Showing papers on "Allelopathy published in 1999"

Essential Oils as Allelochemicals and Their Potential Use as Bioherbicides

Abstract: Essential oils were extracted from 32 aromatic plants and evaluated for allelopathic properties. The major components of the oils were identified by gas chromatography and GC-MS. Extracts from Origanum syriacum, Micromeria fruticosa, and Cymbopogon citratus were selected for further study. The germination of several species, including wheat, was strongly inhibited by essential oils when applied at 20–80 ppm. Essential oils mixed with the top 0.5 cm of soil inhibited germination of wheat and Amaranthus seeds. This effect depended on the type of soil. The possible use of essential oils as herbicides is discussed.

Roles of allelopathy in plant biodiversity and sustainable agriculture

Abstract: Allelopathic compounds are metabolites released from plants that might be beneficial or detrimental to the growth of receptor plants. These compounds are involved in the environmental complex of managed or natural ecosystems. Allelopathic compounds have been shown to play important roles in the determination of plant diversity, dominance, succession, and climax of natural vegetation and in the plant productivity of agroecosystems. The overuse of synthetic agrochemicals often causes environmental hazards, an imbalance of soil microorganisms, nutrient deficiency, and change of soil physicochemical properties, resulting in a decrease of crop productivity. The incorporation of allelopathic substances into agricultural management may reduce the use of synthetic herbicides, fungicides, and insecticides and lessen environmental deterioration. Scientists in many different habitats around the world have demonstrated the above examples previously. It is known that most volatile compounds, such as terpenoids, are re...

Principles and practices in plant ecology : allelochemical interactions

Abstract: Allelopathy: One Component in a Multifaceted Approach to Ecology, K.M.M. Dakshini, C.L. Foy, and Inderjit Methodologies Designing Laboratory Plant Debris-Soil Bioassays: Some Reflections, U. Blum How to Make Bioassays for Allelopathy More Relevant to Field Conditions with Particular Reference to Cropland Weeds, C.L. Foy Bioassays for Allelopathy: Interactions of Soil Organic and Inorganic Constituents, Inderjit and K.M.M. Dakshini Bioassays Applied to Allelopathic Herbaceous Vascular Hydrophytes, S.D. Elakovich The Occurrence and Behavior of Plant Phenolic Acids in Soil Environment and Their Potential Involvement in Allelochemical Interference Interactions: Methodological Limitations in Establishing Conclusive Proof of Allelopathy, B.R. Dalton Biochemical Analysis of Allelopathic Compounds: Plants, Microorganisms, and Soil Secondary Metabolites, G.R. Waller, M.-C. Feng, and Y. Fujii Microspectrofluorimetry of Intact Secreting Cells, with Applications to the Study of Allelopathy, V.V. Roshchina and E.V. Melnikova Interactions among Plant and Microbial Systems Pollen Allelopathy, S.D. Murphy Allelopathic Bacteria, O. Barazani and J. Friedman Allelochemistry in Plankton Communities, K.I. Keating Allelopathy in Benthic and Littoral Areas: Case Studies on Allelochemicals from Benthic Cyanobacteria and Submersed Macrophytes, E.M. Gross Australian Studies on Allelopathy in Eucalyptus: A Review, R. Willis The Study of Allelopathy in Australia: 1988-1993, B.-S. Kil and J.V. Lovett Historical Review and Current Models of Forest Succession and Interference, D.W. Gilmore Ecological Aspects Plant Phenolics and Terpenoids: Transformation, Degradation, and Potential for Allelopathic Interactions, Inderjit, H.H. Cheng, and H. Nishimura Variation of Flavonoid Synthesis Induced by Ecological Factors, N.C. Lobon and J.C. Escudero Catalytic Transformation of Phenolic Compounds in theSoils, P.M. Huang, M.C. Wang, and M.K. Wang The Role of Flavan-3-Ols in Plant Defense, W. Feucht and D. Treutter Microbial Competition and Soil Structure Limit the Expression of Allelochemicals in Nature, S.K. Schmidt and R.E. Ley Lignin-Related Phenolic Acids in Peat Soils and Implications in Tropical Rice Sterility Problem, T. Katase Effect of Plant Polyphenols on Nutrient Cycling and Implications for Community Structure, R.R. Northup, R.A. Dahlgren, T.M. Aide, and J.K. Zimmerman Biochemical, Chemical, and Physiological Aspects Detoxification of Allelochemicals in Higher Plants and Enzymes Involved, M. Schulz and A. Friege Effect of Different Concentrations and Application Period of p--Hydroxybenzoic Acid on Development, Yield, and Yield Components of Spring Wheat, O. Christen and C. Theuer Biochemical Effects of Allelopathic Alkaloids, M. Wink, B. Lutz-Bruning, and T. Schmeller Higher Plant Flavonoids: Biosynthesis and Chemical Ecology, M.A. Berhow and S.F. Vaughn Allelochemical Function of Coumarins on the Plant Surface, A. M. Zobel Biological Significance of Plant Saponins, W.A. Oleszek, R.E. Hoagland, and R.M. Zablotowicz Allelopathic Potential of Grain Sorghum (Sorghum biocolor [L.]Moench) and Related Species, L.A. Weston, C.I. Nimbal, and P.l. Jeandet An Integrated View of Allelochemicals and Amid Multiple Stresses, F.A. Einhellig Biological Control of Plant Disease and Weeds: Applied Aspects Potentially Useful Natural Product Herbicides from Microorganisms, H.G. Cutler Allelopathic Interactions in the Biological Control of Postharvest Diseases of Fruit, C.T. Bull Potential of Cultivar Sunflower (Helianthus annuus L.) as Source of Natural Herbicide Templates, F.A. Macian, R.M. Varcia, A. Torres, and J.M.G. Molinillo Biological Properties of Rue (Ruta graveolens L.): Potential Use in Sustainable AgriculturalSystems, G. Aliotta and F. Cafiero

