Fruits of black walnut tree

Plant Allelopathy

Plant allelopathy is the ‘chemical warfare’ among the plants imposed by one plant on another to suppress the latter and take advantage from that suppression. The word allelopathy comes from two Greek words allelon and pathos; where allelon means ‘each other’ and pathos means ‘to suffer’. Thus in the phenomenon of plant allelopathy, allelopathic plants create adverse conditions to other neighboring plants by reducing their seed germination and seedling growth. The allelopathic plants are very effective in weed killing and known as Nature’s Weed Killers.

Why Allelopathy is Important

Using synthetic chemical compounds in agriculture has brought various types of ecological and environmental disasters to the present world. These toxic chemicals are damaging the ecological balance to a severe extent and introducing many fatal diseases. That’s why the demand of sustainable agriculture and eco-friendly alternatives to chemical compounds has been increased. Plant allelopathy is a great alternative of using toxic chemical herbicides in weed management.

How Allelopathy Works

Competition is a very common phenomenon in the Earth’s Biosphere. Like other living organisms, plants also compete for sunlight, nutrients, water, space etc. and this competition is the basis for allelopathy. Some plants, known as allelopathic plants, use their chemical tools to win the competition and use the available resources more efficiently. Allelopathy can be carried out by allelopathic plants by the following processes-

Allelopathic plants release chemical compounds from their roots into the soil, and these chemicals suppress or even kill the neighboring plants when they are absorbed by the plants. The harmful chemicals released by allelopathic plants are known as allelochemicals. Some allelochemicals change the amount of chlorophyll production in a plant and thus, they slow down or stop the photosynthesis process of that plant which ultimately leads to the suppression or death of that plant.

Many allelopathic plants release allelochemicals in gaseous forms. These gaseous allelochemicals are released from the small pores of their leaves. When the neighboring plants absorb these gasses, they are suppressed or killed.

When leaves drop from the allelopathic plants to the ground, they are subjected to decomposition; when the leaves decompose they release their noxious chemicals as a way to inhibit the growth of other neighboring plants.

Allelopathic Plants

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 Tree-Of-Heaven (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.

Black walnut or latin Juglans nigra isolated on white background.
Black walnut or latin Juglans nigra isolated on white background.

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-

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.

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.

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 p-benzoquinone 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.

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

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.

Caution

Allelopathic plants sometimes create some persistent problems to the soil. For example, the residues of allelochemicals may exist in the soil for a long time after the plant is removed; which results in soil sickness and makes some sites unsuitable for general plant growing.

So, allelopathic plants must be utilized carefully.

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