Allelopathy plants

Inderjit and Callaway, R.M. (2003). Experimental designs for the study of allelopathy. Plant and Soil 256 1-11. 

D) In many examples of allelopathy, plant phenols are implicated as a component of the phytotoxin and therefore it would be reasonable to expect phenols to be Involved in this situation. However, our studies failed to show an accumulation of soluble phenols in the soil. A substantial elevation in soil phenol levels would be required to inhibit seedling development, and phenolics were not detected in root exudates. 

A. C. Thompson, ed.. The Chemisty ofi Allelopathy Biochemical Interactions Among Plants, ACS Symposium Series 268, American Chemical Society,... 

The early literature on naturally occurring plant growth inhibitors and the influence one plant might exert on another by chemical means (allelopathy) is considered comprehensively in various reviews (12, 15, 36, 37, 48, 49, 61, 67, 94, 121, 162). Reviews of studies concerned with the influence of plant exudates on root-infecting fungi (130) and the effects of phytotoxins which arise as decomposition products (113) have appeared recently. Because of the excellent coverage of the topics by others, no attempt is made here to review the early literature exhaustively. Instead, consideration is restricted essentially to specific compounds and to some of the more recent literature. 

Inderjit and R. del Moral, Plant phenolics in allelopathy. Bot. Rev. 62 186 (1996). D. L. Jones, Organic acids in the rhizosphere—a critical review. Plant Soil 205 25 (1998). 

The term allelopathy was coined by Molisch in 1937 Q). Presently, the term generally refers to the detrimental effects of higher plants of one species (the donor) on the germination, growth, or development of plants of another species (the recipient). Allelopathy can be separated from other mechanisms of plant interference because the detrimental effect is exerted through release of chemical inhibitors (allelochemicals) by the donor species. Microbes associated with the higher plants may also play a role in production or release of the inhibitors (2). 

Chemicals with allelopathic potential are present in virtually all plant tissues, including leaves, stems, roots, rhizomes, flowers, fruits, and seeds. Whether these compounds are released from the plant to the environment in quantities sufficient to elicit a response, remains the critical question in field studies of allelopathy. Allelochemics may be released from plant tissues in a variety of ways, including volatilization, root exudation, leaching, and decomposition of the plant residues. 

In many ecosystems, plants tend to pattern themselves as pure stands or as individuals spaced in rather specific densities or configurations. Many desert species show obvious zones of inhibition around which few, if any, alien species are able to invade. These patterns often cannot be adequately explained by competition alone, and are probably caused by a combination of factors including allelopathy. The phenomenon happens with herbaceous plants as well as woody shrubs and trees. 

How weeds produce their effect is the subject of continuing research. As rather recently defined, the negative effect of weeds on crop plants includes both competition and allelopathy (1.) and has been termed interference. Previous weed science research considered the competition component foremost and, usually, solely (e.g. 2). Allelopathy, however, has received some attention recently, with a number of symposia and reviews devoted to allelopathic effects. Some of these cover the effects of weeds on crops (3, 4, 5). In this chapter, I review the possible role of allelopathy in weed interference in crop production in the Mid-South. A second objective of this review is to indicate where research is needed and to suggest potential lines of future research, especially with respect to the developing role of conservation tillage practices in this region. 

One of the features of allelopathy, as pointed out by Rice (18). is that it is specific. Certain species, but not others, are affected by the allelochemics produced by a plant. This suggests that allelopathy by a weed must be positively demonstrated for each crop. References to specific reports of demonstrated allelopathy by a weed on a crop are given in Table 1. In some cases [crabgrass (Digitaria sanguinalis (L.) Scop.), spurge (Euphorbia spp.), etc.], no crop is listed in the allelopathy column. 

Allelopathy (root words ALLELON and PATHOS) is derived from Greek, "allelon" of each other and "pathos" to suffer - the injurious effect of one upon another. The subject matter of this symposiun covers that body of knowledge which concerns the production by one plant of chemicals that induce suffering in another and may also be called chemical pathogenesis. Although the evidence... 

Allelopathy, as Muller pointed out (1 ), must not be confused with physical competition, such as crowding and shading. The total influence of one plant on another should be termed "interference" which in turn includes both physical and chemical effects. 

