Allelopathic activities

K. H. Park, K. Moody, S. C. Kim, and K. U. Kim, Allelopathic activity and determination of allelochemicals from sunflower (Helianilws annuus L.) root exudate.s. 11. Elucidation of allelochemicals from sunflower exudate.s. Korean J. Weed Sci. 12 173 (1992). 

An interesting feature of this list is that some of our most troublesome and difficult to control weeds are not suspected allelopathics. Many weeds, such as morningglory (Ipomoea spp.), cocklebur (Xanthium strumarium L.), and sicklepod (Cassia obtusifolia L.), which make almost all lists of serious weed species, are strong competitors with little or no evidence of allelopathic activity. In fact, cocklebur is probably the epitome of what a competitor can be, being even more devastating than... 

A regression analysis was used to test the johnsongrass rhizome biomass for allelopathic activity. 

Table I. Weeds With Reported Allelopathic Activity Found in Row Crops in the Southern United States...

A crop is listed if a report exists of specific allelopathic activity of that weed on the crop. 

The foregoing examples summarize the allelopathic agents produced by both crop and weed plants which affect the growth of crop plants. However, we must be extremely cautious when implicating the inhibitory (or stimulatory) action of secondary plant products or their metabolites in terms of allelopathic activity, because of the following contradictory reports. 

Aliphatic compounds Several water-soluble simple organic acids and alcohols are cannon plant and soil constituents. They include methanol, ethanol, n-propanol and butanol (40), and crotonic, oxalic, formic, butyric, lactic, acetic and succinic acids (41, 42), all of which inhibit seed germination or plant growth. Under aerobic conditions, however, aliphalic acids are metabolized in the soil and therefore, should not be considered a major source of allelopathic activity (40). 

Fatty Acids and Lipids Although several fatty acids, esters and alcohols are known to be toxic to plant growth, their role in allelopathy is not fully investigated (3). Dihydroxystearic acid (3, 49) is the classic example known to exhibit allelopathic activity. 

Figure 3. Aromatic Compounds Eliciting Allelopathic Activity...

The observation that allelcpathic agents affect plant growth and development leads the physiologist to question the modes of action. While we know that diverse secondary plant products exhibit allelopathic activity, it is difficult to understand the mechanism of action of these plant products partly because of ... 

Soybean leaf extracts have not previously been reported to be phytotoxic however many other leguminous species have been shown to be active (1,17). The possible allelopathic activity of soybean indicated by these bloassays is potentially economically important, since soybean is a major crop world wide. 

Amaranthus Palmeri S. Wats., Palmer amaranth, is a coarse, weedy, drought-resistant, dioecious member of the Amaranthaceae and is related to A. retroflexus L., redroot pigweed, and the ornamentals k. tricolor L., sumner poinsettia, and /. caudatus L., love-lies-bleeding. Palmer amaranth was used by natives of the North American desert as a protein source and cereal to supplement maize and beans. Another member of the same family, A. spinosus L., spiny amaranth, has been reported to exhibit allelopathic activity toward coffee... 

Seeds of lettuce and other species have frequently been used to bioassay for the allelopathic activity of plant exudates (17.18.19). As with the use of cell suspensions, there are certain advantages and disadvantages to this methodology. The experimental simplicity, small amounts of material required and short time frame are certainly attractive qualities. However, species used in such bioassays quite often do not represent the actual target species under consideration. This is especially true when terrestrial crop species are substituted for weeds of aquatic systems. Nevertheless, information obtained from such experiments are often valuable when used in conjunction with results of other assays. 

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... 

Interestingly, okadaic acid diol ester (16) in the culture medium is oxidatively transformed into the more hydrophilic metabolites 18-20 by intact cells of the diatom Thalassiosira weissflogii [52,53]. This transformation is speculated to change the toxic metabolite with allelopathic activity into a less toxic transport form that can be better excreted by the diatom. 

Table 2 The effects of water and ethanol extracts from allelopathically active plant species on the germination of Equisetum arvense microspores (amount of red-fluorescing microspores, % of control)... 

Experiment 2. Effects of terpenoid allelochemicals on pollen germination The effects of the terpenoids found in many allelopathically active plants are dependent on the concentration of the active matter (Fig. 4). Linalool, cymol, citral, gaillardine and austricine excreted by many flowers, stimulated the pollen germination upto 10 7-1()" M concentrations and depress in higher ones. Rodriguez et.al (1976) described the biological effects of some sesquiterpene lactones, which are allelochemicals. 

