27 Allelopathy

Dr Gajendra Singh Vishwakarma

1. Introduction

2. History of Allelopathy

3. Types of Allelopathy

4. Different types of allelopathic chemicals

5. Release of allelochemicals in nature

6. Fate of allelochemicals in the soil

7. Allelopathy in cropping systems

8. Techniques for allelopathy study

9. Mode of action of allelopathy Chemicals

10. List of trees and shrubs showing allelopathic effects are as follow

1.Introduction

Allelopathy is a combination of two words i.e., allelon which means “of each other”, and pathos which means “to suffer”. Simply, allelopathy refers to the chemical inhibition of one plant species by another plant species. The “inhibitory” chemicals are released into the environment and affect the development and growth of nearby plants. As per ‘International Allelopathy Society’ allelopathy is a process that involves the release of secondary metabolites produced by plants, algae, bacteria and fungi. These substances influence the growth and development of agricultural and biological systems. The allelopathic substance was first detected in black walnut tree by Davis in 1928. Allelopathic chemicals can be present in the leaves, flowers, roots, fruits, or stems and may be released from these parts of the plant. Allelochemicals are introduced by the plants into the environment via root exudation, foliar leaching, volatilization, residue decomposition. They can also be found in the surrounding soil. Target species are affected by these toxins in many different ways. The toxic chemicals may reduce the plant growth by inhibiting the shoot/root growth, by blocking the nutrient uptake, or by disturbing the naturally occurring symbiotic relationship thereby destroying the plant’s usable source of a nutrient. Allelochemicals may be involved in plant-plant, plant herbivore or plant insect communication via chemicals.

2.History of Allelopathy

The term Allelopathy was first coined by Hans Molish in 1937. The term Allelopathy is relatively recent, but the concept of allelopathy is too old. For over 2000 years, phenomenon of allelopathy has been reported in the literature as plant interference. Initial observations of weed and crop allelopathy were made by Theophrastus (300 BC) and Pliny II (1 AD). Further in the 16th century, many botanists reported the irregular growth of some plants in the presence of others but no one found any experimental proof. In the seventeenth and eighteenth centuries, botanists relied strongly on a comparative approach and conducted some experiments on the plant growth and its relation to nutrition. A number of botanists, farmers, agronomists and gardeners conducted a series of experiments to describe allelopathic interactions among plants. However, intense interest in the field of allelopathy was revived in 20th century. Work of Schreiner and Reed (1907, 1908) and co-workers revived the interest of many scientists in this field.

3.Types of Allelopathy

Based on the fate of chemicals in soil

• True Type: The release of toxic compounds into the environment in their original form in which they are produced.
• Functional Type: The release of a toxic chemical substance into the environment in modified form. The modification occurs due to the transformation by microorganisms.

Based on the target or recipient

Simple Allelopathy or Teletoxicity: when target plants are other than donor plants.

1. Weed on Crops: When weeds like (Quack grass, wild oat, Bermuda grass etc) cause serious decrease in the yield of important edible and cash crops. The decrease in the yield is mainly due to the release of root exudates, residue remains and toxic allelopathic compounds.
2. Weed on Weeds: When one weed inhibits the emergence of another weeds e.g. logon grass interfere the growth of button grass by releasing inhibitory substance.
3. Crop on Weed: Some crops release the allelopathic chemicals which inhibits the growth of weeds. For e.g. oats pea suppress the growth of lambsquarter. The allelopathic effects of weed and crops on another weed may be applied for the development of natural herbicide.

• Autoallelopathy or Autotoxicity: when target plants are same as donor plants for e.g., seed germination of horse nettle (Solanum carolinense L.) is inhibited by the material extracted from roots, stems and leaves of the same plant incorporated into the soil.