The term "Allelopathy" was coined by Molisch (2 ) to refer to both detrimental and beneficial biochemical interactions anong all classes of plants, including microorganisms. Because the root word "pathy," however, implies detrimental interactions, Rice (3) defines "allelopathy" as follows "Any direct or indirect harmful effects of one plant (including microorganisns) on another through the production of chemical compounds that escape into the envirorment." Perhaps, the term "allelopathy" should be extended to include the manifold mutual effects of metabolic products of both plants and animals. Now Rice includes beneficial interactions (18). 

Fatty Acids and Lipids Although several fatty acids, esters and alcohols are known to be toxic to plant growth, their role in allelopathy is not fully investigated (3). Dihydroxystearic acid (3, 49) is the classic example known to exhibit allelopathic activity. 

In spite of the early work that provided the basis for developing the concept of allelopathy, many plant scientists, especially plant physiologists, are still skeptical about the realities of allelopathic potential because of the conflicting and oftentimes unconvincing reports in allelopathy. A few problems that many of us still face are ... 

The latter problems are of particular interest to chemists, who should devise appropriate methods for resolving the complexity of chemicals, properly identifying them and finally determining their exact composition and makeup. The participation of chemists is needed to verify the concept of allelopathy as a concentration-dependent phenomenon. They should help to reconstitute the chemical composition as it was found in the original and isolated plant samples. This systematic approach leads to verification of the concept as well as to proper assessment of the initial observation with crude extracts, and to final application to the field situation. Once the concept is proven, same simulation experiments need to be performed to maximize the allelopathic effect (toxin action). The concentration of the toxic chemicals is varied to where the threshold levels of chemicals prove to be involved in the exhibition of allelopathy under field conditions. 

Plant physiologists and other biological scientists also have their important role to play in allelopathy. They must devise suitable bioassays to detect the suspected allelopathic compounds, follow the biological activity of the individual and associated chemicals, develop activity profiles for identified chemicals, and determine the conditions (dose/response) for chemicals to arrive at the threshold levels. They must also determine which chemicals contribute... 

Report of the Research Planning Conference on the Role of Secondary Canpounds in Plant Interactions (Allelopathy)."... 

The term allelopathy, when first proposed by Molisch (1 ), referred to either the beneficial or detrimental interaction between all types of plants and microorganisms. As presently used, this definition is generally accepted. Since 1970 a concerted effort has been made to understand the phenomenon of allelopathic interaction. The many interpretations resulting from these studies are well documented in the literature (2-4). An area currently receiving considerable attention is the allelopathic effect resulting from weed-crop and weed-weed interactions (2, 5-7). One study conducted by Wilson and Rice (7) showed that the common sunflower, Helianthus annuus L., possessed allelopathic properties. Realizing the inherent potential... 

Many phytotoxic compounds produced by higher plants are phenolic compounds. Several of these have been implicated in allelopathy. Based on the biosynthetic pathway from which they are derived, phenolic compounds produced by higher plants fall into two general categories 1) terpenoid phenolic compounds derived from five... 

In this chapter we are focusing on two different plants which appear to express allelopathy in quite different fashions. The first, Parthenium hysterophorus Linn, is recognized in many parts of theworld as causing serious agricultural problems due principally to its invasion of crop lands and the subsequent lowering of crop yields (1). Although this plant is native to the North and Central Americas, it has now been spread to many other... 

The beKavior of P. hysterophorus in the field would appear to fall under TKe classic definition of allelopathy as defined by Rice (4), i.e., this plant adversely affects other nearby vegetation by extruding chemicals which inhibit the growth of these other plants (5-10). 

There are, of course, many plants toxic to animals which also may cause dermatitis. Such properties in principle are not necessarily related to allelopathy. 

What makes B. megapotamica unusual is that the chemicals responsible 7or the toxic properties of the plant are produced not by the plant itself but by a fungus associated with the plants. It is this plant-fungus interaction at the chemical level which suggests that B. megapotamica is involved in a most extraordinary form of allelopathy (41). 

Since our principal interest in P. hysterophorus has been to identify the chemicals involved in the allelopathy, we have employed water as the extraction media for the plant material. Presumably, in the environment, chemicals extruded by P hysterophorus find their way to neighboring plants by transport through the water in the soil. 

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