Allelopathic Relationships

Some cyclic oxylipins were shown to be involved into the interplant allelopathic relationships between aquatic plants [25]. For example, Eleocharis microcarpa produces two compounds, inhibiting the growth of blue-green algae [25]. 

Once biological activity has been established (in the laboratory or the field) and once the chemical work has been accomplished, we need to confirm allelopathic activity in the natural environment. To accomplish this end, the effects of soil and microbial flora must be considered. Thus, the disciplines of soil chemistry and microbiology are required. The chapters in this volume deal primarily with the biology and chemistry of phytotoxins isolated from plants however, we hope that these topics will stimulate soil chemists and microbiologists to contribute to solving the problems associated with the study of allelopathy. Thus, the purpose of this volume is not only to bring before the scientific community a representation of research efforts in the area of allelopathy, but also to promote the relationships... 

Such allelopathic activity is developed during maturation of oat plants. Whereas only little activity is found in roots, somewhat more is found in stem and leaves. The highest activity, however, is found in the caryopses during maturation, especially in the husks ( ). In the first part of this paper, we shall describe isolation and structural elucidation of the main inhibitor from the husks of Avena sativa, which can be considered as an allelochemical against seeds of several plants. The second part of the paper deals with allelochemicals from rose seeds which, although of different chemical structure, bear some relationship to the allelochemical from oats. 

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. 

Organic solvents and water extracts prepared from monoculture wheat soils under conventional tillage (CT) and no tillage (NT) indicated that both soils contain some inhibitory compounds. The CGC/MS/DA of some of the organics is presented. Selected organics from CT and NT as well as allelopathic and autotoxic effects are described and discussed. The relationship between the wheat yields in CT and NT and the possible biological stress is indicated. 

Enzymatic complexes involved in flavonoid precursors [Achnine et al., 2004] and flavonoid biosynthesis are associated with the cytoplasmic side of the endoplasmic reticulum (ER) [Stafford, 1974, 1981, 1991 Hrazdina et al., 1978, 1987 Hrazdina and Jensen, 1992 Winkel-Shirley, 1999 Liu and Dixon, 2001 Saslowsky and Winkel-Shirley, 2001]. However, flavonoids accumulate in multiple subcellular compartments and some are even extruded out of the cellular domain. Examples are flavonoids sequestered in the vacuole of specific cell layers and organs where they function as insect attractants or herbivore deterrents [Koes et al., 1994 Mol et al., 1998], extruded flavonoids that participate in allelopathic interactions or the establishment of symbiotic relationships with soil rhizobia [Martinoia et al., 1993 Paiva, 2000], cytosolic... 

Staman et al. (2001) stated that in order to demonstrate that allelopathic interactions are occurring, one must, among other things, demonstrate that putative phytotoxins move from plant residues on or in the soil, the source, through the bulk soil to the root surface, a sink, by way of the rhizosphere. These authors hypothesized that the incorporation of phytotoxic plant residues into the soil would result in a simultaneous inhibition of seedling growth and a stimulation of the rhizosphere bacterial community that could utilize the putative phytotoxins as a carbon source. If true and consistently expressed, such a relationship would provide a means of establishing the transfer of phytotoxins from residue in the soil to the rhizosphere of a sensitive species under field conditions, presently, direct evidence for such transfer... 

Subsequent tests with velvetleaf, Kodkia, Jerusalem artichoke, and cocklebur showed that their allelopathic action altered water balance (55,94,95). Growth reductions in sorghum and soybean seedlings in nutrient solution amended with extracts from these weeds correlated with high diffusive resistances and low leaf water potentials. Stomatal closure occurred in plants treated with the more concentrated extracts. Depressions in water potential were due to a reduction in both turgor pressure and osmotic potential. A lower relative water content was also found in velvetleaf-treated plants. These impacts on water balance were not from osmotic factors. Allelochemicals from these weeds have not been thoroughly ascertained, but the present evidence shows that some contain phenolic inhibitors. Lodhi (96) reported that Kodkia contains ferulic acid, chlorogenic acid, caffeic acid, myricetin, and quercetin. As noted earlier, an effect on plant-water relationships is one mechanism associated with the action of ferulic acid. 

Allelopathic agents—Congresses. 2. Pests— Biological control—Congresses. 3. Insect-plant relationships—Congresses. 

What follows this introduction to plant-plant interactions (Chapter 1) are three additional chapters. The first chapter (Chapter 2) describes the behavior of allelopathic agents in nutrient culture and soil-microbe-seedling systems under laboratory conditions. Simple phenolic acids were chosen as the allelopathic agents for study in these model systems (see justifications in Section 2.2.6). The next chapter (Chapter 3) describes the relationships or lack of relationships between weed seedling behavior and the physicochemical environment in cover crop no-till fields and in laboratory bioassays. Here as well the emphasis is on the potential role of phenolic acids. The final chapter (Chapter 4) restates the central objectives of Chapters 2 and 3 in the form of testable hypotheses, addresses several central questions raised in these chapters, outlines why a holistic approach is required when studying allelopathic plant-plant interactions, and suggests some ways by which this may be achieved. 

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