4.Different types of allelopathic chemicals In different plants and trees, a large number of chemical belonging to different class/nature have been reported as allelopathic chemicals. The list of different types of chemical is as follow:

Phenolic acids

Phenolic compounds are the most common plant allelochemicals. They consist of a hydroxyl group (-OH) bonded directly to an aromatic hydrocarbon group. The phenolic compounds that exhibit the property of allelopathy are derived from coumarins, cinnamic and benzoic acids. The metabolic pathways responsible for the production of phenolic compounds in plants are shikimic acid and acetic acid pathways.They are included in the category of secondary metabolites. The presence of phenolic allelochemicals has been observed both in natural and man-made ecosystems.They cause various ecological and economic problems like replanting problems in orchards, reduction in crop yield, and regeneration failure of natural forests.They exhibit diverse modes of action and may be explored as lead compounds for the development of herbicides or pesticides.The major function of phenolic compounds in plants is to provide scent and color to flower for attracting pollinators or to keep away herbivores or pathogens. The phenolic compounds like chlorogenic acid, protocatechuic acid (3, 4-dihydroxybenzoic acid), gallic acid, 3, 4-dihydroxybenzaldehyde, p-hydroxybenzoic acid, caffeic acid (3, 4-dihydroxycinnamic acid) and 3, 5-dinitrobenzoic acid, isolated from different plants have shown inhibitory effects on other plant species especially weeds.

Organic acids

Organic acids are organic compounds that possess acidic properties. The common examples of organic acids are carboxylic acid, tartaric acid, and citric acid. These acids are reported for their allelopathic nature. Citric, malic, oxalic, and tartaric acids extracted from the leaf aqueous extract were reported for the allelopathic activity against the lettuce seedlings growth.

Flavonoids

Flavonoids are also categorized as secondary metabolites produced by plants. These are biologically active low molecular weight compounds. More than 10,000 structural variants of flavonoids have been reported till now. Flavonoids possess various physical and biochemical properties and therefore, they show interaction with different targets in sub-cellular locations. This interaction elicits several activities in microbes, plants, and animals. Flavonoids are known to play crucial roles in the development of roots and shoots, transport of growth hormones like auxin, and pollination. Flavonoids are also known to possess anticancer, antiviral, antibacterial, and antifungal activities. In plants, the transport of flavonoids occurs within and between plant tissues and cells. Thereafter, they are particularly released into the rhizosphere. In the rhizospheric region, they are involved in allelopathy or plant/plant interactions. However, the role of flavonoids is less characterized as compared to other secondary metabolites. The major flavonoids are kaempferol, quercetin, maackiain, coumestrol, luteolin etc. Methyltransferases and glycosyltransferases are responsible for the methylation and glycosylation of flavonoids. These modifications are responsible for the alteration of their reactivity, solubility, and stability. In nature, flavonoids are present in the form of glycosides.

Alkaloids

Alkaloids are secondary metabolites that are widely distributed in plants. These compounds are mainly involved in defense mechanism of plants against insects, microorganisms and mammals. However, few of them have also been recognized as allelopathic compounds. Examples include paraxanthine, theobromine, mimosine, nupharolutine and caffeine. Alkaloids have also been employed in pharmacy, food preparation and as poisons. Recent investigations suggest that plant-produced alkaloids contribute to interference with other plants and that primary allelopathic activity may disrupt a biochemical pathway in plants which is analogous to one more usually associated with animals.

Terpenes

Terpenes are wide-spread secondary plant metabolites. These are well known for their allelopathic nature. Greater part of plant Essential Oils (EOs) is constituted by Terpenes. Terpenes are further categorized as monoterpenes and sesquiterpenes with C10 and C15 atoms, respectively. The EOs can be extracted from leaves, roots, wood, bark, stem, flowers, and fruits, etc. The common examples are cineoles, α and β-pinenes, camphene, dipentene, eucalyptol, thyme, terpinen-4-ol, cerene, carvone, geraniol, limonene and p-cymene etc. The terpenes are known to inhibit seed germination in plants. These may also cause morphological and physiological changes in plant seedlings.

Amino Acids: Certain amino acids like lysine, methionine and tryptophan, produced by plants have also been reported to inhibit plant growth by inhibition of metabolic pathways inside plants. Apart from their action against weeds, the amino acids have also been reported for their inhibitory effects against some fungi.

