What is allelopathy? Discuss its role in major cropping systems of irrigated agriculture.

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Ghissu Editorial · Accepted Answer

Allelopathy is the effect of one plant on another through chemicals released from roots and residues, and in irrigated systems it both suppresses weeds and depresses crops. Rice roots exude momilactone B against barnyard grass, while freshly incorporated rice straw can depress wheat germination.

**Introduction**

Allelopathy, a term coined by Hans Molisch in 1937, is the inhibitory or stimulatory effect of one plant on another through allelochemicals released by root exudation, leaching or residue decomposition. In irrigated cereal systems it is both a tool and a constraint.

**Allelopathy as a Weed and Pest Tool**

SKETCH: flow | Allelopathy at Work
- Roots and residues
- Allelochemicals released
- Weed seeds suppressed
- Less herbicide needed

1. **Allelopathic rice**: Rice roots exude momilactone B, which suppresses barnyard grass, one of the worst weeds of paddy, so weed-suppressive cultivars can cut weeding costs in the rice-wheat belt.
2. **Mulches for wheat**: Where Phalaris minor turned resistant to isoproturon in Punjab and Haryana, sorghum and sunflower residues, and sorghum water extract (sorgaab), let farmers lower the herbicide dose.
3. **Brassica break crops**: Mustard residues release isothiocyanates that suppress soil pathogens and nematodes, a biofumigation effect that makes mustard a useful rotation crop in irrigated north-west India.

**Allelopathic Constraints in Sequential Cropping**

1. **Residue toxicity**: Phenolic acids from rice straw incorporated just before sowing can depress wheat germination, so residue timing decides whether straw is a mulch or a yield drag.
2. **Crop autotoxicity**: Yield decline in successive sugarcane ratoons is partly attributed to the crop's own residues, an argument for rotation with pulses.
3. **Weeds and bund trees**: Parthenin from Parthenium and leachates from boundary eucalyptus inhibit the germination and growth of adjoining crops.

**Conclusion**

Allelopathy is a double-edged force. Harnessed through weed-suppressive cultivars, mulches and rotations, and managed where it harms, it can make the rice-wheat and sugarcane belts less herbicide-dependent and more sustainable.

What is allelopathy? Discuss its role in major cropping systems of irrigated agriculture.

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