The majority of inhibiting chemicals are produced as secondary substances by plants and released into the soil through the roots or leaf wash. The suppression of growth through the release of chemicals by a higher plant is known as allelopathy. Thus, alleopathy (allelon = each other + pathy = suffering) means chemical control of distribution among plants.
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. Allelopathic chemicals can be present in any part of the plant.
They can be found in leaves, flowers, roots, fruits or stems. They can also be found in the surrounding soil. Target species are affected by these toxins in many different ways. The toxic chemicals may inhibit shoot/root growth. They may inhibit nutrient uptake or they may attack a naturally occurring symbiotic relationship thereby destroying the plant’s usable source of a nutrient.
Allelopathy is a form of chemical competition. The allelopathic plant is competing through “interference” chemicals. Competition by definition takes one of two forms exploitation or interference. When organisms compete with one another, they create the potential for resource limitations and possible extinctions. Allelopathic plants prevent other plants from using the available resources and thus influence the evolution and distribution of other species. One might say that allelopathic plants control the environments in which they live.
Though a considerable number of plants in nature show allelopathic behavior. Allelopathy is not a common phenomenon for all plant species. Some plants and trees those are well known as allelopathic are Black Walnut (Juglans nigra), Ailanthus or TreeOfHeaven (Ailanthus altissima), Fragrant Sumac (Rhus aromaticus), Rice (Oryza sativa), Pea (Pisum sativum), sorghum etc. Black
walnut is an expert allelopathic plant which contains allelopathic properties within its leaves, buds, roots and nut hulls, it is also known to secrete a substance into the soil called juglone that is a respiratory inhibitor to some plants. Though most of the allelopathic plants store their chemical weapon, allelochemicals, within their leaves, allelopathic properties can be stored within a number of organs of the allelopathic plants. The allelopathic characters can be found in roots, barks, flowers, fruits, seeds, pollen, foliage etc. of the allelopathic plants.
Advantages of Allelopathy
Allelopathic plants can be introduced in agroecosystems to get some advantages from that
1. Allelopathy can be used for beneficial purpose through using allelochemicals as natural herbicides or pesticides. Various allelochemicals classes including alkaloids, flavonoids, cyanogenic compounds, cinnamic acid derivatives, benzoxazines, and ethylene and some other seed germination stimulants can be isolated from various families of terrestrial and aquatic plants. These allelochemicals are readily or potentially phytotoxic to many unnecessary plants.
2. Using allelopathic plants in companion cropping may bring a great advantage to an agroecosystem. A selectively allelopathic plant can be used as a companion plant with a certain crop plant. The selectively allelopathic plant will suppress certain weeds and will not disturb the growth of the main crop. The introduction of a number of crop species such as corn, lupin, oats, beets, wheat, peas, millet, barley, rye etc. in companion cropping has been proved effective in suppressing a number of weeds.
3. Some parasitic weeds produce seeds which germinate in response to chemical compounds released from their hosts. For instance, Striga, a parasitic plant to cereals, germinates in response to pbenzoquinone compound released from its natural host sorghum. Ethylene is also effective to stimulate Stirga to germinate. Thus, ethylene can be applied to make Stirga germinate in the absence of a host. Using allelochemicals to stimulate the suicidal germination of weed seeds reduces the number of dormant seeds in soil.
4. The allelopathic characteristics of wild types plants can be transferred into the commercial crops to boost up their allelopathic traits for weed suppression.
5. Selectively toxic plant residues can be managed in a proper manner to control weeds efficiently. Using allelopathic crops in crop rotation, cover cropping with smother crops, using phytotoxic mulches etc. can be the examples of some good allelopathic residue management practices.