Plants allelopathic compounds

C. S. Tang and C. C. Young, Collection and identification of allelopathic compounds from the undisturbed root. system of Bigalte Lompograss iHemarthia alti.s-sima). Plant Physiol. 69 155 (1982). 

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... 

The germination stimulant or stimulants from host plants have not yet been identified, but research on isolation and identification of these allelopathic compounds continues. Other nonhost plants, such as cotton, also release chemicals which stimulate the germination of witchweed seed and these crops can replace the cereal crops in witchweed-infected fields. If no acceptable host is present, the witchweed plant is unable to mature and produce seed. The importance of cereal crops as a staple food in underdeveloped countries makes growth of nonhost crops only partially acceptable, and there are numerous wild hosts that allow the witchweed to germinate, mature, and produce more seed (several thousand seeds can be produced by a single plant). Nevertheless, application of either natural or synthetic stimulants in the absence of a host plant is an effective way of reducing and eventually eliminating the witchweed problem. 

Allelopathic compounds act as repellents for herbivorous pests, so the same strategy used in weed control could be effective against pests and pathogens. Only allelopathy is not possible to use the complete control of weeds, pests or diseases it is necessary to combine it with other methods of plant protection. 

An interesting new area of work is the biotransformation of plant products to new compounds by their endophytic fungi. A known allelopathic compound, lepidimoide, was synthesized by an endophytic Colletotrichum sp. from okra Hibiscus esculentus) polysaccharide [(1 —> 4) — O — a-(D-galactopyranosyluronic acid)-(l - 2) —O — a-L-rhamopyranose]... 

Evidence indicates that allelopathic compounds get out of plants by volatilization, exudation from roots, leaching from plants or residues by rain, or decomposition of residues ( 5). 

Allelopathy in agriculture. Schreiner and his associates published several papers shortly after 1900 which indicated that certain crop plants produce compounds inhibitory to growth of the same and other crop plants (2). McCalla and Duley (43.44) reported the allelopathic effects of decaying wheat residues in 1948-1949, and many papers on allelopathic effects of crop plants have been published in the past three decades. 

Ouglone is the only quinone identified as an allelopathic compound from higher plants (5). It is produced by walnut trees and is a potent inhibitor. Numerous antibiotics produced by microorganisms are quinones, including the tetracycline antibiotics such as aureomycin (80). 

It is important to identify the allelopathic compounds in the substrate (soil or water) of the allelopathic plant and to determine whether these compounds have come from the plant, are produced by partial decomposition of other compounds, or are synthesized by microorganisms using carbon sources from the plant. It is important to keep in mind that the allelopathic compounds produced by bacteria, fungi, and algae are just as much a part of the science of allelopathy as are those produced directly by plants. 

A moderate amount of information is available concerning the factors affecting concentrations of phenolics in plants, and a little research has been completed concerning factors affecting concentrations of alkaloids and terpenoids. Little information is available concerning factors affecting concentrations of other types of allelopathic compounds thus, research is urgently needed in this area. 

Several authors have obtained circumstantial evidence that allelopathic compounds reduce mycorrhizae formation (20-23). Kovacic and associates ( ) have shown that understory plants in a live ponderosa pine stand are largely nonmycorrhiza-forming species. They hypothesized that this was due to inhibition of the vesicular-arbuscular mycorrhiza necessary for the growth of herbaceous mycorrhizal plants, under living pines. They demonstrated that more mycorrhizal plants occurred under dead pines, bioassay plants formed mycorrhizae in soils beneath dead pines but not in soil beneath live pines, and mycorrhizal inoculum appeared to be absent from the live pine stand. 

It seems unlikely that the allelopathic chemicals that may be extracted from plant material are actually those that reach the host plant, yet nearly all our information on allelopathic compounds is derived from extracts that have never been exposed to the soil. Some compounds, such as juglone, may remain unchanged in the soil under some circumstances ( ), but many compounds, such as ferulic or salicylic acid, are converted to other chemicals in the soil. 

The above classification of detoxication reactions has been developed for the metabolism of synthetic pesticides In plants. However, the same reactions can occur with natural exocons, such as allelopathic compounds, that have the same functional groups as synthetic pesticides. Most allelopathic chemicals contain functional groups that can be conjugated by Phase II reactions. Thus, detoxication of allelopathic compounds can be expected to proceed by conjugation with the omission of Phase I reactions. The remainder of this review will be concerned with the conjugation of allelopathic compounds. 

A characteristic feature of allelopathy is that the inhibitory effects of allelopathic compounds are concentration dependent. Dose-response curves with known compounds show an inhibition threshold. Below this level either no measurable effect occurs, or stimulation may result. Although the concentration of a compound required to exceed the inhibition threshold varies extensively according to different sensitivities among species and also among phases of the growth cycle for higher plants, the concept of an inhibition threshold seems consistent. Thus, it is reasonable to evaluate how, and if, a subthreshold concentration of an allelochemical may contribute to allelopathic interference. Also in need of evaluation is how environmental conditions may influence the deleterious action of an allelochemical and the concentration required for an effect. Such interactions are especially pertinent for those environmental situations that place some degree of stress on plant functions. 

Thus, the potential Impact of an allelochemical on plant growth should be evaluated with regard to both the presence of associated allelopathic compounds and the influence of other chemical and physical conditions in the environment. Certainly allelochemical action is not an isolated event, and from the standpoint of plant functions, the controversy between competitive and allelochemical Interference loses some of its significance. Allelochemical action needs to be regarded with a holistic view where one stress may reinforce, or magnify, another. From this perspective, inhibition of plant growth is not so much a matter of which factor is most detrimental instead it is determined by the interaction of multiple stresses. 

There are many examples of crop rotation sequences that passively suppress nematode populations which will not be reviewed here. Examples of active nematode suppression in crop rotation sequences are typically found with plant species that produce and excrete allelopathic compounds. These compounds then affect plant-parasitic nematodes in the rhizosphere either directly or indirectly by altering rhizosphere microbial populations (Halbrendt, 1996). For the purpose of this chapter, allelopathic... 

Waller, G.R., M.C. Feng, Y. Fujii. Biochemical analysis of allelopathic compounds plants, microorganisms, and soil secondary metabolites. In Principles and Practices in Plant Ecology Allelochemical interactions, Inderjit, Dakshini and Foy C.L. eds. CRC Press, Boca Raton, FL, US A. 1999, pp. 

Chung, I.M., Ahn, J.K., Yun, S J. Identification of allelopathic compounds from rice (Oryza sative L.) straw and their biological activity. Can J Plant Sci 2001 81 815-819. 

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