ALLELOPATHY AND WEED MANGEMENT

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.

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.

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

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

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

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

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

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

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

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

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

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 10.6-33.6 2.2-16.5 1.6-10.7 Cheema et al. (2003), Naseem et al. (2010)

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 42.8-91.3 37.4-94.1 39.9 Jabran et al. (2008, 2010) (Sorghum + Sunflower + Rice) + Butachlor Rice Echinocloa crusgalli, Cyperus iria 74.0-67.0 66.0-76.0 61.0 Rehman et al. (2010)

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

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