Concept and Definition

• Allelopathy is the phenomenon where one plant produces and releases secondary metabolites (allelochemicals) into the environment that influence germination, growth, survival, or reproduction of other plants (crops, weeds, or same species).
• Term coined by Hans Molisch (1937) from Greek words:
  • “Allelon” = of each other
  • “Pathos” = to suffer
• Sometimes also called teletoxy (chemical warfare among plants).

Mechanisms of Allelochemical Release

Plants release allelochemicals through:

• Volatilization (Vapour release) Volatile compounds released from leaves evaporate into atmosphere. Example: Eucalyptus oils, members of Labiateae and Compositae families.
• Leachates from Foliage Rainfall or dew washes soluble chemicals from leaves, which reach the soil and inhibit germination of nearby seeds. Example: Eucalyptus leachates → suppress Cynodon dactylon and Cyperus rotundus.
• Root Exudation Roots secrete chemicals into rhizosphere. Example: Cirsium arvense (Canada thistle) → inhibits oats. Sorghum roots → release sorgoleone, which suppresses Echinochloa crus-galli.
• Residue Decomposition When leaves, stems, or roots decay, allelochemicals are released into soil. Example: Sunflower and Rice residues → suppress germination of several weeds.

Types of Allelochemicals

• Phenolics – ferulic acid, caffeic acid, vanillic acid.
• Terpenoids – cineole (Eucalyptus), parthenin (Parthenium hysterophorus), sorgoleone (Sorghum).
• Alkaloids – quinoline, pyridine derivatives.
• Flavonoids, coumarins, tannins.

👉 These compounds interfere with cell division, nutrient uptake, photosynthesis, enzyme activity, and hormone balance in susceptible plants.

Examples of Allelopathy

(a) Weeds on Crops (Negative Impact)

• Parthenium hysterophorus → parthenin → inhibits wheat, maize, pulses.
• Cyperus rotundus (nut sedge) → reduces maize and groundnut growth.
• Cirsium arvense (Canada thistle) → root exudates suppress oats.
• Chenopodium album → interferes with wheat germination.

(b) Weeds on Other Weeds

• Imperata cylindrica → root exudates suppress annual weeds (Borreria hispida).
• Parthenium can inhibit other broad-leaved weeds, allowing its dominance.

(c) Crops on Weeds (Beneficial for Weed Control)

• Sorghum bicolor → sorgoleone (from roots) → inhibits Echinochloa crus-galli.
• Sunflower residues → suppress Amaranthus, Chenopodium.
• Wheat and Oats → root exudates suppress Chenopodium album.
• Rice straw incorporation → inhibits germination of certain weeds.

(d) Trees on Weeds

• Eucalyptus globulus → leaf leachates suppress Cyperus rotundus and Cynodon dactylon.
• Acacia → phenolics in litter suppress understory weeds.

Allelopathic Control of Weeds (Botanicals)

Plants themselves (extracts, powders, residues) can be used as bio-herbicides:

• Tagetes erecta (Marigold) → suppresses Parthenium.
• Dry Dodder (Cuscuta powder) → controls Eichhornia crassipes (water hyacinth).
• Sorghum and Sunflower mulches → reduce weed flora in crops.
• Neem, Mustard, Rice bran extracts → have selective weed suppression effects.

Ecological & Agronomic Significance

Negative Effects (Problem in Cropping System):

• Weeds release toxins that reduce germination and growth of crops.
• Example: Parthenium reducing cereal yields; Cyperus suppressing groundnut.

Positive Effects (Weed Management Potential):

• Some crops naturally suppress weeds – can be exploited in:
  • Allelopathic crop rotations (e.g., sorghum before maize).
  • Residue mulching (sunflower, rice straw).
  • Botanical extracts (natural herbicides).
  • Cover crops & green manures (mustard, sesbania).

Role in Integrated Weed Management (IWM)

• Allelopathy provides a sustainable and eco-friendly tool in weed control.
• Reduces dependence on synthetic herbicides, thereby preventing resistance and environmental pollution.
• Integrated use of allelopathic crops + herbicides + cultural practices → ensures better weed suppression.