In human societies one man of certain class or level tries to check or inhibit the growth and development of the other man of the same or different class or level. Not all humans tend to do so, but some do. In 2001, two students of High School namely Overcast and Cox showed through their studies that when Kochia plant (Kochia scoparia) preceded spring wheat (Triticum aestivum), it inhibited growth of the spring wheat in terms of delayed emergence of seedlings, decreased rate of growth, decreased final height and reduced average dry weight of the spring wheat. The study was done in North Montana (U.S.). Overcast and Cox presented their study in the International Science and Engineering Fair (ISEF) in 2001 and were awarded a First Place team prize in the international event. Later, through a standard and larger study it was shown that Kochia seemed to produce allelopathy on some other crops too in the same area. Now, what is allelopathy?
The biological phenomenon by virtue of which some organisms like algae, bacteria, coral and fungi influence the growth and development of other organisms by the help of some biochemicals they produce; is called as allelopathy. The biochemicals produced in this process are called allelochemicals.
In other words, the production of specific biomolecules by one plant to induce suffering in or to offer benefit to another plant is termed as allelopathy. In this case, the biomolecules produced for causing harm or benefit to a neibouring plant, are allelopathic chemicals. The effects of allelochemicals may either be beneficial or detrimental to the neighbouring organism. The allelochemicals are called Secondary Metabolites as they are not required for primary life processes. Based on beneficial or harmful effects, allelopathy has been classified as positive and negative.
Benefits from allelopathy
Some plants acquire more of the available resources like nutrients, water and sunlight from the surrounding environment without producing any chemical and initiating any chemical reaction. This process is called as competition. Since this process does not require any chemical action, this has not been put in the category of negative allelopathy. However, competition and negative allelopathy can act together and enhance the survival rate of a species.
Image :2 Garlic Mustard(Alliaria peteolata, Mustard family)
It has been reported that allelopathic interactions play a crucial role in natural as well as man made ecosystems. Allelopathy is an important factor which contributes in determining distribution of species and their abundance within communities. Allelopathy is also helpful in the success of many invasive species. The spotted knapweed (Centaurea maculosa, family Asteraceae), Nut sedge (Cyperus sp. Family Cyperaceae), and Garlic mustard (Alliaria peteolata, Mustard family) are three specific examples.
Image :3 Nut sedge (Cyperus sp. Family Cyperaceae)
Some specific examples of allelopathic weeds
C. maculosa is reported to contain Catechin and stereoisomer of Catechin which act as herbicide to inhibit competition by a wide range of other plant species. This compound is phytotoxic which is reported to inhibit seed germination and growth by making phosphorus more available in certain soils. As a result acidification of cytoplasm of the cells of competing plants starts which kills the cells. However some plants like Gallardia grandiflora and Lupinus sericeus are reported to be resistant to Catechin-induced toxicity.
Friedman and Horowitz(1971), Horowitz and Friedman (1971) and Singh et al.(1970) found that purple nutsedge tubers produced phytotoxins inhibiting the growth of other plants in the neighbouring areas. Chwen- Ming Yang, 1991, through an important study on “Allelopathic Potential of Purple Nutsedge (Cyperus rotundus L.), and Barnyardgrass (Echinochloa crus-galliBeauv.) on Corn. I. Inhibition of weed extracts on germination” showed that Aqueous extracts from field-grown mature plants of purple nutsedge and barnyardgrass were employed to study their allelopathic potential on corn (Zea mays L. cv. Tainung 1) germination. Such effects in response to temperature gradient were also investigated. The results confirmed the existence of allelopathic potential on corn germination in both extracts at concentration of 1 g ground powder/10 ml double distilled water. It has been reported that Water extracts of shoot of common lambs quarters (Chenopodium album), yellow nutsedge (Cyperus esculentus) and sunflower (Helianthus annuus) at 1% level significantly reduced soybean seed germination.
As for Garlic Mustard following report can be quoted as a ready reference -
The study report published in 2006 concluded that Garlic Mustard produces allelochemicals that harm mycorrhizal fungi that many North American plants, including native forest trees, require for optimum growth. Additionally, because White-tailed Deer rarely feed on Garlic Mustard, large deer populations may help to increase its population densities by consuming competing native plants. Trampling by browsing deer encourages additional seed growth by disturbing the soil. A complication to the eradication of Garlic Mustard from an area is the longevity of viable seeds in the ground. Seeds contained in the soil can germinate up to five years after being produced. Garlic mustard has been classified as Magnoliopsida.
Garlic mustard produces a variety of secondary compounds, including the flavonoid isovitexin 6″-O-β-d-glucopyranoside as a feeding deterrent to Pieris napi oleracea defense proteins, glycosides, and glucosinolates that reduce its palatability to herbivores.Research published in 2007 shows that, in Northeast Forests, garlic mustard rosettes increased the rate of native leaf litter decomposition, increasing nutrient availability and possibly creating conditions favorable to garlic mustard's own spread.
