Weeds are out of place plants and not intentionally grown. Weeds have naturally strong competitive behavior and those weeds that can best compete always tend to dominate. There are approximately 250,000 species of plants worldwide; of those, about 3% or 8000 species behave as weeds. Weeds are also known as invasive plants. The weeds typically produce huge amounts of kernels, assisting their spread and are exceptional at surviving and germinate in disturbed environments. It can be specie that is not common or a native species that settle and keeps recurring in an ecosystem in which it did not previously exist. Weeds can inhabit all environments; from our towns and cities through to our oceans, deserts, and alpine areas.

Allelopathy effects of weeds is a biological phenomenon by which a plant produces one or more biochemical compounds that influence the germination, growth, survival, and reproduction of other plants. Allelopathy is made up of two Greek words one is ‘allele’ and other ‘pithy’. Allelo means mutual or one another. Pathy means suffering. In nature many plant secrets secondary metabolites or phytotoxins that may constrain the growth of diverse other plants. This phenomenon of nature is called allelopathy and the effect may be directly by a living plant or indirectly through the process of plant decomposition. If the metabolite or phytotoxin produces inhibitory effect they are known as allelochemicals. Allelopathy may have beneficial (positive allelopathy) or detrimental (negative allelopathy) effects on the target organisms and the community

Allelopathy is characteristic of certain plants, algae, bacteria, coral, and fungi. Allelopathic interactions are an important factor in determining species distribution and abundance within plant communities and are also thought to be important in the success of many invasive plants.

Using allelochemicals to stimulate the suicidal germination of weed seeds reduces the number.  Weeds are a menace in all crops as they compete for light, water and nutrients and harbor diseases and insects.  A high volume of herbicide usage induces numerous changes in plant growth like inhibition of growth, foliar chlorosis, albinism, and necrosis. Many herbicides persist in the environment and cause biomagnifications. So there is every need to develop biodegradable herbicides.

Herbicides developed from the plants will be safer and they are biodegradable. Allelopathy holds potentials for selective biological weed management. The phenomenon of allelopathy refers to chemical interactions between all types of plants. In this process, the chemical exudates or leachates released from leaves, stems, or roots of a plant can inhibit the growth of a neighboring one. Vigna mungo (L.)  Hepper is commonly known as black gram or urdbean. It is one of the important crops cultivated of dormant seeds in the soil.

The allelopathic characteristics of wild types of plants can be transferred into the commercial crops to boost up their allelopathic traits for weed suppression.

Allelopathic Effects Of Weeds

The inhibitory effect of allelopathy is a complex process. Different classes of chemicals involved in allelopathy such as flavonoids, phenolic compounds, terpenoids, steroids, alkaloids, amino acids, and carbohydrates alone or with mixtures and may involve the interaction of different compounds. The mixture sometimes having a greater allelopathic effect than individual compounds alone.

Furthermore, environmental and physiological stresses, diseases, pests, solar radiation, herbicides, and less optimal nutrient, moisture, and temperature levels can also affect the allelopathic weed effect. The allelopathic activity can be found on different plant parts, including leaves, flowers, leaf mulch, and leaf litter stems, bark, roots, soil, and soil leachates and their derived compounds. These chemicals are released in four different ways.

  1. Volatilization: the release of chemicals into the atmosphere. These chemicals may be diluted or absorbed in the vapour by surrounding plants or directly can  condensate in dew or may reach the soil and be taken up by the roots;
  2.  Leaching: Chemicals may leach with dew or rainfall, irrigation from the superficial parts of plants that are deposited on the soil or other plants. Leaching may also occur through plant residues laid on earth.
  3. Root exudation: from plant roots, bark into the soil environment, or on other plant parts. Shedding off the roots is not clearly understood whether these compounds are actively exuded, leaked or arise from dead
  4. Decomposition: plant residues decomposition is difficult to determine whether produced by microorganisms or toxic substances are contained in residues and simply released upon decomposition.

Allelochemicals can be present in soil and can affect crop plants beside it and those to be planted in succession. Furthermore, the effect can vary over a growing season. Although allelochemicals may be more biodegradable than traditional herbicides, they may have undesirable effects on non-target species, necessitating ecological studies before widespread use.

Allelochemicals of crops and plants may also have selective activity. For example, Leucaena leucocephala, the miracle tree helps for water and soil conservation, revegetation, and livestock nutrition; The toxic amino acid (non-protein) present in this tree leaves inhibit the growth of other trees but not its seedlings. Leucaena species have also been shown to reduce the yield of wheat but increase the yield of rice. The allelopathic effects of fresh shoot aqueous extract of Tithonia diversifolia showed that the radicle and plumule lengths of the maize (Zea mays L.) seedlings were significantly inhibited. The same extract stimulated the growth of older plants of two weeks old and above.

In the laboratory, leachates and plant extracts are usually screened for their effects on seed germination with further isolation & identification of allelochemicals from greenhouse tests and field soil, confirming laboratory results. Interactions among host crops, allelopathic plants, and other non-target organisms must also be considered. Furthermore, allelochemistry may provide templates or basic structures for developing new synthetic herbicides.

Various studies have elucidated specific allelochemicals involved in weed suppression, including diterpenoid momilactones in rice; benzoxanoids in the rye; alkaloids and flavonoids in fescue; tabanone in cogongrass; naphthotectone and anthratectone in teak (Tectona grandis); abscisic acid beta-d-glucopyranosyl ester in red pine; cyanamide in hairy vetch; and fatty acid (cyclopropene) in hazel sterculia (Sterculia foetida).

An allelopathic weed can effectively be used to control bothersome weeds near crop by planting a variety with allelopathic qualities, either as a smother crop, when left as a residue mulch or, in a rotational sequence, or, especially in low-till systems, to control subsequent weed growth. For instance, in one study, rye mulch had no effects on common lambsquarters and velvetleaf but had suppressive effects on common purslane and pigweed. In a multi-season field study, when applied as a soil amendment, mustard seed meal derived from white mustard (Sinapis alba) was effective for weed suppression in organic sweet onion, but crop injury was also significant.

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