5.Release of allelochemicals in nature

Allelopathy is about the production and release of allelochemicals in nature. The release pathway varies for different species.

(1) Exudation of volatile compounds and their deposition on the surface of leaves by rainfall.

(2) Exudation of volatile compounds from the green parts of the plant.

(3) Dead decaying organic matter of plant residues (e.g., litter fall or dead roots)

(4) Root exudation

Root exudation: Root exudates are the chemicals secreted by roots into the surrounding environment of soil (i.e. the rhizosphere). Root exudates are the major repository of allelochemical input in the soil. Through the exudation, roots not only regulate microbial community in its close vicinity, but also change the chemical and physical properties of the soil for the regulation of the growth of neighboring plant species.

Volatilization: The essential oils (EOs) found in various plants are liberated through this mode. The EO escapes out through aerial parts of the plant. There are two way of recipient plant exposure to the essential oils. First, the recipient plant may be exposed to these oils directly through respiration or direct contact. Second, the EOs get absorbed on soil particles and further plants absorb them from the soil solution. These are complex mixtures of hydrocarbons and their oxygenated compounds, and thus impart characteristic odor or taste to the essential oil. Volatile oils after their release exhibit high phytotoxicity towards a number of plants.

Leachation Leachation is the movement of large quantities of plant metabolites from aerial parts of the plants by the action of rain, dew, mist or fog in form of aqueous solutes. Leachation is an important mechanism of liberation of allelochemicals. It is as an effective way of releasing toxins by plants, thus avoiding autotoxicity and allelopathy to occur simultaneously. The materials that can be leached out include a diverse array of compounds, which may be organic or inorganic. Among these are amino acids, sugars, vitamins, phytohormones and allelochemicals such as phenolic acids, organic acids, alkaloids, terpenoids etc. The leachation of inorganic nutrients, sugars, phytohormones are beneficial for plant growth. However, the release of toxic substances has inhibitory effects on being absorbed. External factors including temperature, light, rain period and intensity etc. may also influence leachation.

Residue Decomposition

The old leaves, barks and branches of plant that have turned into the litter or residue are also responsible for the release of allelopathic compounds. The leaves and other plant parts fall onto the ground and are decomposed by weathering or soil microbes that release allelochemicals into the environment. The decomposing residues provide the largest quantity of allele-chemicals into the rhizosphere that can induce the growth inhibition of plant seedlings. Commonly in agriculture, the plant residues are incorporated into the soil to improve soil fertility. However, the growth and productivity may also be reduced by this incorporation.

6.Fate of allelochemicals in the soil

As soon as allelochemicals are released into the soil, they enter a complex plant-soil system. In this system, various factors affect their availability and effectiveness on target plants. Some processes are responsible for the addition of allelochemicals to the system whereas some processes like microbial breakdown, plant uptake and leaching are responsible for their reduction. Soil microbes take up the allelopathic compounds released from plants and are responsible for their degradation by secreting extra-cellular and intercellular microbial enzymes. Microbial transformations may lead to the production of more phytotoxic allelochemicals. Physiochemical degradation or oxidation of plant residues can also detoxify or produce additional allelochemicals.

7.Allelopathy in cropping systems

The phenomenon of allelopathy in crops has both positive and negative implications for cropping systems. Negative effects of allelopathy include changes in the distribution pattern of crops, reduction in yield and difficulty in replantation of crops. When a particular species releases allelochemicals that can cause damage to other plant species, this phenomenon is known as heterotoxicity. However, when a plant releases such type of allelochemicals that inhibit its own germination and development, this phenomenon is known as autotoxicity. The probable reason of autotoxicity is natural selection, where older plants avoid competition for light, water, nutrients etc. with younger individuals, by maintaining them at certain distance. For example, uniform spatial pattern can be seen in desert plant populations, where plants are spaced at even distance.

