1. ALLELOPATHY AND WEED MANGEMENT

2. Allelopathy
Allelopathy is a phenomenon involving either direct or indirect and either beneficial or adverse effects of a plant (including microorganisms) on another plant through the release of chemicals in the environment.
Allelopathy is derived from two Greek words, ‘Allelon’ means each other and ‘Pathos’ means to suffer i.e. injurious effects of one upon another.

4. Allelochemicals
Allelochemicals refer mostly to the secondary metabolites produced by plants and are by products of primary metabolic processes (Levin, 1976). They have an allelopathic effect on the growth and development of the same plant or neighboring plants.

7. Sources of allelochemicals
Stem
Leaves&
Flowers
Fruit&
Seed
Root&
Rhizome

8. Release allelochemicals
Allelochemicals may volatilize from the plants to the atmosphere. The volatile vapours may be absorbed directly from the atmosphere by plants, the adsorbed vapours may condensate in dew and fall to ground, these volatile compounds may be absorbed on the soil particles and subsequently taken by plants from the soil solution.
Volatilization
Leaching is the removal of substances from plants by the action of aqueous solvents such as rain, dew, mist, fog and snow.
Leaching

9. Release allelochemicals Decomposition of plant residues
Many compounds are exuded form the roots, which may influence the growth of microorganisms and associated higher plants.
Root exudates
The decomposition of plant residues adds the largest quantity of allelochemicals to the soil. At plant death, materials compartmentalized in cells are released into the environment.
Decomposition of plant residues

10. Changes in Membrane Permeability
Mode of action
allelochemicals can also lead to increased cell membrane permeability. Consequently, cell contents spill and there is increased lipid peroxidation. Finally, there is slow growth or death of plant tissue. In addition, allelochemicals can also inhibit plants from absorbing nutrients from surroundings and affect the normal growth of plants.
Changes in Membrane Permeability

11. Inhibition of Cell Division and Elongation
Mode of action
It was found that allelochemicals could inhibit plant root elongation, cell division, change cell ultra-structure, and then interfere with the normal growth and development of the whole plant.
Inhibition of Cell Division and Elongation

12. Effects on Plant Photosynthes and Respiration
Mode of action
The impact of allelochemicals on the respiration of plants has mainly been shown to involve weakened oxygen absorption capacity, while the impact on photosynthesis has mainly been to reduce the chlorophyll content and photosynthetic rate.
Effects on Plant Photosynthes and Respiration

13. Effects on Various Enzyme Function and Activities
Mode of action
allelochemicals enter through the plant cell membrane and change the activity and function of certain enzymes. can inhibit activities of peroxidase (POD), catalase and cellulose.
Effects on Various Enzyme Function and Activities

14. Effects on Synthesis of Plant Endogenous Hormones
Mode of action
allelochemicals can reduce or inactivate the physiological activity of plant hormones, which may then inhibit the normal physiological process of plants. gibberellins - indoleacetic acid - ethylene synthesis
Effects on Synthesis of Plant Endogenous Hormones

15. Effects on Protein Synthesis
Mode of action
Some phenolics (i.e., ferulic acid and cinnamic acid) can inhibit protein synthesis. allelochemicals from O. sativa can inhibit amino acid transport and protein synthesis, and the subsequent growth of treated plants. All phenolics could reduce integrity of DNA and RNA
Effects on Protein Synthesis

17. Weed control through allelopathic crop water extracts
Allelopathic extract
crop
Weeds controlled
Weed control
Yield increase over control
Reference
Reduction in weed density (%)
Reduction in weed dry weight (%)
Sorghum
Rice
Echinocloa colonum,
Cyperus rotundus,
Cyperus iria -
40.4
12.5
Wazir et al. (2011)
Sunflower
Wheat
Avena fatua,
Phalaris minor,
Rumex obtusifolius
Cheema et al. (2003), Naseem et al. (2010)

18. Weed control through combination of allelopathic water extracts and reduced doses of herbicides
Allelopathic extract + herbicide (1/2 dose)
crop
Weeds controlled
Weed control
Yield increase over control
Reference
Reduction in weed density (%)
Reduction in weed dry weight (%)
(Sorghum + Brassica) + Pendimethalin
Canola
Cyperus rotundus, Chenopodium album
39.9
Jabran et al. (2008, 2010)
(Sorghum + Sunflower + Rice) + Butachlor
Rice
Echinocloa crusgalli,
Cyperus iria
61.0
Rehman et al. (2010)

19. Natural Herbicide Development
Continued isolation and identification of compounds implicated in allelopathy has been emphasized as a major research effort to develop natural herbicides or growth regulators that may possess novel modes of action and therefore, may more be effective than many current herbicides to which weeds have developed resistance

20. Selection and Breeding for Allelopathy
allelopathic activity is particularly high in cereal crops and should be used as an effective selection criterion in breeding programmers to enhance the competitive ability.
Integrated Sustainable Weed Management
Integrated sustainable weed management rely on many available strategies to reduce weed seed banks, prevent weed emergence and minimize the competition of weeds with growing crop