Allelopathic agents Allelopathy

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 ). 

Phenolics may influence competition among plants, a phenomenon called allelopathy. A series of experiments in both the field and the laboratory have indicated a role for a number of phenolic derivatives (hydroquinone, hydroxybenzoates and hydroxycinnamates) as allelopathic agents. These are chemicals excreted by the plant, which may be autotoxic or affect the growth of other plants in the environment [6,7]. 

Mandava, N.V. Chemistry and biology of allelopathic agents. In The Chemistry of Allelopathy. A.C. Thompson ed., Washington DC, USA American Chemical Society, 1985. 

Allelochemicals. 2. Allelopathic agents. 3. Allelopathy. I. Marfas, Francisco A., Galindo, Juan C.G., Molinillo, Jose M.G., and Cutler, Horace G. 

The number and diversity of compounds implicated in allelopathy are rapidly growing. Acetic acid and a few other major intermediates of metabolism have been reported as allelopathic agents, but most of those that have been identified are secondary compounds that arise from the shikimic acid and acetate pathways, or result as hybrids from these synthesis pathways. Rice (1 ) classified the compounds into fourteen chemical categories, plus a miscellaneous group. 

Jarvis BB, Pena NB, Rao MM, Comezoglu SN, Comezoglu TF, Mandava NB (1985) Allelopathic Agents from Parthenium hysterophorus and Baccharis megapotamica. In Thompson AC (ed.) The Chemistry of Allelopathy. Biochemical Interactions. ACS Symposium Series 268, p. 149... 

Fischer, N. H. and L. Quuano, Allelopathic agents from common weeds, in The Chemistry of Allelopathy (A. C. Thompson, ed.), ACS Symposium Series 268, 133-147, American Chemical Society, Washington, DC, 1985. 

Molisch (1937) coined the term allelopathy to describe both positive and negative chemical interactions of plants mediated through the environment (Wilhs 1985, 2007 Molisch [translation] 2001). His definitions of allelopathy and allelopathic agents as translated by LJ La Fleur and MAB Mallik (2001) were as follows,... 

While virtually all plants contain chemical constituents which have allelopathic potential, only those chemical constituents that are released to the surrounding environment can be considered potential allelochemicals. A potential allelochemical can only be designated as an allelopathic agent after it has undergone a rigorous proof of demonstrated phytotoxic activity. The chemical nature of recognized allelochemicals is diverse, and almost every class of secondary plant metabolite has been implicated in allelopathy. 

Table I provides general information on the identified allelopathic weeds and the crops they affect. More detailed information on two of these weeds, johnsongrass and purple nutsedge, will be presented to show the tenuous nature of the evidence for allelopathy. These two species are undoubtedly allelopathic, at least under certain conditions. Rigorous proof that allelopathy is the agent responsible for even a specific portion of the interference exerted by them is not easily attained, however, even though these are the weeds with the best research data available of those weeds that occur in the Mid-South.

Except direct use of allelopathic crops as cover crops, smother crops, and intercrops, applications of allelopathy for weed control include the use of allelopathic residues as an herbicide agent, e.g., pellets flours, water extracts, etc. The most common example of crop residue utilization is application of straw on the soil surface (mulching), e.g., rice straw inhibited germination of field bindweed (Convolvulus arvensis L.), winter wild oat (Avena ludoviciana Durieu), and little-seed canarygrass (Phalaris minor Retz.) (Lee et al. 1991 Tamak et al. 1994 Young et al. 1989). 

These days, allelopathic plants as catch crops or trap crops found utilization in plant protection of tropical regions against parasitic weeds. They do not eliminate the parasite completely but decrease the seed bank in the soil. Other applications of allelopathy for weed control include the use of plant residues as a natural herbicide agent, e.g., water extracts, pellets, flours, by-products of crop processing, etc. The strategies for use of volatile compounds as soil fumigants are developed. 

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