Crop cultivars with allelopathic capability.

Abstract: Allelopathy has potential in integrated weed management. Crop plants have the capability to produce and exude allelochemicals into their surroundings to suppress the growth of weeds in their vicinity. Selection for superior genotypes with allelopathic potential has been carried out in several field crops, and evidence has accumulated that crop cultivars differ significantly in their ability to inhibit the growth of certain weed species. To date, progress has been made in understanding the genetics of crop allelopathic activity, and successful genetic manipulation of this trait has also been demonstrated. However, much more research needs to be carried out in order to have a thorough understanding of the genetic control of allelopathic activity. Several genes might be involved in regulating the production and exudation of allelochemicals. Concerted efforts using advances in plant biotechnology will help to unveil the genetics of this trait. Once the allelopathic genes have been located, a breeding programme could be initiated to transfer the genes into modern cultivars to enhance their allelopathic activity for weed suppression, thereby reducing over-reliance on herbicides.

Biological-control herbivores may increase competitive ability of the noxious weed Centaurea maculosa

Abstract: Biocontrol organisms are generally applied in an attempt to reduce the vigor of target species and provide native species with an competitive advantage. We tested the effectiveness of a widely used biocontrol moth, Agapeta zoegana (knapweed root moth) for two years in the field and found that it had no significant direct effect on the biomass of Centaurea maculosa (spotted knapweed), one of the most destructive invasive plants in North America. Instead of releasing a native grass from competition, the reproductive output of Festuca idahoensis planted with Centaurea was significantly lower when neighboring Centaurea had been attacked by Agapeta. In a greenhouse experiment, we found that Festuca planted in pots with Centaurea that had been attacked by Trichoplusia ni (another nonnative herbivore) had significantly smaller root systems than when they were planted with Centaurea that were protected from herbivory. Root systems of Centaurea that had been attacked by Trichoplusia exuded higher levels of total sugars, but not total phenols. We hypothesize that moderate herbivory stimulated compensatory growth, induced the production of defense chemicals that also had allelopathic effects, or stimulated root exudates that altered the relationship between Centaurea and Festuca via soil microbes. Our data suggest that herbivory may increase the negative effects of C. maculosa on neighboring plants, and that some biocontrols may have indirect negative effects on native species that are not currently recognized.

Weed-suppressing rice cultivars – does allelopathy play a role?

