The term allelopathy is the interaction between plants, plants with microbes like viruses, fungi etc. plants releases compounds that have allelopathic potential.

By Maliha Nawaz Mokal, Madiha Rashid, Mansoor Hameed


Hans Molisch firstly used the term allelopathy in 1930s. Further studies modified the definition of allelopathy as the positive and negative effects of one plant on another. These direct and indirect effects are usually due to the release of allelochemicals. The allelopathy includes both the negative and positive effects but mostly researchers consider it as harmful effects of donor plants.


Living organisms produce metabolites that are secondary in nature and are non-nutritional. These metabolites serve as the allelochemicals. These allelochemicals are present in every part of plant like root, stem, leaves and flowers. in other plants only one or two location contain these allelochemicals. The allelochemicals include the metabolites like terpenoid, phenolic compounds, alkaloids, organic cyanide, long chain fatty acids and many more. Some time for an allelochemicals to exert its effect on plants, have to undergo some changes. These changes can be environmental or structural for the complete biological activity of that allelochemicals. The toxicity level of some allelochemicals reaches to the death of other plants.

Mode of action

Allelochemicals largely effect physiological processes and phenotypic responses. All these effects are secondary in nature rather than the primary one. The mode of action of most of the allelochemicals are secondary. They cause alteration in biochemical and physiological processes along with the modification of cell ultrastructure and molecular biology of cells. Knowing about the mode of action of allelochemicals can greatly help the researcher to increase the production of crops. They can also give a good source of information about the agro ecological services applicable to agronomy.

Allelopathic Effects


Effects of allelochemicals are studied for their effects on germination of seeds and capability of seeds. allelochemicals cause inhibitory effects on germination and seedling growth of most of plant species. In cereal crops the inhibitory effects are mainly due to the change of amylase activity by allelochemicals. Amylase cause the breakdown of starch which are very important for the respiration because they serve as respiratory metabolites. Therefore allelochemicals effect the normal cellular metabolic processes rather than effecting the cell organelles.


Photosynthetic process which is considered to be the most important physiological process in plants is also effected by allelochemicals. Allelochemicals largely effect the physiology of plants that is why they are studied on large scale for organic agriculture practices. These practices include the weed management where allelochemicals are used as natural herbicides.


Secondary metabolites acting as allelochemicals effect the mitochondrial processes. Allelochemicals decrease ATP production. ATP is required for every process occuring in cell.This ATP is require for every energy demanding process. All these effects indirectly cause the decrease in the growth and development of plants. Allelochemicals cause effects on membrane bounded enzymes mostly the ATPase that is proton pump present on membrane. It was found that the allelochemicals related to phenolic compounds perform the auxin protecting activity which in turn increase the auxin production in plants which is a growth hormone. Allelochemicals cause the increased production of auxin.

ROS production

Oxidative stress is one of the major stress on plants in which the production of reactive oxygen species (ROS) become uncontrolled. One of the negative impact of allelochemicals are the production and accumulation of reactive oxygen species. As for ROS there is always a defense system present in plants include the production of antioxidants which cause the scavenging of harmful ROS. Allelochemicals also increased the production of antioxidants in plants to inhibit the effects of ROS. Allelochemicals cause the initiation and activation of antioxidants in plants. As another defensive mechanism plants also produce bioactive metabolites. This process of production of metabolites protect the plants against the biotic as well as a biotic stresses.

Effect of allelochemicals concentration

Most of the stimulatory and inhibitory effects of allelochemicals are concentration dependent. Some allelochemicals that cause decrease in growth in one plant at one concentration can increase the growth of other plant at another concentration. Therefore it is very important to study that which concentration has negative impact and which have the stimulatory effects on different plant species. The inhibition of growth is related to the mitotic activity of cells. As different plant have different combination of allelochemicals therefore their allelochemicals potentials are also vary from plant to plant and also among different plant parts.

Gene induction

One of the major effect of allelochemicals is that it cause the induction of genes that are related to the production of antioxidant. Plasmolysis and the similar type of effects like plasma membrane separation from cell wall also caused by allelochemicals in plants. Allelochemicals effects the production of amino acids and proteins.

Other Effects

In Allelopathic interactions the widely studied inhibitory effects occur on the various physiological functions. These processes involve the photosynthesis, respiration, water balance in plants, stomata distribution and function of stomata complexes. Photosynthesis. Allelochemicals also effect the cell division, permeability of membrane and enzyme activity of plants.