Interactions betweenKalmia and black spruce: Isolation and identification of allelopathic compounds

Aqueous extracts of fresh leaves and organic soil of northern sheep laurel (Kalmia angustifolia var.angustifolia) were found to be inhibitory to the growth of black spruce (Picea mariana) germinants. Primary root growth of black spruce was more affected by the extracts than was shoot growth. The growth inhibition caused by the leaf extract was most pronounced under acidic conditions (pH 3–4). The aqueous extract ofKalmia leaves contained ferulic, vanillic, syringic, gentisic,m-coumaric,p-coumaric,o-hydroxyphenylacetic, andp-hydroxybenzoic acids as well as some other unknown compounds. These compounds were isolated from the aqueous extract ofKalmia leaves by ethyl acetate extraction and identified using thin-layer chromatography (TLC) and high-performance liquid chromatography (HPLC). Bioassay indicated that the overall toxicity of the phenolic compounds to black spruce appeared to increase in the order ofo-hydroxyphenylacetic,p-hydroxybenzoic, vanillic,p-coumaric, gentisic, syringic, ferulic, andm-coumaric acids.     

Allelopathy of yellow fieldcress (Rorippa sylvestris): Identification and characterization of phytotoxic constituents

Both the neutral and acidic fractions of the acetone extract of yellow fieldcress (Kireha-inugarashi,Rorippa sylvestris Besser) inhibited lettuce seed germination. Salicylic,p-hydroxybenzoic, vanillic, and syringic acid were identified in the acidic fraction. In the neutral fraction, hirsutin (8-methylsulfinyloctyl isothiocyanate), 4-methoxyindole-3-acetonitrile, and pyrocatechol were identified. Bioassay using a root exudate recirculating system showedR. sylvestris during flowering inhibited the lettuce seedling growth. Hirsutin (13g/plant/day) and pyrocatechol (9.3g/plant/day) were the major compounds released into the rhizosphere. Several combinations of pyrocatechol,p-hydroxybenzoic acid, vanillic acid, and hirsutin reduced lettuce seedling growth. These compounds seemed to be allelochemicals.     

Phytotoxic substances in root exudates of cucumber (Cucumis sativus L.)

The addition of activated charcoal to a nutrient solution for the hydroponic culture of cucumber resulted in significant increases in the dry weight of the plant and fruit yield. Hydrophobic root exudates were collected at different growth stages with Amberlite XAD-4 resin and bioassayed with lettuce seedlings. The exudates at the reproductive stage were more phytotoxic than those at the vegetative stage. The exudates contained organic acids such as benzoic,p-hydroxybenzoic, 2,5-dihydroxybenzoic, 3-phenylpropionic, cinnamic,p-hydroxycinnamic, myristic, palmitic, and stearic acids, as well asp-thiocyanatophenol and 2-hydroxybenzothiazole, all of which, except 2-hydroxybenzothiazole, were toxic to the growth of lettuce.     

Possible allelopathic constituents ofCoffea arabica

The aqueous extracts of leaves, stems, and roots ofCoffea arabica significantly inhibited the seed germination and radicle growth of rye grass, lettuce, and fescue. When the extracts were diluted to 1% solution, significant suppression of lettuce growth was still found and was particularly pronounced in the extract of young seedlings. The paper chromato-gram of the ether fraction of an aqueous extract of coffee leaves was bioassayed with lettuce seeds and revealed a remarkable inhibition throughout the chromatogram except for the segment ofR _f 0.00–0.12. Paper without spotting extract was used as a standard. The phytotoxins present in coffee tissue were identified by paper and thin-layer chro-matography and mass spectrometry. The compounds include caffeine, theobromine, theophylline, paraxanthine, scopoletin, and chlorogenic, ferulic,p-coumaric,p-hydroxybenzoic, caffeic, and vanillic acids. All compounds except caffeic acid exhibited significant phytotoxicity to lettuce growth at a concentration of 100 ppm.Paper no. 222 of the Scientific Journal Series, Institute of Botany, Academia Sinica, Taiwan, and Journal Article 3582 of the Agricultural Experiment Station, Oklahoma State University, Stillwater, Oklahoma. The abstract was published in the Proceedings of 63rd Annual Meeting of American Societies for Experimental Biology, held on April 1–10, 1979, Dallas, Texas.     

Kairomones and their use for management of entomophagous insects. XV.

Acids found in moth scales of laboratory-rearedHeliothis zea (Boddie) moths are hexanoic, heptanoic, octanoic, nonanoic, 2- (or 3-) furan carboxylic, phenylacetic, benzoic, sorbic, and 4-hydroxybenzoic acid. The last two of these acids are preservatives added to the artifical diet as sorbic acid and methyl-p-hydroxybenzoate. FemaleTrichogramma pretiosum Riley exhibited increased rates of parasitization ofH. zea eggs in the presence of some of these compounds in laboratory experiments. Exposure to a mixture of all of these compounds did not increase parasitization, and the elimination of acids from the crude moth-scale extract did not reduce parasitization by the wasps.Lepidoptera: Noctuidae.Hymenoptera: Trichogrammatidae.In cooperation with the University of Georgia College of Agriculture Experiment Stations, Coastal Plain Station, Tifton, Georgia 31793.Mention of a commercial or proprietary product in this paper does not constitute endorsement by the USDA.     

