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Allelopathy refers to the beneficial or harmful effects of one plant on another plant, both crop and weed species, from the release of biochemicals, known as allelochemicals, from plant parts by leaching, root exudation, volatilization, residue decomposition, and other processes in both natural and agricultural systems. Allelochemicals are a subset of secondary metabolites not required for metabolism (growth and development) of the allelopathic organism. Allelochemicals with negative allelopathic effects are an important part of plant defense against herbivory (i.e., animals eating plants as their primary food) (Fraenkel 1959; Stamp 2003).
The term allelopathy is from the Greek-derived compounds allelo and pathy (meaning “mutual harm” or “suffering”) and was first used in 1937 by Austrian scientist Hans Molisch in the book Der Einfluss einer Pflanze auf die andere - Allelopathie (The Effect of Plants on Each Other) (Willis 2010). First widely studied in forestry systems, allelopathy can affect many aspects of plant ecology, including occurrence, growth, plant succession, the structure of plant communities, dominance, diversity, and plant productivity.
Commonly cited effects of allelopathy include reduced seed germination and seedling growth. Like synthetic herbicides, there is no common mode of action or physiological target site for all allelochemicals. However, known sites of action for some allelochemicals include cell division, pollen germination, nutrient uptake, photosynthesis, and specific enzyme function. For example, one study that examined the effect of an allelochemical known in velvetbean, 3-(3’,4’-dihydroxyphenyl)-l-alanine (l-DOPA), indicated that the inhibition by this compound is due to adverse effects on amino acid metabolism and iron concentration equilibrium.
- Phenolic acids
- Coumarins - block mitosis in onion by forming multinucleate cells
- Terpinoids
- Flavinoids
- Scopulatens - inhibits photosynthesis without significant effect on respiration
Allelopathic chemicals are released from the plants as:
- Vapour - from roots and leaves through stomata
- Foliar leachate
- Root exudates
- Breakdown/decomposition product of dead plant parts
- Seed extract
Allelopathic trees release a chemical in gas form through small openings in their leaves.
Some plants store protective chemicals in their leaves. When the leaves fall on the ground, they decompose and give off chemicals that protect the plant. Water-soluble phytotoxins may be leached from roots and above-ground plant parts or may be actively exuded from living roots. Rye and quack grass release allelopathic chemicals from rhizomes or cut leaves.
Some plants release defensive chemicals into the soil through their roots. The released chemicals are absorbed by the roots of nearby trees. Exuding compounds are selective to other plants. Exudates are usually phenolic compounds (e.g., coumarins) that tend to inhibit development.
- Agropyron repens (Quack grass): Generates ethylene in rhizomes due to microbial activity in soil, interfering with the uptake of nitrogen and potassium in maize, ultimately decreasing its yield.
- Avena fatua (Wild oat): Residues inhibit germination of certain herbaceous species like wheat.
- Cynaodon dactylon (Bermuda grass): Residues remain in the field, inhibiting seed germination, root, and top growth of barley.
- Sorghum halpense (Johnson grass): A perennial weed in sugarcane, soybean, maize, etc. Root exudates from decaying Johnson grass have an inhibitory effect on these crops.
- Imperata cylindrica (Cogon weed): Inhibits the emergence and growth of annual broadleaf weed, i.e., Borreria hispada by exudation of inhibitory substances through rhizomes.
- Sorghum halpense (Johnson grass): Living and decaying rhizomes inhibit the growth of Setaria viridis (Giant Foxtail), Digitaria sanguinalis (Large crabgrass), and Amaranthus spinosus (Spiny amaranth).
- Oat, pea, wheat: Suppress the growth of Chenopodium album (Lambsquarter).
- Coffea arabica (Coffee): Releases 1,3,7 trimethylxanthin, which inhibits the germination of Amaranthus spinosus (Spiny amaranth).
- Varieties: There can be a great deal of difference in the strength of allelopathic effects between different crop varieties.
- Specificity: The crop that shows a strong allelopathic effect against one weed may show little or no effect against other weeds.
- Autotoxicity: Sometimes plant species may also suppress the germination and growth of its own species. E.g., Lucerne.
- Crops on Crop Effect: Residues from allelopathic crops can hinder the germination and growth of following crops as well as weeds.
- Environmental Factors: Low fertility increases allelopathic effects due to more production of allelochemicals. Warm, wet conditions can cause faster decline of allelopathic effects as opposed to slower decline under cold and wet conditions.