Abstract: In laboratory studies, water-soluble extracts of the litter of four shrub and three conifer species had variable effects on the growth of four species of ectomycorrhizal fungi. In general, low concentrations (parts per thousand) stimulated fungal growth; while, high concentrations (parts per hundred and parts per ten) either stimulated growth, inhibited growth, or had no effect, depending on both fungal and litter species. In greenhouse studies, litter applied to the surface of a sand-soil mixture reduced the formation ofRhizopogon sp. on Douglas-fir seedlings. Allelochemicals in the litter may inhibit seedling growth and suppress fungai growth and root colonization in the field, thus explaining the failures of reforestation by conifer species in disturbed sites.

Abstract: Salicylic acid, p-hydroxybenzoic acid, caffeine, hydroquinone, and umbelliferone were evaluated in the greenhouse for their effects on shoot dry-weight accumulation of several crop and weed species. With the exception of caffeine, all the chemicals reduced shoot growth in oats (Avena sativa L. ‘Goodfield’). Chemicals applied preplant incorporated, preemergence, or postemergence were effective, depending upon the rate of chemical. When applied preplant incorporated at rates as high as 56.0 kg/ha, most of the chemicals reduced growth of corn (Zea mays L. ‘B73 × Mo17′), soybean [Glycine max (L.) Merr. ‘Corsoy’], velvetleaf (Abutilon theophrasti Medic.), redroot pigweed (Amaranthus retroflexus L.), and wild proso millet (Panicum miliaceum L.). Exceptions were caffeine on corn and soybean and hydroquinone on soybean. At 11.2 kg/ha the chemicals inhibited the weed species more than the crop species. Postemergence applications of caffeine and hydroquinone inhibited growth of the weed species more than the crop species. Hydroquinone at 1.1 kg/ha inhibited redroot pigweed, but rates as high as 11.2 kg/ha did not inhibit soybean. These experiments show that growth of agronomically important crops and weeds can be inhibited differentially by allelopathic chemicals.

Abstract: Several experiments have been conducted at the University of Illinois, Urbana, Illinois, to determine the role of allelopathic (autotoxicity) effects of alfalfa (Medicago sativa L.) on the establishment of alfalfa. By means of correcting any soil fertility differences and protecting the alfalfa seedlings from fungal attack, results suggest that there is a release of phytotoxic factors from the previous crop on reestablishing alfalfa. Field results indicate that the best preceding crop for alfalfa establishment is corn (Zea mays L.), followed by various small grains, soybeans (Glycine max L.), and the worst preceding crop is alfalfa. Results indicate there were no major genetic differences among cultivars for resistance to autotoxicity. There was no evidence supporting saponins as being phytotoxic to alfalfa. Exudates of different saponin level cultivars indicated that saponin content is not the suspected phytotoxic factor.

Abstract: The concept that some crop plants may be allelopathic to common weeds of agricultural lands is receiving greater attention as an alternative weed control strategy. Several crops showing promise are: grain and forage species such as barley (Hordeum), oat (A vena), fescue (Festuca), and sorghum (Sorghum), and the agronomic species of corn (Zea) and sunflower (Helianthus). Among the problems that hinder the conclusive demonstration of allelopathic effects of crop plants are the loss of that capacity through selection and the variability among cultivars. Recent studies to evaluate the allelopathic potential of crop plants have shown that several sunflower varieties inhibit the germination and growth of associated weeds and to a greater extent than found in several biotypes of native sunflower. Aqueous extracts of dried sunflower and rape tissue inhibited or stimulated germination and growth of weeds, and the response depended upon the source of extract, the extract concentration, and the weed species tested. The validity of bioassay results was tested in a 5-year field study with sunflower and oat grown in rotation. Weed density increased in all plots but the extent of increase was significantly less in plots of sunflower than in control plots. The use of crop plants with increased allelochemical production could limit the need for conventional herbicides to early season application with late season control provided by the crop.

Abstract: Inhibitors of germination or of growth, highly diversified chemicals are commonly found in higher plants. They occur in vegetative organs as well as in seeds or other dispersal units. Nonprotein amino acids, when present, are mainly found in seeds where they can occur in extremely high concentrations. Density of seeds, rate of emanation of inhibitors, their amount and effectiveness, all determine allelopathic potential of seeds. To induce allelopathy, rate of emanation of inhibitors must be fast and of sufficient duration. Our observations in coffee seedsCoffea arabica L. indicate that rate of emanation of the inhibitor caffeine is highly enhanced during senescence of seeds, suggesting that when allelopathic potential of seeds is evaluated the presence of both young and old seeds should be considered. In many plants seeds are liberated close to the parent plant, the zone where seed-induced allelopathy may occur. Large numbers of seeds are usually produced in order to ensure establishment; greater number and mass of seeds may also increase allelopathic inhibition of competing vegetation.

