Isolating allelopathic effects

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. 

This does not mean that analysis of allelopathy in an arid environment should be done differently from that which is customary in a humid environment yet it is important to estimate the extent to which inorganic salts (excreted by the plant or released from its litter) are involved in the allelopathic effect. So far as secondary metabolites are concerned, it should be of interest to compare their production under humid and stressed conditions. It is suggested that for the evaluation of the allelochemical effect, species suppressed in their natural habitat should be preferred over any other standard seeds commonly used for evaluating germination inhibitors. Also, efforts to isolate allelochemicals from soils will assist in the establishment of allelopathy on a more concrete basis than is available at present. 

The allelopathic effects are cyclic and seem to involve the following series of events Materials accumulate in the soil in the dry season. Rain carries the toxins into the soil where they are absorbed into the top 3 cm of soil. The effectiveness of procedures for isolation of phenolic acids from soil have been evaluated (Dalton et al., 1987). These biologi-... 

Isol. from the l oot of Polygonum hydropiper. Constit. of folk medicinal plant with anticancer allelopathic effects. Amorph. 

C39H40O17 780.735 Isol. from the root of Polygonum hydropiper and of Polygonum lepathifolium. Constit. of folk medicinal plant with anticancer allelopathic effects. Amorph. powder prisms (as octa-Ac). Mp 84-84.5° (octa-Ac). [a]o +61.9 (c, 0.31 in MeOH). X, ax 228 (e 15000) 315 ( 26000) (MeOH) (Derep). 

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. 

Once biological activity has been established (in the laboratory or the field) and once the chemical work has been accomplished, we need to confirm allelopathic activity in the natural environment. To accomplish this end, the effects of soil and microbial flora must be considered. Thus, the disciplines of soil chemistry and microbiology are required. The chapters in this volume deal primarily with the biology and chemistry of phytotoxins isolated from plants however, we hope that these topics will stimulate soil chemists and microbiologists to contribute to solving the problems associated with the study of allelopathy. Thus, the purpose of this volume is not only to bring before the scientific community a representation of research efforts in the area of allelopathy, but also to promote the relationships... 

Suzuki Y, Takabayashi T, Kawaguchi T, Matsunaga K (1998) Isolation of an allelopathic substance from the crustose coralline algae, Lithophyllum spp., and its effect on the brown alga, Laminaria religiosa Miyabe (Phaeophyta). J Exp Mar Biol Ecol 225 69-77 Svensson CJ, Pavia H, Toth G (2007) Do plant density, nutrient availability, and herbivore grazing interact to affect phlorotannin plasticity in the brown seaweed Ascophyllwn nodosum. Mar Biol (in press), doi 10.1007/s00227-007-0649-5 Swanson AK, Druehl LD (2002) Induction, exudation and the UV protective role of kelp phloro-tannins. Aquat Bot 73 241-253... 

As pointed out above, the bioassay design depends on the objective(s) of the study. A bioassay to determine allelopathic interactions in the field or in an ecological setting may have a quite different design than one used to determine PGR activity of a compound or to determine its molecular mode of action. Specific bioassays can be used to follow the isolation/purification of allelochemicals, evaluate their phytotoxic (or growth simulation) effects (i.e., visual effects), determine their host range/selectivity, evaluate allelopathic action of volatile compounds, or examine physiological/biochemical effects, such as photodynamic and membrane effects, effects on photosynthesis, specific enzyme sites, and effects at the ultrastructural level to locate receptor sites or sites of injury. Several examples of useful bioassays will be presented later. 

There are a number of allelochemicals among the mono-, sesqui-, and diterpenoids. In particular, plants in arid and semiarid regions produce diverse volatile terpenoids with allelopathic activity.5 Among the volatile monoterpenes, 1,8-cineole (4) and camphor (5) exhibit strong growth inhibitory effects on plants and are considered to be involved in plant competition. 1,4-Cineole (6), a minor isomer of 1,8-cineole, is a potent inhibitor of asparagine synthetase.6 -Menthane-3,8-diols (fir 7 and frw .r8), -menth-2-en-l-ols (cis 9 and trans 10), thymol (11), carvacrol (12), 1,8-cineole, cr-pinene (13), and /3-pinene (14) were isolated as allelopathic monoterpenes from Eucalyptus species.7 Eucalyptus trees also produce allelopathic sesquiterpenes including spathulenol (15), and a-, (3-, and 7-eudesmols (16-18).7... 

Literally hundreds of plant extracts have been tested for bioactivity and shown to be active. In fewer cases, compounds have been isolated from these extracts and tested, and individually they often have shown activity. Scores of commercially available natural products also have been tested in a myriad of bioassays. Paper after paper has been written purporting that many of them are allelopathic. Such studies are done quickly and, unfortunately, often carelessly. The common flaw of most of these laboratory reports is that they fail to consider such obvious essentials as the need to test for activity of chemicals at concentrations in which they occur in nature use appropriate bioassay species that occur in the field, rather than the traditional, easily obtained crop species such as lettuce or tomato and test mixtures of compounds that simulate nature s release of chemicals into the soil, and in which synergistic effects are often commonplace. Thus, because of such neglect, there is both garbage and confusion in the literature. Many reported effects are not real, and probably many real ones have been overlooked simply because the bioassay used was faulty. 

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