8.Techniques for allelopathy study

a) Bioassay using Petridishes

The most common technique for studying the allelopathy is bioassays with Petridishes. Bioassay studies involve the evaluation of plant extracts, essential oils, and isolated chemicals compounds on the germination and initial seedling growth of a target plant. The plant extracts can be made from any part of a plant like flowers, stems, roots, fruit and seeds. Steps to carry out bioassay

1. Bottom of the Petridish is lined with filter paper.
2. Moistened with distilled water or emulsion of allelopathic compound
3. Seeds are lined on the filter paper so that they come in contact with tested compound.
4. The Petridishes can be placed in growth chamber having a programmed photoperiod. Germination percentage and seedling length are the most common parameters evaluated in these bioassays.

b) Competition

Competition effects have been categorized into four types:

• Additive
• substitutive
• neighborhood
• systematic

among these, additive type can be used to identify allelopathic species. This type of identification involves a change in the density of one species while the density of other species remains constant. Once it gets confirmed that a particular species is showing  allelopathic behavior, and then other types of competition experiments can be performed to select those cultivars that are having higher allelopathic potential.

c) Residual toxicity in the soil

Soil samples are collected from the rhizospheric region where allelopathic species are present. These samples are used as substrate for the germination of target species. Similarly, soil samples from nearby areas where allelopathic species are not present are used as control.

d) Detoxification of the substrate

Activated carbon has a peculiar property that when mixed into the soil, it can adsorb various organic compounds. This property can be used to evaluate allelopathic activity. On the basis of this property, it has been assumed that the allelopathic potential of a particular species can be diminished or eliminated by the presence of activated carbon. Activated carbon can be mixed with plant extracts, incorporated into the soil or hydroponic solution, or placed directly on the soil surface. To assess the allelopathic potential, a comparison is made between the treatments and control (without the addition of activated carbon).

d) Hydroponic experiments

Investigation of allelopathy via hydroponics is an interesting tool. A hydroponic apparatus provides a medium for the diffusion and delivery of allelo-chemicals to target plants.

e) Amendment of plant residues

In this method, plant materials are added in different amounts to the substrate to assess the allelopathic effect and the release of allelochemicals on the target species. This technique has been successfully used in greenhouse and field experiments.

f) Plant box method

This method is entirely based on the principle of dose response. This method establishes a direct links between growth inhibition and concentration of root exudates in the media. Agar is used as diffusion medium in this method, since agar allows the dispersion of allelochemicals from roots of the donor to the target plant.

Procedure

• Place the donor plant inside cellulose tube.
• Place the tube in the corner of a plant box.
• Keep the box in ice and fill it with cooled agar.
• After the gelatinization of the agar, seeds of target plant are placed concentrically near the donor plant.
• Seal the container to avoid evaporation.
• Place the boxes in pot so that the roots of the target seedlings get darken.
• Keep it in a Biochemical Oxygen Demand (BOD) chamber.
• Control consists of a plant box without the donor plant.

9 Mode of action of Allelopathy Chemicals

The morphological changes and restriction in the development of roots and shoots may arise due to the changes in the hormone levels. The concentration of plant hormone indole acidic acid and zeatinriboside which are responsible for plant growth (shoots and roots) decreased on treatment with allelopathic chemicals like essential oils (EOs) and phenols. The germination inhibition may be the consequence of blockage in the water uptake, or disruption of the metabolic enzymes that are involved in the glycolysis and oxidative pentose phosphate pathways. Moreover, disruption of mitochondrial respiration system may be another mechanism for the inhibition of seed germination and radicle elongation, due to the interference of volatile essential oils. As a result, ATP production gets decreased. Allelopathic compounds are also known to inhibit the root growth by interfering with cell proliferation in root apical meristem and restricting the mitotic activity of cell. Allelopathic compounds cause generation of reactive oxygen species (ROS) like singlet oxygen and superoxide, hydroxyl as well as hydroperoxyl radicals. These ROS leads to disruption of membrane permeability, which cause damage to DNA, proteins and enhance the accumulation of lipid peroxide molecules.

10. List of trees and shrubs showing allelopathic effects are as follow