Allelopathic agents

Following this terminology, all allelopathic agents described in this paper are kolines since we confine the subject to higher plants affected by other plants grown in the immediate environment, e.g., crop plants whose growth is inhibited by weeds. 

The foregoing examples summarize the allelopathic agents produced by both crop and weed plants which affect the growth of crop plants. However, we must be extremely cautious when implicating the inhibitory (or stimulatory) action of secondary plant products or their metabolites in terms of allelopathic activity, because of the following contradictory reports. 

According to Robinson (38), Whittaker and Feany (39), and Rice (5j, a great majority of secondary plant products are biosynthesized from acetate and shikimic acid (38), many of which have been implicated as allelopathic agents, and the main groups of compounds are described below. 

Arcmatic compounds phenols, phenolic acids, cinnamic acid derivatives, coumarins, flavonoids, quinones, and tannins, all of which are aromatic compounds, comprise the largest group of secondary plant products. They are often referred to as "phenolics" and have been identified as allelopathic agents in more instances than all of the other classes of compounds combined 5). 

It has been documented (5J that allelopathic agents are released into the environment by (a) exudation of volatile chemicals from living plant parts, (b) leaching of watersoluble chemicals from above-ground parts in response to the action of rain, fog or dew,... 

Table 2. Allelopathic Agents Isolated Fran Various Sources (46)...

Several procedures have been reported for extraction of the suspected allelopathic agents from donor plants. Essentially all the procedures that were employed attempted to simulate the routes of entry of toxic substances into the natural environment. As shown previously, the allelopathic agents are released through leaves and roots, or escape into the environment as volatile materials. Table 3 suratBrizes the different extraction and bioassay procedures employed to isolate and detect the toxic chemicals (17). For extraction, the investigators used either the plant parts from the donor plants or the intact donor plants from which the suspected chemicals were leached through leaves, stems or roots. 

Table 3. Extraction and Bioassay Methods for Allelopathic Agents (17)...

Various bioassay methods have been used to detect the "natural" release of allelopathic agents. Sane authors preferred, after partial purification, to assay the extracts by petri dish methods for gemination, growth of roots or shoots and other symptoms of seedlings. The bioassays also included tests in soil or sand and also in nutrient solution (Table 3). 

Chromatographic methods were extensively used to characterize the allelopathic agents. Because a great majority of the suspected toxins were already known (and several of them are ccrmercially available), simple chromatographic methods were thought to be sufficient for their characterization from the extracts. However, no detailed investigations were ever undertaken to characterize those inhibitors that may be present in minute amounts. 

The lack of understanding of the effect of the allelopathic agents on whole plant photosynthetic processes, namely, changes in stanatal opening, membrane permeability, water content and marry other processes that affect the overall photosynthetic processes. 

Determine effects of allelopathic chemicals on ecological community, including the use of growing plants as allelopathic agents. 

Investigate the most premising allelopathic agents for harmful side effects as required by current EPA procedures. 

Proper understanding of allelopathic crop and weed plants including their growth stages at which toxin production occurs and characterization of allelopathic agents frcm these plants provide new avenues for developing technologies in weed control, crop efficiency, pest control and plant diseases. 

Once we know the chemical nature of the allelopathic agents and their effects on plant growth dynamics, as well as on health and environment, we can apply genetic manipulation and biotechnology to develop toxin-resistant plants and to reduce the toxin levels frcm the donor plants. These approaches serve a dual purpose because they contribute to increased agricultural productivity and help to minimize the potential risks on health and environment. 

In a search for allelopathic agents from common weeds, Amaranthus palmerl S. Wats (Palmer amaranth) and Ambrosia artemisiifolia L. (Louisiana annual ragweed) have been analysed for their organic natural products. From A. palmerl phytol, chondrlllasterol, vanillin, 3-methoxy-4-hydroxynitrobenzene and 2,6-dimethoxy- benzoquinone were isolated. From the roots of Ambrosia artemisiifolia four polyacetylenes, a mixture of sesquiterpene hydrocarbons, methyl caffeate, and a mixture of 8-sitosterol and stlgmasterol were obtained. 

A mechanism Is proposed by which water-insoluble plant lipids (sterols etc.) may act as allelochemicals via micelle formation with long-chain fatty acids. By this process plant lipid solubility and transport In the aqueous medium are enhanced. This might suggest a reevaluation of water-insoluble plant constituents such as sterols as potential allelopathic agents. 

FISCHER AND QUIJANO Allelopathic Agents from Common Weeds... 

Allelopathic Agents from Parthenium hysterophorus and Baccharis megapotamica... 

There are numerous reports of allelopathy in the literature, but often the identity of the allelochemical(s) is unknown. There are, however, many cases where specific compounds or groups of compounds have been implicated as allelopathic agents. Table 1 summarizes some examples of sources and identities of allelochemicals that directly inhibit plant growth. These secondary compounds have been implicated as a driving force in ecological succession ( 1 ). 

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