Abstract: A very important component of the aggressive nature of weeds is allelopathic interference, the full potential of which is just being realized in the management of agroecosystems. Research results are presented which demonstrate the allelopathic interactions involved in a wide range of cropweed combinations occurring in a great variety of habitats. This includes crops planted in weed control, crops with allelopathic potential, and noncrop plants of beneficial use for weed control as a result of allelopathic interference. Allelopathy can play a beneficial role in multiple cropping systems, crop rotations, and cover cropping. The potential role for allelopathic interactions in the design of biological weed control is proposed.

Abstract: We previously reported thatPolygonum aviculare has a strong allelopathic action againstCynodon dactylon (L.) Pers. and other test species. Moreover, we found that several phenolic compounds appeared to be important allelochemics in this activity. We have now isolated other potential inhibitors fromP. aviculare residues and soil underPolygonum stands, and none of these occurred in soil underC. dactylon stands. GC-MS analysis revealed that these additional inhibitors are long-chain fatty acids with 14–22 carbons. Nine were identified inP. aviculare residues and seven in soil underP. aviculare. Sodium salts of all the identified fatty acids inhibited seedling growth ofC. dactylon and at least some test strains of the nitrogen-fixing bacteria,Azotobacter andRhizobium.

Abstract: Black alder trees were interplanted with black walnut on a 2.4 × 3.7-m spacing at four locations in Illinois and Missouri. In every plantation, the alders suddenly declined and died after 8–13 years. Alternative causes for the mortality were investigated–competition from walnut, adverse soil properties, frost, insects, disease, unsuitable seed source, and allelopathy—but only allelopathy could be substantiated. The decline in alder survival and diameter growth was strongly related to walnut above-ground dry biomass and walnut crown competition factor. We propose that the black walnut allelopathy most likely results from a combination of (1) sufficient walnut biomass to contribute substantial amounts of juglone to the environment, and (2) wet soil that greatly restricts aerobic metabolism by soil microorganisms, allowing juglone to build up to toxic levels.

Abstract: Wild oat plants may produce toxic substances that suppress the growth and development of desirable species, thus accounting for severe yield loss in infested fields. The purpose of this study was to determine the allelopathic potential of wild oat (Avena fatua) on the growth of spring wheat (Triticum aestivum var. Fieldwin) in the absence of plant competition. Wild oat and spring wheat plants were grown separately in 250-ml beakers in a sand medium. Root exudates were extracted from wild oat medium at the 1-, 2-, 3-, and 4-leaf stages of wild oat development and added to beakers containing spring wheat in temporally corresponding stages of development. Spring wheat root and leaf dry weights were measured to determine if one or more allelochemical agents were released from wild oat roots. Spring wheat leaf and root dry weights were significantly reduced by exudates from wild oat plants at the 2- and 4-leaf stages of development, respectively. Allelochemicals were isolated from wild oat root exudates at various stages of plant development. Paper chromatography analysis indicated that at least two unknown compounds were present.R f values in benzene-acetic acid-water of the two unknown compounds (0.825 and 0.930) were similar to scopoletin (7-hydroxy-6-methoxycoumarin) and vanillic acid (4-hydroxy-3-methoxybenzoic acid), respectively. Additional tests using diazotizedp-nitraniline, ultraviolet absorption spectra, and gas chromatography analysis also indicated that the unknowns were coumarin-related compounds such as scopoletin and vanillic acid.

Abstract: Seeds of Typha latifolia were exposed to leaf extracts, leaf pieces, and soil water from adult plants of T. latifolia. In liquid culture, only extracts of concentration greater than or equal to 3% (dry weight to volume) inhibited seed germination. Adding soil to the liquid cultures increased the inhibition of seed germination by extracts. The inhibitory effects of extracts were correlated with the development of water molds in the cultures; prior to the development of water molds, even 15% extracts were not inhibitory to germination. In contrast to results from extracts; neither 3% concentrations of senesced leaf pieces nor soil surface water from one year old pots of T. latifolia had any detrimental effects on germination. Seeds sown into pots containing established T. latifolia germinated as well as seeds sown into control pots. These results do not support the contention that T. latifolia inhibits the germination of its seeds by the release of allelopathic substances.

