Allelopathic effects

Assessment of the Allelopathic Effects of Weeds on Field Crops in the Humid Midsouth... 

How weeds produce their effect is the subject of continuing research. As rather recently defined, the negative effect of weeds on crop plants includes both competition and allelopathy (1.) and has been termed interference. Previous weed science research considered the competition component foremost and, usually, solely (e.g. 2). Allelopathy, however, has received some attention recently, with a number of symposia and reviews devoted to allelopathic effects. Some of these cover the effects of weeds on crops (3, 4, 5). In this chapter, I review the possible role of allelopathy in weed interference in crop production in the Mid-South. A second objective of this review is to indicate where research is needed and to suggest potential lines of future research, especially with respect to the developing role of conservation tillage practices in this region. 

The research strategies recommended by the USDA Research Planning Conference for EPA-related activities include studies ons (1) allelopathic effects on environment in terms of allelochemical persistence, activity and effectiveness, (2) ecological consequences due to alle-lochemicals, and (3) potential risks to human and animal health (37). In addition to these approaches, we should also pursue studies toward understanding the effect of ccrmercial allelochemicals, alone or in combination with other chemicals such as pesticides, on human health, safety and environment. 

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. 

The term allelopathy, when first proposed by Molisch (1 ), referred to either the beneficial or detrimental interaction between all types of plants and microorganisms. As presently used, this definition is generally accepted. Since 1970 a concerted effort has been made to understand the phenomenon of allelopathic interaction. The many interpretations resulting from these studies are well documented in the literature (2-4). An area currently receiving considerable attention is the allelopathic effect resulting from weed-crop and weed-weed interactions (2, 5-7). One study conducted by Wilson and Rice (7) showed that the common sunflower, Helianthus annuus L., possessed allelopathic properties. Realizing the inherent potential... 

Maximal levels for -coumaric and ferulic acids of 30.0 and 6.5 pmol/1 0 g of soil ha e been reported (158) and concentrations of 4 x 10 M and 3 x 10 M, respectively, for these two acids in other soils (161)Other gtudies indicate a similar concentration range of 2.3 x 10 to 10 M for -hydroxybenzoic, vanillic and j>-coumaric acids (169). These levels may be too low to have direct measurable allelopathic effects on plants in greenhouse or growth chamber studies (non-rhizosphere soils, low microbial population). However, in field rhizosphere soils (high microbial population) these levels could be sufficient to influence microbial growth... 

The resins of conifer plants contain terpenoids and phenols, which demonstrate allelopathic effects. 

The allelopathic effects of dominant plants on other plants in phytocoenosis are caused by phenolic phytotoxins present in all parts of plants, but the highest amount of these compounds is accumulated in the leaves. Leaves of dominant trees represent the main components of the litter in the forest, thus analysis of phenolic compounds and measurements of their content in leaves and leaf litter is considered as very important. 

Lodhi, M.A.K. and Rice, E.L. (1971). Allelopathic effects of Celtis laevigata. Bulletin of Torrey Botanical Club 98 83-89. 

Murphy, S. D. and Aarssen, L. W. (1995). In vitro allelopathic effects of pollen from three Hieracium species (Asteraceae) and pollen transfer to sympatric Fabaceae. American Journal of Botany 82 37-45. 

Sukhada, K.D. and Jayachandra (1980b). Allelopathic effects of Parthenium hysterophorus L. Part IV. Identification of inhibitors. Plant and Soil 55 67-75... 

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