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). 

Discover, identify, synthesize and evaluate allelopathic compounds for potential use in agriculture to increase the control of weeds, diseases, insects and nematodes, and to reduce the cost of their control. 

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... 

Shettel and Balke (27) have further demonstrated that allelopathic compounds can selectively suppress certain weed species. 

Aliphatic acids such as butyric acid have been previously implicated as being allelopathic compounds (46, 47, 23). Chou and Patrick (23) isolated butyric acid from soil amended with rye and showed that it was phytotoxic. Hydroxy acids have also been shown to possess phytotoxic properties (48) but have not been implicated in any allelopathic associations. Since SHBA is a stereo isomer, and the enantiomer was not identified because of impurity, all bioassays were run using a racemic mixture. The D-(-) stereo isomer of SHBA has been isolated from both microorganisms and root nodules of legumes and is suspected to be a metabolic intermediate in these systems (49). It is likely that only one enantiomer was present in the extract therefore, the true phytotoxic potential of this compound awaits clarification of the phytotoxicity of the individual enantiomers. 

Figure 2. Structure of dihydroactinidiolide, an allelopathic compound isolated from Eleocharis coloradoensis.

The identification of an allelopathic compound(s) and its mode of action may eventually permit the economical use of spikerush as a biological control measure in aquatic systems too fragile for chemical control, e.g. recreational water, drinking water, and certain irrigation canals. 

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. 

Ortega, R.C., Anaya, A.L. and Ramos, L. (1988). Effects of allelopathic compounds of corn pollen on respiration and cell division of watermelon. Journal of Chemical Ecology 14 71-86. 

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. 

Allelopathic companion crops, 73 352 Allelopathic compounds, as herbicides, 73 329-331... 

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). 

Allelopathic compounds consist of a wide variety of chemical types which arise through either the acetate or the shikimic acid pathway (5 ). These compounds range from very simple gases and aliphatic compounds to complex multi-ringed aromatic compounds. Oniy a few examples are mentioned below. 

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 revealing to consider microbial decomposition of allelopathic compounds in relation to synergism. As discussed above, partial decomposition of one compound may result in the presence of several active compounds, which may exert synergistic allelopathic effects. Thus, partial decomposition could increase allelopathic activity, rather than decrease it. 

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. 

The surface has just been scratched in determining the mechanisms by which the different kinds of allelopathic compounds exert their actions. Therefore, it is important that much more research be done in this area of allelopathy. 

All species studied inhibited the growth of certain test species. This confirms the selectivity of the allelopathic compounds (15). 

The production of allelopathic compounds in tropical zones, particularly if they are continuously released into the environment, may contribute to the elimination of secondary species already established and to the selection of those that are beginning to establish in the habitat. 

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. 

The toxin may undergo microbial degradation either while it is free in soil solution or while it is adsorbed. This could destroy all or part of the toxin, and there is evidence that most of the natural organic chemical groups that contain allelopathic compounds can be metabolized by some microorganism. The possibility always exists, however, that the microbial degradation product from the metabolism of an active toxin will itself be an allelopathic chemical. 

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. 

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