Abstract: The effects of corn pollen and aqueous leachates of pollen upon the radicle growth ofBidens pilosa, Cassia jalapensis, andRumex crispus are shown Extractions of pollen with various solvents and methods were carried out so as to assess its active principle and its effect uponC jalapensis The preliminary steps to separate and identify the allelopathic compounds of the sonicated and macerated pollen extracted with methylene chloride are described The strongest inhibitory effect was produced by the hexane fraction The allelopathic effect of corn pollen upon the growth ofC jalapensis in several substrates is shown The possible structure of some of the active fractions is discussed as well as the possibility that the allelopathic potential of pollen might actually occur in nature

Abstract: Nitrogen-fixing nurse crops and cocrops of plant species nodulated byFrankia andRhizobium have been used to promote the growth of black walnut trees. Although walnut is known to inhibit the growth of certain associated plants due to its allelopathic derivative juglone (5-hydroxy-1,4-napthoquinone), juglone inhibition of symbiotic, nitrogen-fixing soil microorganisms had not been investigated. This research revealed that a concentration of 10−3 M juglone absolutely inhibited the growth in vitro of aFrankia isolate from root nodules of red alder and ofRhizobium japonicum strain 71. Lesser concentrations of juglone inhibited the growth of these bacteria relative to the controls. The more-rapidly growingRhizobium strain exhibited slight growth at 10−4 M juglone concentration, whereasFrankia growth was completely inhibited. Considering both the susceptibility of the host plant and nitrogen-fixing endophyte to the allelochemical juglone, caution should be exercised in selecting nitrogen-fixing plants as nurse crops for black walnut.

Abstract: The allelopathic effects of Engelmann spruce bark and bark extacts on seed germination and seedling growth of several conifer species were examined. Extracts were hydroxystilbenes (isorhapontin and astringin) and condensed tannins. Experiments included the following: (i) seed germination in petri dishes with various concentrations of stilbenes and tannin–stilbenes; (ii) seed germination in petri dishes containing Engelmann spruce bark fragments; (iii) seedling growth in varying mixtures of peat/Engelmann spruce bark, the latter added either before or after seedling establishment; (iv) seedling growth in samples of Engelmann spruce forest soil horizons. In all experiments and in all species there was an allelopathic response to bark extracts. This response was expressed through inhibition of seed germination (seed-coat rupture and radicle emergence), necrotic discoloration and protoplasmic disruption of root-tip cells, suppression of root hairs, and repressed growth in general. Among the conifers tested Pi...

Abstract: The experiments on Eucalyptus trees reported here demonstrate that allelopathic effects can be mediated by insects grazing on foliage We show that the allelochemical nature of insect frass suppresses germination, growth and survival of herb layer species, that plant species vary in their tolerance of this material, and that as a result, the structure and composition of associated, herbaceous understory plant communities are markedly affected by frass fall

Abstract: Western ragweed (Ambrosia psilostachya DC.) top growth and rhizome extracts were inhibitory to germination and growth of seedling shoots and roots of test plants in almost all cases. Germination of all plants tested was reduced an average of 19.5% by the ragweed extracts. Shoot and root growth of plants tested was reduced an average of 56.8% by the ragweed extracts. The combined effects of germination plus growth reductions resulted in the tested plants producing only 34.8% in the extracts compared to production in distilled water.

Abstract: The Labiatae is a large family of diverse morphology and habit. Almost all members contain phytochemicals, volatile oils which contribute variously to commercial value, problems of toxicity and allelopathic activity. Weed status in the Labiatae depends primarily on competition for environmental resources and on characteristics dependent on the phytochemical content. In a number of genera, notably Salvia, allelopathic activity has been documented.

Abstract: Laboratory experiments were conducted to determine juglone sensitivity of 16 species (Trifolium incarnatum, Coronilla varia, Vicia villosa, Lespedeza stipulacea, L. cuneata, Acer ginnala, Caragana arbor-escens, Elaegnus angustifolia, E. umbellata, Lonicera maackii, Quercus alba, Fraxinus americana, Liriodendron tulipifera, Alnus glutinosa, Pinus strobus, andP. sylvestris) being considered for mixed plantings withJugions nigra (black walnut). All species were sensitive to juglone, but seed germination and radicle elongation were less affected than shoot elongation and dry weight accumulation. Seed germination and radicle elongation were affected by juglone in 6 and 11 species, respectively, mainly by the higher concentrations (10−3 M and 10−4 M). Shoot elongation and dry weight accumulation of all species were affected by juglone; many species were sensitive to concentrations as low as 10−6 M. Seedlings of all species were severely wilted and eventually killed by 10−3 M juglone, and most were chlorotic and severely retarded by 10−4 M juglone. Seedlings inhibited by 10−6 M and 10−5 M juglone did not showany visible signs of injury. Based on the effects on seedling shoot elongation and dry weight accumulation, the five species found to be most sensitive to juglone were:Lonicera maackii, Lespedeza cuneata, Trifolium incarnatum, Alnus glutinosa, and Elaeagnus umbellata.