Abstract: A range of 111 rice cultivars was studied for weed-suppressing ability in field experiments with a sown infestation of Echinochloa crus-galli. Cultivars differed significantly in their ability to suppress the growth of E. crus-galli, and the differences were reasonably reproducible over three seasons. The same rice cultivars were tested in a laboratory screening for allelopathic potential, which showed significant differences in the ability to reduce root growth of E. crus-galli. Correlation between the laboratory screening and the field experiments showed that field performance could be described to some extent by E. crus-galli root length reduction in the laboratory. Plant height in the field experiment was correlated with weed biomass 8 weeks after seeding. Even among the most weed-suppressing rice cultivars, however, all heights were represented. None of the measured growth parameters from greenhouse studies could explain the distribution of weed-suppressing rice cultivars. This indicates that allelopathy in combination with competitive ability determines the weed interference outcome of a given rice cultivar.

Effect of phenolic compounds on the germination of six weeds species

Abstract: Low molecular weight phenolic compounds have been identified in fresh leaves and in soils in which leaves of five varieties of Capsicum annuum L. were decomposing. Six phenolic compounds were tested in laboratory bioassays for their allelopathic effects on germination and seedling growth of six weeds. Ferulic acid, gallic acid, p-coumaric acid, p-hydroxybenzoic acid, vanillic acid, and p-vanillin were bioassayed in concentrations of 10, 1, 0.1, and 0.01 mM. Equimolar mixtures containing all these phenolics were prepared at the final total concentration of 10, 1, 0.1, and 0.01 mM to test for possible interactive effects. Chenopodium album L., Plantago lanceolata L., Amaranthus retroflexus L., Solanum nigrum L., Cirsium sp. and Rumex crispus L. were the selected target weeds. The highest concentration of the compounds inhibited the germination of all these weeds, but lower concentrations had no effect or were stimulatory. However, effects varied with the weed species, the concentration of the compound tested and the compound itself. In assays with the mixture of phenolics we found evidence of some additive effects.

Allelopathy: Principles, Procedures, Processes, and Promises for Biological Control

Abstract: Allelopathy can be defined as chemical interactions between and among both plants and microorganisms via releases of biologically active chemical compounds into the environment. During the past three decades this scientific field has received growing attention from soil scientists, microbiologists, ecologists, plant physiologists, biochemists, botanists, weed scientists, agronomists, and natural product chemists. Although a few studies are acknowledged to have demonstrated probable allelopathy in nature, many appeared limited to in vitro circumstances. This difficulty reflects the complexity of allelopathic interactions. Allelopathic effects are often modified by additional biotic and abiotic stress factors, uncertain meteorological events, or physical, chemical, and biological soil factors, all of which can influence the residence time, persistence, concentration, and fate of allelopathic compounds in the environment. Special emphasis is given to an overview of the allelopathic activities of weed and crop species, especially via crop residues in the agroecosystem, and to consideration of the biotic and abiotic factors which influence the expression of Allelopathy A brief discussion of secondary metabolites with allelopathic activities and of the mechanisms of action of allelopathic compounds is also provided. The allelopathic potential of certain weed and crop species can influence the growth and distribution of associated weed species and the yield of desired plants, and allelopathy has been employed successfully in biocontrol programs focusing on control of problematic weeds and plant diseases. Thus, it plays an important role in an agroecosystem and it is clear that a better understanding of allelopathy can help both in crop improvement and in developing more sustainable agriculture

Allelochemicals released in soil following incorporation of rapeseed (Brassica napus) green manures.

Abstract: Plant-derived allelochemicals such as those produced by glucosinolate hydrolysis in Brassica napus, or rapeseed, are viable alternatives to synthetic compounds for the control of soil-borne plant pests. However, allelochemical production and residence times in field soils have not been determined. Soil samples were taken at 0-7.5 and 7.5-15 cm during a period of 3 weeks following plow-down of two winter rapeseed cultivars (Humus and Dwarf Essex). Soil samples were extracted with dichloromethane and analyzed using gas chromatography. Nine glucosinolate degradation products were identified-five isothiocyanates, three nitriles, and one oxazolidinethione. Maximum concentrations were observed 30 h after plow-down. Compounds derived from 2-phenylethyl glucosinolate, the principal glucosinolate in rapeseed roots, dominated the profile of degradation products. Shoot glucosinolates left few traces. This indicates that rapeseed roots may be a more important source of toxic fumigants than above-ground parts of the plant.

Inhibition of seedling survival under Rhododendron maximum (Ericaceae): could allelopathy be a cause?

