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7. Allelopathic effects of weeds on crop plants
Root exudates of Canada thistle (Cirsium sp.) injured oat plants in the field.
Root exudates of Euphorbia injured flax. But these compounds are identified as
parahydroxy benzoic acid.
Maize
Leaves & inflorescence of Parthenium sp. affect the germination and seedling growth.
Tubers of Cyperus esculentus affect the dry matter production Quack grass produced
toxins through root, leaves and seeds interfered with uptake of nutrients by corn.
Cirsium
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8. Sorghum
Stem of Solanum affects germination and seedling growth.
Leaves and inflorescence of Parthenium affect germination and seedling growth.
Wheat
Seeds of wild oat affect germination and early seedling growth.
Leaves of Parthenium affects general growth.
Tubers of C. rotundus affect dry matter production.
Green and dried leaves of Argemone mexicana affect germination & seedling growth.
Sunflower
Seeds of Datura affect germination & growth.
Argemone mexicana
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9. Effect of weed on another weed
Thatch grass (Imperata cylindrica) inhibited the emergence and growth of an annual broad
leaf weed (Borreria hispida).
Extract of leaf leachate of decaying leaves of Polygonum contains flavonoides which are
toxic to germination, root and hypocotyls growth of weeds like Amaranthus spinosus.
Inhibitor secreted by decaying rhizomes of Sorghum halepense affect the growth of
Digitaria sanguinalis and Amaranthus sp.
In case of Parthenium, daughter plants have allelopathic effect on parent plant. This is called
Autotoxy.
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10. Effect of crop on weed
Root exudates of wheat, oats and peas suppressed Chenopodium album. It increased
catalase and peroxidase activity of weeds and inhibited their growth.
Cold water extract of wheat straw reduces growth of Ipomea & Abutilon.
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11. Stimulatory effect
Root exudates of corn promoted the germination of Orbanchae minor; and Striga
hermonthica.
Kinetin exuded by roots sorghum stimulated the germination of seeds of Stirga asisatica.
Strigol – stimulant for witch weed was identified in root exudates from cotton.
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12. Development of Herbicides from Allelochemicals
With the increase use of herbicide, weed became resistant against these herbicides. So
herbicide with new mode of action is needed today.
Various allelochemicals with herbicidal activity have been isolated from different
Crops and allelochemicals with herbicidal activity can be categorized into two major
groups: phenolics and terpenoids .
One research study reported that phenolic compounds showed their allelopathic potential by
inhibiting symbiotic relationship between rhizobium and legume.
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13. These natural phytotoxins offer a great opportunity to develop herbicides with a safe mode
of action . Another study investigated that Veronica persica (Lour.) Merr. had effective
herbicidal activity; thus a safer herbicide from allelochemicals can be developed.
Eucalyptus globulus leaf aqueous extract was tested against the germination potential and
early growth of Lactuca sativa and Agrostis stolonifera.
Results showed that this extract has an inhibitory effect on both target species. Also spraying
treatment reduced both aerial and root biomass and reduced protein contents and
chlorophyll concentrations.
HPLC analysis confirmed the presence of eight phenolic compounds (chlorogenic, two ρ-
coumaric derivatives, ellagic, hyperoside, rutin, quercitrin, and kaempferol 3-O-glucoside)
and other five low weight organic acids .
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14. Research study reported bioherbicidal potential of leaf extract of Delonix regia on
germination and seedling growth of field Convolvulus arvensis and Triticum aestivum L. .
Result shows that lower aqueous concentration (2.5% and 5%) and ethyl acetate (50, 500,
1000 ppm) of leaf extract of Delonix regia inhibited germination, root length, shoot length,
and seedling dry biomass of Convolvulus arvensis.
Convolvulus arvensis
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