Allelopathic interactions

Flavonoids and other phenolic compounds released by plant roots have important functions in plant-pathogenic interactions, feeding deterrence, nematode resistance, and allelopathic interactions they also serve as signal molecules for the establishment of symbiotic associations (72,149). However, a detailed analysis of signaling pathways involved in these interactions is currently available in only a limited number of cases (see also Chap. 7). 

Although allelopathic interactions have been observed for centuries, the science of allelopathy is in its infancy. Much needs to be accomplished, and it will require joint efforts of scientists from several disciplines. Although by no means a complete list, the following areas need intensive study. 

Root exudates A wide variety of chemicals, such as sugars, amino acids, and aromatics, is excreted by roots of plants. Very little information is available on the allelopathic interaction of root exudates with the higher plants, except for the identification of a few products in isolated cases (46). 

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... 

Although many physiological and biochemical processes In plants are affected by various allelochemicals, In most Instances the details of the mechanism of action of a particular allelochemical have not been elucidated. Because soil mediates the transfer of most allelochemicals (except perhaps volatile compounds) from a donor to a receiver, plant roots are often the first tissues to contact an allelochemical. Thus, It Is not surprising that root growth and development are Inhibited In many Instances of allelopathy (1.-3) One of the primary physiological functions of plant roots Is the absorption of mineral nutrients. Therefore, It Is logical that the Influence of allelopathic Interactions on mineral absorption by plant roots has been Investigated. 

Several general characteristics of the results compiled in Table I are worthy of mention. Compared to the variety of chemicals postulated to be involved in allelopathy (1), few specific compounds have been tested for inhibition of mineral absorption. The most extensively studied compounds are the phenolic acids, probably because of their being ubiquitously found in nature (1). Also, several flavonoids are inhibitory to mineral absorption (Table I). Both of these groups of compounds are often cited as being responsible for allelopathic interactions between plants. 

If mycorrhizae are sites of action for allelochemicals, this is an important indirect aspect of allelopathic interaction among plants. Inhibition of mycorrhizal formation or a reduction in the efficiency of mycorrhizal association would reduce the nutrient level of the mycorrhizal plant and subsequently its competitiveness, stress tolerance or nodulation. Although allelochemicals have been implicated in the reduction of nodulation and nodule size, possible mycorrhizal involvement has not been examined. This is a difficult area of research but one that will provide better understanding of this complex situation. 

Other roles for noxious metabolites produced by certain phytoplankton species include mediation of allelopathic interactions [47]. Allelopathy covers biochemical interactions among different primary producers or between primary producers and microorganisms. These can provide an advantage for the producer in the competition among different photoautotrophs for resources. Although not directly involved in chemical defense, allelopathic metabolites can affect the dominance and succession of species in phytoplankton therefore they are crucial for understanding plankton composition. In contrast to the fresh water environment, the location of many studies on the role of allelopathic interactions, which have identified active compounds [47], only few studies have addressed this topic in the marine environment. 

In allelopathy studies, the allelochemicals first influence the physiological and biochemical processes in cells. Till now there is no book of methods to study allelopathic interactions in the cells. The activity of cells influence various important physiological processes like seed germination, plant growth and development, photosynthesis and respiration, senescence and abscission are included in this volume. To understand the basic mechanisms of various physiological processes, being affected by allelochemicals at the cellular level enzyme activity and metabolite studies are essential. 

The plant cell is a basic unit for germination, growth and development of plants. The allelochemicals first come in contact with the cell and then allelopathic interactions take place. Some allelochemicals have broad-spectrum activity that extends to the tissues of host plants, where their effects may be either beneficial or deleterious to plant germination, growth, development or yield. Hence, to understand the mechanisms of such intercations interactions the study of the cell and its various processes is very necessary. Therefore, this book has been been prepared (a) to make available all methods for such studies and (b) scientists can understand the scope of allelopathic research in relation to the cell. Hence, we have explained and discussed various techniques to study cell processes etc. 

While more commonly used to count or otherwise characterize cells for medical applications, Coulter Counters and flow cytometry technique can also be applied to the analysis of pollen grains in allelopathic studies. They are quite useful in determining the size and number of pollen grains. The technique is often used for assessing the production and size of pollen from the originating individual rather than how much was transferred to heterospecific stigma, as would be needed in a basic assessment of potential allelopathic interactions. 

Allelopathic interactions of plants can provide weed control by (a) use of allelopathic crops as cover crops, mulches or green manure, (b) use of allelopathic plants in crop rotations, (c) crop mixtures and intercropping, (d) varieties with strong allelopathic potential, and (e) use of allelopathic crop water extracts and other agents. 

Singh HP, Batish DR, Kohli RK, Allelopathic interactions and allelochemicals New possibilities for sustainable weed management, Crit Rev Plant Sci Tl Siy)—... 

The goals of this paper are to discuss some of the major generalizations that can be made about allelopathic interactions, provide some examples of suggested roles, and focus on desirable future research applications. Only a few of the pertinent investigations are cited in illustrating major principles. 

In 1979, we decided to extend our studies to one of the agroecosystems of greater Importance in Mexico the coffee plantations. These studies were realized within the Program of Agroecosystems at the-Institute Nacional de Investigaciones sobre Recursos Bioticos. We worked at the coffee plantations in Coatepec, Veracruz, which are characterized by the presence of shade trees which resemble the structure of the deciduous temperate forests, with three well defined strata the herbaceous layer, the shrub layer represented by coffee plants and the tree layer. The main objective of this study was to assess the allelopathic interactions among the species that constitute this community, in particular the coffee plants (19). 

Allelopathic interactions may occur throughout the life of a stand, but are most commonly observed during reforestation or regeneration. Allelopathy prevents some tree species from regenerating, but most regenerate in spite of it. The allelopathic plants of abandoned fields are not common forest species. In contrast, however, Douglas-fir, jack pine, black and white spruce, wild cherry, and slash and loblolly pine seedlings appear to be inhibited by species common in the forest. In such cases... 

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