Abstract: In the southern Appalachian mountains a subcanopy species, Rhododendron maximum, inhibits the establishment and survival of canopy tree seedlings. One of the mechanisms by which seedlings could be inhibited is an allelopathic effect of decomposing litter or leachate from the canopy of R. maximum (R.m.) on seed germination, root elongation, or mycorrhizal colonization. The potential for allelopathy by R.m. was tested with two bioassay species (lettuce and cress), with seeds from four native tree species, and with three ectomycorrhizal fungi. Inhibitory influences of throughfall, fresh litter, and decomposed litter (organic layer) from forest with R.m. (+R.m. sites) were compared to similar extractions made from forest without R.m. (-R.m. sites). Throughfall and leachates of the organic layer from both +R.m. and -R.m. sites stimulated germination of the bioassay species above that of the distilled water control, to a similar extent. There was an inhibitory effect of leachates of litter from +R.m. sites on seed germination and root elongation rate of both bioassay species compared with that of litter from -R.m. sites. Native tree seed stratified in forest floor material from both forest types had a slightly higher seed germination rate compared with the control. A 2-yr study of seed germination and seedling mortality of two tree species, Quercus rubra and Prunus serotina, in field plots showed no significant influence of litter or organic layer from either forest type. Incorporating R.m. leaf material into the growth medium in vitro depressed growth of one ectomycorrhizal species but did not affect two other species. Leaf material from other deciduous tree species depressed ectomycorrhizal growth to a similar or greater extent as leaf material from R.m. In conclusion, R.m. litter can have an allelopathic effect on seed germination and root elongation of bioassay species as well as some ectomycorrhizal species. However, this allelopathic affect is not manifest in field sites and is not likely to be an important cause for the inhibition of seedling survival within thickets of R.m.

Cultivar and age differences in the production of allelochemicals by Secale cereale

Abstract: Concentrations of DIBOA [2,4-dihydroxy-1,4-(2H)-benzoxazine-3-one] and BOA [2-(3H)-benzoxazolinone], described previously as major allelochemicals in Secale cereale (rye), were determined in eight field-grown cultivars, harvested at booting, using high-performance liquid chromatography (HPLC). Allelochemicals were also quantified in greenhouse-grown cultivar ‘Bates’ harvested 30, 45, 60, and 75 days after planting (DAP). The total production of DIBOA and BOA from field-grown S. cereale ranged from 137 to 1,469 μg g−1 dry tissue among the eight cultivars. ‘Bonel’ cultivar had the highest hydroxamic acid (HA) content and ‘Pastar’ the lowest. Bonel also showed the highest activity on Eleusine indica (goosegrass) and Pastar the least, in culture plate bioassays using aqueous extracts. HA content in shoot tissue varied with S. cereale maturity. The greatest level of HA in greenhouse-grown Bates was obtained 60 DAP compared to 30 DAP.

Determination of allelochemicals in spring cereal cultivars of different competitiveness

Abstract: Competitive cereal cultivars are less susceptible than others to weed interference. Their characterization may provide selection criteria that can be used as guidelines to develop new, even more competitive cultivars. Root exudates are a potential means by which competitive cultivars reduce weed growth. The objectives of this study were to evaluate the effect of cereal root exudates on Brassica kaber (DC.) L. C. Wheeler growth, to isolate and characterize the allelochemical compounds released by spring cereal cultivars, and to determine if a relation exists between these allelochemicals and cultivar competitiveness. Highly competitive (HC) and lesser competitive (LC) cultivars of four crop kinds (Triticum aestivum L. [wheat], Avena sativa L. [wild oat], two- and six-rowed Hordeum vulgare L. [barley]) were selected based on previous work. Exudates from undisturbed root systems of B. kaber and cereals were collected and used in a bioassay test with B. kaber. Root exudates were analyzed for 16 common phenolic compounds using high-performance liquid chromatography (HPLC). Bioassays indicated that cereal exudates had no negative effect on B. kaber germination, but all concentrations of cereal root exudates inhibited B. kaber root and hypocotyl growth. As cereal root exudate concentration increased, B. kaber growth decreased. For each crop kind, B. kaber growth inhibition was greater with HC cultivars than with LC cultivars. The root exudates of all crop kinds and cultivars contained benzoic, caffeic, ferulic, o-coumaric, and vanillic acids as well as scopoletin. Para-hydroxybenzoic acid was found in exudates from T. aestivum, A. sativa, and two-rowed H. vulgare cultivars. Para-coumaric acid was not identified in root exudates from LC H. vulgare cultivars. Gentisic acid was produced by A. sativa and H. vulgare. Vanillic and o-coumaric acids along with scopoletin may be responsible for the allelopathic effects of H. vulgare, T. aestivum, and A. sativa cultivars. These three compounds may be useful as possible indicators of allelopathic potential of genotypes under development and thus considered for use in breeding programs.