If the allelochemical is hydrophylic, it cannot enter into the cell and act from outside by binding with chemoreceptors. The compounds from allelopathically active plants may serve as chemosignals and their signalling occurs via alternative pathways (i) Chemoreceptor (sensors) — transducers (G-proteins) —> secondary messengers (Ca2+, cyclic AMP or GMP, inositol triphospate, etc) —> organelles or (ii) Chemoreceptor (sensors) —> ion channels —> action potential organelles, or (iii) Chemoreceptor (sensors) —> ion channels —> cytoskeleton— organelles (Roshchina, 2005 a). What is the effect of acted allelochemical on the pathways, could be analysed to study the effects of substances on separate sites of the transduction chain. 

Blue colour ring concentrated on plasmalemma of pollen (A). Arrows on B (lower part) shows a difference in the colour between cellular surface (blue color) and apperture for the output of pollen tube (red colour). Pistil excreted blue colour product, which covers the red coloured surface of pistil. The cholinesterase activity in plants is considered as sensitive test to study the allelopathic activity (Roshchina and Roshchina 1993 Roshchina,1999 2001a). 

First of all, the allelopathic activity of a compound (or of a mixture of compounds) isolated as per the above methods should be tested in agar diffusion assays using different algal strains The selection of an assay alga depends primarily on its ability to grow satisfactory under the chosen... 

Rue infusion was tested for its allelopathic activity in vitro on water uptake and germination of radish seeds. Seeds of radish (Raphanus sativus L.) Saxa, collected during 2003, were purchased from Improta Co., Naples. 

Experiment 1. The secretory trichomes of allelopathically active species. 

Experiment 8. The observation of the secretory hairs and their secretion Cell-donor of allelochemicals releases the substances out. The process may be seen with the help of LSCM technique as the study of the fluorescence of various external secretory structures. Such structures are glandular cells, which contained many potentially fluorescent substances (Roshchina and Roshchina, 1993). One of the example is shown for secretory leaf hair of allelopathically active species Solidago virgaurea L. (Fig.ll). 

Experiment 1. Visual observation of secretory cells in luminescence microscope Various types of secretory cells in allelopathically active plants contain fluorescing secondary products (Fig. 1). Secretions from the above ground parts of plant (in leaves, flowers, stems) were concentrated in secretory hairs and glands. Whereas secretions of roots are in secretory reservoirs and idioblasts (ordinary cells which accumulate secretory products) or may be released by the secretory surface of the root tip (Fig. 1.). The fluorescence appears to change, when allelopathically active cell of other plant species (cell-donor) interacts with acceptor cell (Roshchina and Melnikova, 1999). 

Secretory hairs of allelopathically active species contain various secondary metabolites (allelochemicals) which may fluoresce in blue-green on the red... 

WALLER, G.R., JURZYSTA, M., THORNE, R.L.Z., Allelopathic activity of root saponins from alfalfa (Medicago sativa L.) on weeds and wheat, Bot Bull. Academy. Sin, 1993,34, 1-11. 

On the other hand, allelopathic effect was negatively influenced by rainfall (Shiming 2005). The inhibition process was mitigated by shading and consequent moisture conservation (Muller 1966). Allelopathic activity can vary as well with photoperiod (Peng et al. 2004). Harder et al. (1998) found out that an increasing availability of nutrients decreased the concentration of allelopathic effective phenolic compounds in the plants of two winter wheat varieties. In soil, allelochemicals can be adsorbed by soil particles, decomposed by microorganisms and move with water. 

For next development of allelopathy utilization, especially such ways as breeding for stronger allelopathic potential is very hopeful. Hybridization could be a promising method of breeding. However, allelopathic activity was identified as a quantitative trait and therefore this characteristic is affected by both genetic effects and environmental conditions. The main disadvantage of the application of allelopathy is considerable variability in the dependence on environment. Therefore all results achieved in laboratory should be compared with effects of allelopathic crops in field conditions. 

Dilday RH, Yan WG, Moldenhauer KAK, Gravois KA (1998) Allelopathic activity in rice for controlling major aquatic weeds. In Olofsdotter M (ed) Proceedins of the workshop on Allelopathy in Rice, Manila, Philippines, International Rice Research Institute, Makati City, Philippines, pp 7-26... 

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