Allelochemical interference

Phenolic acids interfere with many major physiological processes of higher plants (35). These disruptions of function include an alteration of plant water balance. We found depression of leaf water potential to be an early indicator of allelochemical stress from ferulic and p-coumaric acids (42). Likewise one mechanism of allelopathic action by cultivated sunflower, velvetleaf Abutilon theophrasti Medic.), Koahia [Koahia saoparia (L.) Schrad.], and several other weeds was water stress (43-45). Since some allelochemicals interfere with plant-water relationships, it seemed logical that their action might be most critical at times when plants are under water stress from other causes. 

Although indirect and probably quite rare, another route has been reported for allelochemical interference with plant-water relationships. Lovett and Duffield (47) identified benzylamine as an allelochemical in the leaf washings from the cruciferous weed Cametina sativa (L.) Crantz. Subsequent work showed benzylamine induced hydrophobic conditions in the soil, and these conditions could reduce water availability for plant growth (48). Thus, indirect action through changes in soil structure could be partially responsible for adverse effects on linseed (Linseed usitatissimm L.) and could enhance more direct allelopathic effects. 

We designed experiments to determine if residual concentrations of atrazine and near-threshold levels of phenolic acids have a cooperative action (51). Procedures in these studies were similar to the ones used in investigations described earlier for determining moisture-allelochemical interference, except oat (Avena sativa L.) seedlings were utilized and these were treated 9 days after germination. Ferulic acid was chosen as a representative allelochemical. Treatments of atrazine and ferulic acid were made as amendments to the nutrient solution in which the oat plants were grown. 

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. 

Inderjit, Weiner, J. Plant allelochemical interference or soil chemical ecology Persp Plant Ecol Evol System 2001 4 3-12. 

Dalton, B. R., 1999. The occurrence and behavior of plant phenolic acids in soil environments and their potential involvement in allelochemical interference interactions Methodological limitations in establishing conclusive proof of allelopathy. In Inderjit, Dakshini, K. M. M., and Foy, C. L., (Eds.), Principles and Practices in Plant Ecology Allelochemical Interactions, CRC Press, Boca Raton, FL, 57-74... 

The term allelopathy was coined by Molisch in 1937 Q). Presently, the term generally refers to the detrimental effects of higher plants of one species (the donor) on the germination, growth, or development of plants of another species (the recipient). Allelopathy can be separated from other mechanisms of plant interference because the detrimental effect is exerted through release of chemical inhibitors (allelochemicals) by the donor species. Microbes associated with the higher plants may also play a role in production or release of the inhibitors (2). 

The results of these experiments support the hypothesis that inhibitory allelochemicals may interfere with the water balance of seedlings. Alterations in the water status of grain sorghum seedlings were caused both by known phenolic acids and by allelopathic weeds. 

When these algae are transferred into animal cultures (as food), it is suggested that the binder would interfere with the animal s utilixation of ambient Bjj in a manner that inhibits the development of reproductive viable progen . If correct, this could be interpreted in natural circumstances as either primar or secondar allelochemical activit (or both). It is primar in that the production of a B 2-binder would offer a variet of advantages in terms of... 

Australian workers have reported allelopathic phenomena in native plant communities, both undisturbed and managed. In agriculture, allelochemicals have been identified with plant interference during life and, from their residues, after death. Bacteria are involved in examples of allelopathy from these several milieux. 

The biochemical mechanisms through which allelochemicals exert deleterious or toxic effects on plants are, for the most part, unknown (1). Some phenolic acids, cinnamic acids, coumarins, and flavonoids have been reported to inhibit photosynthesis and respiration of intact plants and microorganisms. However, the mechanisms, at the molecular level, through which the compounds interfere, remain to be ascertained. Some phenolic acids, coumarins, and flavonoids were reported to Inhibit C02-dependent 0 ... 

Results obtained from the partial reactions, chlorophyll fluorescence, and binding studies did not provide any clues relative to interactive sites on the electron-transport pathway for the allelochemicals. No evidence was obtained to specifically implicate Interference with the protein of PS II. Additionally, insofar as they could be analyzed, none of the allelochemicals affected PS I-associated electron transport between the site of donation by DPIPH and acceptance by methyl vlologen. 

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. 

Opportunity exists in agroecosystems for two sources of chemical interference, natural and synthetic. The origin of allelochemicals... 

Several lines of evidence support the conclusion that allelopathic inhibition of germination and plant growth typically occurs from the joint action of several allelochemicals. Additive or synergistic effects have been shown in bioassays with combinations of monoterpenes, organic acids, and several classes of phenolic compounds. These experiments demonstrate that a specific compound may be present below its threshold for inhibition and still be active in allelopathic interference by its effect in concert with other allelochemicals. 

We should determine which plant pests inflict their damage through production of phytotoxins. It now appears that several pathogenic fungi may do this. Numerous weed species may impose interference on crop growth, at least in part through allelochemicals. More than 70 species have now been alleged to have allelopathic potential. 

An obvious place for intensive work on allelopathy is in the weed science area. Here, plant interference is either our problem or our opportunity. We should be clever enough to exploit allelopathy as a weed-suppression strategy. This could be accomplished with crops that release allelochemicals through exudation or by crop residues placed into sequential cropping systems. My research team and others have already developed some promising leads in this area. 

Jimenez-Osornio J. J., S, R. Gliessman. Allelopathic interference in a wild mustard (Brassica campestris L.) and broccoli (Brassica oleracea L. var. italica) intercrop agroecosystem. In Allelochemicals, Role in Agriculture and Forestry, Waller G.R. ed. ACS Symp. Ser. 330. American Chemical Society, Washington DC, USA, 1987 pp. 262-274. 

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