Garlic mustard poses a severe threat to native plants and animals in forest communities in much of the eastern and midwestern U.S. Many native wild flowers that complete their life cycles in the springtime (e.g., spring beauty, wild ginger, bloodroot, Dutchman's breeches, hepatica, toothwort, and trilliums) occur in the same habitat as garlic mustard. Once introduced to an area, garlic mustard out-competes native plants by aggressively monopolizing light, moisture, nutrients, soil and space. Wildlife species that depend on these early plants for their foliage, pollen, nectar, fruits, seeds and roots, are deprived of these essential food sources when garlic mustard replaces them. Humans are also deprived of the vibrant display of beautiful spring wildflowers. Garlic mustard also poses a threat to one of our rare native insects, the West Virginia white butterfly (Pieris virginiensis). Several species of spring wildflowers known as "toothwort" (Dentaria), also in the mustard family, are the primary food source for the caterpillar stage of this butterfly. Invasions of garlic mustard are causing local extirpations of the toothwort, and chemicals in garlic mustard appear to be toxic to the eggs of the butterfly, as evidenced by their failure to hatch when laid on garlic mustard plants.
The History of allelopathy
The negative effects of one plant on the other plant was reported for the first time far back around 300BC by Theophrastus who studied inhibitory effects of pigweed on alfalfa. Yang and Tang in China describe 267 plants having pesticidal abilities and allelopathic effects. De Candolle, the eminent Swiss Botanist reported in 1832 about soil sickness caused by plant exudates.
The term allelopathy is a synthesis of two Greek words – allele and pathy, meaning mutual harm or suffering. This word was first used in 1937 by Professor Hans Molishch of Austria in his book entitled “Der einfluss einer pflanze anf die andere – Allelopathic L.” (Qillis, Rick J.2007). In this book the term allelopathy has been used by the author to describe biochemical interactions through which growth of one plant is inhibited by a neighbouring plant (Roger et al. 2006).
Whittaker and Fenny studied biochemical interactions among plants in 1971. In their research report which was published in the journal Science they have defined allelochemicals as all chemical interactions among organisms. In 1984, Elroy Leon Rice included all direct positive and negative effects of a plant on another plant or on microorganisms by releasing biochemicals into the natural environment. Thus he elaborated the definition of allelopathy given by Whittaker and Fenny in 1971. After Elroy Leon Rice the term allelopathy remained in use to describe broader chemical interactions between organisms.
The International Homoeopathy Society in 1996 described the term allelopathy as mentioned below –
“Any process involving secondary metabolites produced by plants, algae, bacteria and fungi that influence the growth and development of agriculture and biological systems.”
Later, Zoologists too started using the term to describe chemical interactions that occur among invertebrates like corals and sponges.
Ecologists the world over argue that the effect of competition can not be distinguished from allelopathy. They are of the opinion that competition is a negative effect which is produced when two or more organisms attempt to use the same resource directly. However, some researchers in 1970s reported that competition was different from allelopathy and produced sufficient facts in favour of their reports.
Role of allelopathy in agriculture
Researches done in the fields of ecology and agriculture show sufficient amounts of study on the effect of allelopathy in agriculture. These researches have been focussed on the effects of weeds on crops, effects of crops on weeds, and effects of crops on crops. Some of the modern researches done in the field are helpful in determining the possibility of using allelochemicals as growth regulators and natural herbicides so as to promote sustainable agriculture. Now, a number of allelopathic chemicals are available in markets. For example, Leptospermone is a thermo chemical in Lemon bottle brush (Callistemon citrinus). On commercial basis, this chemical no longer remained effective as a herbicide. However, its analog mesotrione was proved effective on experimentation. Mesotrione is now sold in markets and is prescribed for the control of broad leaved weeds in corn. It has also been proved effective in controlling crab grass in laens. In 1993, Sheeja studied the allelopathic interaction of Chromolaena odoratum and Lantana camara on some crops.
Application of allelopathy on desert shrubs
In 1964, a study of Salvia leucophylla deserved the place of cover story of the reputed journal Science. The research showed that bare zones around these shrubs were caused by volatile inter penetrates emitted by the shrubs. However, this story was criticized for being based on artificial laboratory experiments and unwarranted extrapolations to natural ecosystems. In 1970, a research published in Science showed that when the shrub was caged, grass grew in the bare zone. It was inferred that rodents and birds could not reach to the bare zone, grass grew un damaged (Halsey, 2004).
1. Bais HP, Walker TS, Stermitz FR, Hufbauer RA, Vivanco JM (April 2002).”Enantiomeric dependent phytotoxic and anti-microbial activity of Catechin. Arhizosecreted racemic mixture from spotted Knapweed” Plant Physiol. 128 (4): 1173–9.
2. Cyperus rotundus is one of the most invasive weedsknown, having spread out to a worldwide distribution in tropical and temperate regions. It has been called "the world's worst weed" as it is known as a weed in over 90 countries, and infests over 50 crops worldwide.
3. DROST, D.C. & DOLL, J.D., 1980. The allelopathic effect of yellow nutsedge (Cyperus esculentus) on corn (Zea mays) and soybean (Glycine max). Weed Science 28, 229-233.
4. Friedman,T. and Horowitz, M.1971. Biologically active substances in subterranean parts of purple nutsedge. Weed Sci.19:398-401.
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8. TAMES, R.S., GETSO, M.D.V. & VIEITEZ, E., 1973. Growth substances isolated from tubers of Cyperus esculentus var aureus.Physiology of Plants 28, 195-200.