Allelopathic Interactions in Agroforestry Systems

Abstract: Agroforestry is a modern tool to develop sustainable land use and to increase food production by growing woody species (trees, shrubs, palms, bamboos, etc.) with agricultural crops and/or animals in some form of spatial arrangement or temporal sequence. Because these species co-exist with the agricultural crops, their allelopathic compatibility may be crucial to determine the success of an agroforestry system. A survey of the available information reveals that most of the agroforestry species (AF species) have negative allelopathic effects on food and fodder crops. Therefore, it is desirable to do further research in this direction so that AF species with no or positive allelopathic effects on the companion crops may be promoted for agroforestry programs. As AF species remain a part of the agroecosystem for a longer period, and most of them produce a large amount of leaves and litter, their allelochemicals may play an important role in developing an eco-friendly pest management strategy. Besides these generally studied aspects of allelopathy, some comparatively newer aspects of research have been identified, such as evaluation of qualitative yield of agroforestry systems, selective behavior of the allelochemicals, effect on soil quality, and the role of tree allelochemicals in animal and human nutrition. If given due consideration, allelopathy could play a pivotal role in conservation of the highly threatened environment, biodiversity, natural resource base, and making agriculture more sustainable through broadening the scope of agroforestry.

Allelopathic potential and chemical constituents of volatile oil from Ageratum conyzoides

Abstract: Fresh leaves and volatile oil of the important weed Ageratum conyzoides in south China were highly inhibitory to seedling growth of various cultivated crops, especially in an adverse habitat. The constituents of the volatile oil were analyzed by GC-MS. Eleven components were identified and six main components, precocene I, precocene II, 3,3-dimethyl-5-tert-butylindone, β-caryophyllene, γ-bisabolene, and fenchyl acetate, were isolated by means of column chromatography. Precocene I, precocene II, β-caryophyllene, and 3,3-dimethyl-5-tert-butylindone inhibited seedling growth of acceptor plants. Inhibitory activity of the volatile oil was more intense than that of the pure components. Fenchyl acetate and γ-bisabolene have no inhibitory activity, but when mixed with precocene II, they increased the inhibitory activity to growth of acceptor seedling plants. Experiments show that allelopathic synergism exists among allelochemicals of Ageratum conyzoides.

Allelopathy in Northern Temperate and Boreal Semi-Natural Woodland

Abstract: Allelopathy has been widely investigated in forest ecosystems around the world. This ecological process takes part in the vegetation dynamics and partially explains the success or failure of seedling establishment and survival. Therefore, allelopathy could be a common cause of natural regeneration failure, due to adverse impact of understory species directly on trees species (inhibition of germination and seedlings primary growth). This problem has been investigated mainly in coniferous forests, but it also occurs in deciduous forests. Furthermore, allelopathy can involve forest soil microorganisms. They can act as allelochemical mediators, where they are able to inactivate these compounds by means of metabolization, as they are able to produce biologically active compounds using inactive molecules as a substrate. Special attention is given to mycorrhizal fungi and mycor-rhizae, because, in theory, they are able to protect seedlings against allelopathy. Applied aspects of allelopathy in forest ecosystems ...

Allelopathic Bacteria and Their Impact on Higher Plants

Abstract: The impact of allelopathic, nonpathogenic bacteria on plant growth in natural and agricultural ecosystems is discussed. In some natural ecosystems, evidence supports the view that in the vicinity of some allelopathically active perennials (e.g., Adenostoma fasciculatum, California), in addition to allelochemicals leached from the shrub's canopy, accumulation of phytotoxic bacteria or other allelopathic microorganisms amplify retardation of annuals. In agricultural ecosystems allelopathic bacteria may evolve in areas where a single crop is grown successively, and the resulting yield decline cannot be restored by application of minerals. Transfer of soils from areas where crop suppression had been recorded into an unaffected area induced crop retardation without readily apparent symptoms of plant disease. Susceptibility of higher plants to deleterious rhizobacteria is often manifested in sandy or so-called skeletal soils. Evaluation of phytotoxic activity under controlled conditions, as well as ways to appl...

