Allelopathy: Meaning and Importance in Crop Production

Meaning / Definition; Allelopathy is the biochemical interaction among plants, microorganisms, or both, in which one plant releases chemical compounds (called allelochemicals) that influence the growth, germination, or development of another plant — either positively or negatively.

Molisch (1937) first coined the term “Allelopathy” from the Greek words:

• “Allelon” = mutual
• “Pathos” = suffering or feeling
  Hence, Allelopathy = Mutual effects (positive or negative) among plants.

Definition (Rice, 1984): “Allelopathy refers to any direct or indirect, harmful or beneficial effect of one plant (including microorganisms) on another through the production of chemical compounds that escape into the environment.”

Allelochemicals; Allelochemicals are the secondary metabolites released by plants into the environment, affecting other organisms.
These chemicals may be released through:

• Volatilization (e.g., terpenes from eucalyptus)
• Leaching (from leaves and litter by rainwater)
• Root exudation (from living roots into the rhizosphere)
• Decomposition of residues (release of phenolic acids, alkaloids, etc.)

Major Chemical Classes of Allelochemicals

Types of Allelopathic Interactions

 Mechanisms of Allelopathic Action

Allelochemicals influence physiological and biochemical processes in other plants, including:

1. Inhibition of seed germination
2. Reduction of root and shoot growth
3. Alteration of cell membrane permeability
4. Suppression of photosynthesis and respiration
5. Inhibition of enzyme activity (amylase, catalase, peroxidase)
6. Disturbance in nutrient uptake and water absorption

Examples of Allelopathic Effects in Crops

Importance of Allelopathy in Crop Production

1. Weed Management (Natural Herbicide Source)

• Certain crops like sorghum, sunflower, and rice release allelochemicals that suppress weed germination and growth.
• Sorghum water extract (SWE) or Sorghum residue mulch acts as a bio-herbicide in integrated weed management.

Example: Sorgoleone (from sorghum roots) suppresses growth of Echinochloa crus-galli and Cyperus rotundus.

2. Crop Rotation Planning

• Knowledge of allelopathy helps in choosing compatible crops in rotation.
• Avoid rotation where residues of one crop inhibit the next (e.g., sunflower → wheat causes suppression).
• Legumes are generally beneficial because they release growth-promoting compounds.

3. Residue and Mulch Management

• Crop residues should be properly decomposed before sowing the next crop to prevent allelopathic inhibition.
• Incorporation timing matters — e.g., allow 2–3 weeks between incorporation and sowing.

4. Natural Growth Regulation

• Some allelochemicals promote growth or seed germination at low concentrations (known as stimulatory allelopathy).
• Example: Certain phenolics at low doses enhance root growth and nodulation in legumes.

5. Sustainable Agriculture and Organic Farming

• Use of allelopathic cover crops (e.g., rye, sorghum, mustard) helps in weed suppression without chemicals.
• Supports eco-friendly weed control and reduces dependency on synthetic herbicides.

6. Soil Health and Microbial Balance

• Some allelochemicals influence soil microbial activity — can suppress harmful pathogens or enhance beneficial microbes.
• Example: Brassica species release isothiocyanates that suppress soil-borne diseases.

Role in Agroecosystem

Applications in Modern Agriculture

• Bioherbicide Development: Extracts of allelopathic plants (e.g., sorghum, sunflower, eucalyptus) are used to develop natural herbicides.
• Allelopathic Cover Crops: Crops like rye, barley, oats, and sorghum used as green manures suppress weed emergence.
• Integrated Weed Management (IWM): Combining allelopathic crops with reduced herbicide doses for sustainable control.
• Breeding for Allelopathic Varieties: Development of crop varieties with strong allelopathic potential (e.g., allelopathic rice for weed suppression).

🔹 Recent Research Highlights (India)

• ICAR & PAU (Ludhiana) have developed rice and wheat varieties showing allelopathic suppression against weeds.
• Sorghum and sunflower residues studied as natural weed suppressants in maize and wheat systems.
• Rice allelopathy is being explored as a tool for eco-friendly weed management in direct-seeded rice (DSR) cultivation.

Example:

• Crop-to-Crop Allelopathic Examples

Autotoxic (Same Crop Affected by Its Residue)

3. Allelopathy Between Crops and Weeds

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