Allelopathy is the detrimental effects of chemicals or exudates produced by one (living) plant species on the germination, growth or development of another plant species (or even microorganisms) sharing the same habitat.
Allelopathy does not form any aspect of crop-weed competition, rather, it causes Crop-Weed interference, it includes competition as well as possible allelopathy.
Allelo-chemicals are produced by plants as end products, by-products and metabolites liberalized from the plants; they belong to phenolic acids, flavonoides, and other aromatic compounds viz., terpenoids, steroids, alkaloids and organic cyanides.
These allelochemical’s action is in interfering with cell elongation, photosynthesis, respiration, mineral ion uptake and protein and nucleic acid metabolism.
Allelopathy technique can be applied in biological control of weeds by using cover crop for biological control and using alleopathic chemicals as bio-herbicides.
Factors influencing allelopathy
(a) Plant factors
• Plant density: Higher the crop density the lesser will be reaction due to allelochemicals it.
• Life cycle: If weed emerges later there will be less problem of allelochemicals.
• Plant age: The release of allelochemicals occurs only at critical stage. For e.g., in case of Parthenium, allelopathy occurs during its rosette and flowering stage.
• Plant habit: The allelopathic interference is higher in perennial weeds.
• Plant habitat: Cultivated soil has higher values of allelopathy than uncultivated soil.
• Climatic factors: The soil and air temperature as well as soil moisture influence the allelochemicals potential.
• Soil factors- Physico-chemical and biological properties influence the presence of allelochemicals.
• Stress factors– Abiotic and biotic stresses may also influence the activity of allelochemicals.
(i) Effect of weeds on crops
Maize – Leaves and inflorescence of Parthenium sp. affect the germination and seedling growth and tubers of Cyperus esculentus affect the dry matter production.
Sorghum – Stem of Solanum affects germination and seedling growth and 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 and green and dried leaves of Argemone mexicana affect germination and seedling growth.
Sunflower – Seeds of Datura sp. affect germination and growth.
(ii) Effect of crop plants on weeds
• Root exudation of maize inhibits the growth of Chenopodium album.
• The cold water extracts of wheat straw when applied to weeds reduce germination and growth of Abutilon sp.
(iii) Effect of weeds on weeds
• 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.
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