Allelopathic Substance Exuded from a Serious Weed, Germinating Barnyard Grass (Echinochloa crus-galli L.), Roots.

Abstract: The allelopathy of a serious weed, barnyard grass (Echinochloa crus-galli L.), was investigated. Root exudates of young barnyard grass showed allelopathic effects and plant-selective activity and inhibited root elongation of all plants tested. With respect to shoot growth, the exudates did not show inhibition of barnyard grass only. The allelopathic substance was isolated and identified as p-hydroxymandelic acid by NMR. p-Hydroxymandelic acid strongly inhibited shoot growth and root elongation of all plants tested. The effects of three congeners of p-hydroxymandelic acid were tested on rice shoot growth. In the biological activity exhibited in rice, shoot growth was related to the hydroxyl groups.

Allelopathic effects of endophyte‐infected perennial ryegrass extracts on white clover seedlings

Abstract: Two glasshouse experiments were conducted to evaluate possible allelopathic effects of perennial ryegrass/endophyte (Lolium perennel Neotyphodium lolii) associations on white clover (Trifolium repens). The influence of between‐species competition, environmental stress, and soil fauna was eliminated by the use of a sterile sand nutrient culture technique to sustain potted clover seedlings, to which aqueous extracts from moisture‐stressed perennial ryegrass pseudostem were applied. In Experiment 1, extracts from two ryegrass cultivars ('Grasslands Pacific’ and ‘Grasslands Nui'), each hosting in separate seed lines two endophyte strains (ES, WT), suppressed clover growth (mean suppression 22% at 100% concentration) relative to extracts from endophyte‐free lines of the same cultivars. The degree of suppression increased with increasing extract concentration, differed between cultivars, but did not differ between endophyte strains. In Experiment 2, extracts at 100% concentration from a third ryegrass ...

Allelopathic potential of Verbesina encelioides root leachate in soil

Abstract: Verbesina encelioides (Cav.) Benth. & Hook. F. ex. A. Gray (Asteraceae) is a perennial weed that interferes with the growth and establishment of crop species in semiarid regions of India. The present research was designed to understand the probable involvement of allelopathy in its interference mechanism. We studied the effect of soils amended with different dilutions of V. encelioides root leachate (full strength and 1:2 and 1:4 (v/v) ratios of root leachate to water) on the growth of radish seedlings (Raphanus sativus L.). Soils that were not amended were used as controls. We also investigated the influence of different levels of N fertilization (1, 5, and 10 mM) on the modification of the allelopathic potential of amended soils, in terms of their effect on soil total phenolics and radish seedling growth. The addition of both full strength and 1:2 dilution of V. encelioides root leachate resulted in significant (P < 0.05) suppression of root (-25.7 and -17.2%, respectively) and shoot (-21.3 and -13.8%, ...

Allelopathic potential of Argemone mexicana L. a tropical weed

Abstract: Effect of Argemone mexicana L. on seed germination and early seedling growth of six crop species was investigated. Aqueous extract of A. mexicana inhibited germination, root and shoot growth of all the test species. The species showed differential response. Germination was reduced by the aqueous shoot extract in the order: pearlmillet > mustard > wheat> carrot> corn > turnip. Root length was reduced in the order:corn> pearl-millet> wheat > mustard > turnip> carrot, whereas shoot length was reduced in the order: pearl-millet> wheat> corn > turnip> mustard > carrot. Decaying A. mexicana in sandy loam soil at a rate of 5,10,20 g / 400 g soil substantialy inhibited germination and seedling growth of pearl-millet. Bioassay of the extract of A. mexicana revealed three phenolic inhibitors and paper chromatography revealed the presence of p-hydroxybenzoic, vanillic and salicylic acids.

Potentialités autotoxiques et allélopathiques de Cistus albidus L.

Abstract: This study focuses on how allelopathic mechanisms may be involved in the floristic composition and dynamics of Cistus albidus shrubbery. Allelopathic potential of essential oils was measured for Lactuca sativa (toxicity for other species) and Cistus albidus (self-toxicity). These oils have little effect on germination but significantly increase, even in low quantities, the number of unviable seedlings and inhibit normal seedling growth. These results explain, at least partially, the existence of monospecific stands over several years and the lack of natural regeneration of the Cistus albidus populations. Bioassays performed using natural aerial leachates of Cistus albidus revealed no significant effects on the above-mentioned parameters. Leaching does not seem to be an important means of allelochemicals release. Nevertheless, essential oils have been quantified in litter and soil, indicating a possible occurrence of allelopathy in such ecosystems.

Allelopathic Potential of Aquatic Plants Associated with Wild Rice (Zizania palustris): I. Bioassay with Plant and Lake Sediment Samples

Abstract: The allelopathic potential of eight aquatic plants associated with wild rice was investigated using lettuce and wild rice seedling bioassays. Rhizome aqueous extracts of Scirpus acutus, Potamogeton natans, Nymphaea odorata, Nuphar variegatum; shoot extract of Eleocharis smallii; whole plant extract of Myriophyllum verticillatum; and leaf extract of P. natans significantly reduced the root length of lettuce and wild rice seedlings. The lettuce seedling bioassay was more sensitive than the wild rice bioassay. Shoot growth was less affected than the root growth. Water extract of sediments associated with the aquatic plants had little growth inhibitory effect on wild rice. Our study did not yield any conclusive evidence that the wild rice-associated aquatic plants have allelopathic effects on wild rice. We emphasize the use of target species as a bioassay material in allelopathic studies. Further investigation on allelopathic effects of lake sediments associated with the neighboring plants of wild rice is necessary to evaluate their ecological significance.

Developing new herbicide models from allelochemicals

Abstract: Plants contain allelochemicals which are their own defence systems and can act as herbicides. Selected examples of guaianolides and heliannuols, which are sesquiterpenes, are discussed in the context of their potential use as natural herbicide templates.

Allelopathic Potential of Aquatic Plants Associated with Wild Rice: II. Isolation and Identification of Allelochemicals

Abstract: Aqueous extracts of rhizomes of Scirpus acutus and shoots of Eleocharis smallii were analyzed for the presence of phytotoxic compounds using ethyl acetate extraction. The organic fractions of the extract of Scirpus rhizomes contained lactic, succinic, fumaric, 2-hydroxysuccinic, 2-phenyl lactic, m-hydroxybenzoic, p-hydroxybenzoic, protocatechuic, dehydroabietic, and ferulic acids; p-hydroxybenzyl alcohol, p-hydroxyphenyl ethanol and catechin. The extract of Eleocharis shoots contained 4-methoxy phenol, benzofuran, benzene acetic acid, 1-hydroxy-5-methyl acetophenone, and 1,3,4-dimethoxyphenol ethanone identified by GC-Mass spectroscopy. The potential allelopathic effect of these compounds on wild rice under field conditions was discussed.

Saponins produced during the life cycle of mungbeans and their role as allelochemicals

Abstract: Continuous cropping of mungbean ( Vigna radiata L.) can lead to plant growth inhibition. We have found that allelopathy may be the cause of as much as 10–25% of the inhibition in mungbean plants grown following mungbean plants. Mungbean plants are allelopathic, and the surrounding soil is often toxic. The phytotoxic activity is found primarily in the stems and the aerial parts (excluding the stems). We partitioned stem extracts with water and with organic solvents: the water extracts produced the greatest inhibition of mungbean and lettuce; the organic solvents caused both inhibition and stimulation. The discovery of enhancement of mungbean growth by crude mungbean saponins was serendipitous; the plants showed quicker germination and enhanced growth, but such treatment did not increase the yield. Leaves, stem, and roots of mungbeans ( Vigna radiata L.) were harvested at 2, 4, 7, and 10 weeks during the spring growth period of the crop in Taiwan. Crude saponins were isolated from defatted plant resodue with 80% EtOH, partially with 1-BuOH extraction, and analyzed using TLC, HPLC, LC/MS, FAB/MS, and FAB/MS/MS. Soyasaponin I [(SS1) the main saponin], soyasaponin III, 3-O-[β-D-galactopyranosyl-(1→2)-β-D-glucuronopyranosyl] sophradiol, were identified and several mono- and bi-desmosides saponins were not identified. The BuOH extracts and SSI were stimulatory at concentration's of 1, 10, and 100 ppm when tested using seeds of mungbeans and lettuce( Lactuca sativa L.) in a 72-h bioassay on filter paper with distilled water as control. At 1,000 ppm the growth of both plants were inhibited. The bioassay results were significant at the 95% level of confidence. A mechanism of action is proposed.