Oviposition stimulants of the mothEctomyelois ceratoniae: The effect of short-chain alcohols

Females of the mothEctomyelois ceratoniae prefer to oviposit on carobs,Ceratonia siliqua L., infested with the fungusPhomopsis sp. (Gothilf, 1964). An extract of the steam distillate of fungus-infested carobs was more effective in stimulating female moths to oviposit than was an extract of uninfested carobs. The extract of the fungus-infested carobs was found to be composed mainly of simple alcohols: ethanol (60%),n-propanol3 (15%), 2-propanol (2.5%), isobutanol (15%),n-butanol (2.5%), and isopentanol (5%). In addition, the extract contained a small proportion (1–2%) of unidentified compounds. All of the alcohols, with the exception of isopentanol, stimulated oviposition by female moths. However, the alcohols, when tested separately or as a mixture, were not as stimulating as the total extract. The unidentified compounds were less stimulating than the alcohols.     

Allelopathic compounds in leaves ofGliricidia sepium (Jacq.) kunth ex walp. and its effect onSorghum vulgare L.

Allelochemicals fromGliricidia sepium were extracted, identified, and quantified using HPLC. Fifteen toxic compounds, namely gallic acid, protocatechuic acid,p-hydroxybenzoic acid, gentisic acid,-resorcyclic acid, vanillic acid, syringic acid,p-coumaric acid,m-coumaric acid,o-coumaric acid, ferulic acid, sinapinic acid (trans andcis forms), coumarin, and myricetin were identified and quantified. These compounds from the plant extracts were tested on the seeds of the crop plant,Sorghum vulgare. Rate of germination of the seeds and root elongation were found to be inhibited by the various compounds of the extract. Different quantities ofGliricidia leaf mulch, viz., 400, 800, and 1200 g/m² applied to theSorghum grown fields, were found to effectively control weeds. Mulching improved the total yield ofSorghum. Leaf manuring and mulching showed better crop yield when applied up to 800 g ofGliricidia leaf/m². Crop yield was better in mulch-applied fields when compared to the manure-applied ones.     

Effects of phenolic acids on germination and early growth of herbaceous woodland plants

Four herbaceous plant species from woodland (clearings),Deschampsia flexuosa, Scrophularia nodosa, Senecio sylvaticus, andChamaenerion angustifolium, were tested for their sensitivity to phenolic acids. Seven commonly occurring phenolic compounds were used in a germination experiment in concentrations ranging from 0.01 to 10 mM, i.e., salicylic,p-hydroxybenzoic, syringic, caffeic, vanillic,p-coumaric, and ferulic acids. Germination was delayed rather than inhibited. Radicle elongation was strongly affected; at lower concentrations stimulatory effects were observed, whereas at high concentrations radicle elongation was severely reduced. Salicylic acid was the most effective phenolic compound, whereas caffeic acid caused no effects. Early growth was studied in more detail in a second experiment withDeschampsia flexuosa andSenecio sylvaticus and the phenolic acids, ferulic and p-coumaric acid. Primary root length, number and length of secondary roots, and dry weight were stimulated at 0.01 mM but were inhibited at 10 mM of both compounds. The results are discussed in view of the allelopathic relations between trees and herbaceous understory vegetation.     

Effect of Phenolic Acids in Soil under and Between Rows of a Prior Sorghum (Sorghum bicolor) Crop on Germination, Emergence, and Seedling Growth of Peanut (Arachis hypogea)

Sorghum bicolor is an allelopathic crop that reduces the yield of succeeding crops. We have assessed its effect on the germination, emergence, and seedling growth of Arachis hypogea sown in soil that had had a prior sorghum cropping. A. hypogea was sown on rows and interrows of a previous sorghum crop in 1997 and 1998 in Senegal. Seedling establishment (germination rate and seedling weight) was better between rows than on rows of the previous crop. The highest concentrations of phenolic compounds occurred in the rows in 1998, while contents of row and interrow soils were similar in 1997. Vanillic acid was the main component of the six chemicals found in 1997 soils, whereas the 1998 soil samples contained mainly p-hydroxybenzoic acid, p-hydroxybenzaldehyde, vanillic, and p-coumaric acids (10 phenolics identified). The germination of peanut seeds in water (control), soil water extracts, and mixtures of pure phenolics (equivalent to those in 1997 and 1998 soil samples) was tested. All extracts inhibited germination compared to controls, but there was no significant difference among treatments, i.e., the inhibition was the same for seeds in soil solutions and those in the respective phenolic mixtures. Similarly, there were no significant differences among the germination rates in soil water extracts of rows and interrows or in the pure phenolic mixtures of rows and interrows. We propose a geometrical sowing pattern for peanuts between the rows of the previous sorghum crop to escape the latter's "allelopathic heritage."     

Allelopathic potential of sorghum-sudangrass hybrid (sudex)

Experiments were conducted under controlled conditions to investigate the apparent allelopathic effects of sudex [Sorghum bicolor (L.) Moench ×Sorghum Sudanese (P.) Stapf, cv. FFR 201] on weed and vegetable species. Allelopathic potential, as measured by radicle elongation of herbaceous indicator species, decreased with increasing sudex age. Greatest potential allelopathic activity of sudex shoot tissue was observed when sudex was collected at 7 days of age. Small-seeded broadleaf species were more inhibited in the presence of sudex shoot tissue than were grass species. Two major phytoinhibitors were isolated from aqueous extracts of sudex shoot material by partitioning with diethyl ether, followed by thin-layer and liquid column chromatography. Phytoinhibitors were identified asp-hydroxybenzoic acid andp-hydroxybenzaldehyde, potentially the enzymatic breakdown products of the cyanogenic glycoside dhurrin. The I₅₀ values of these compounds using a cress (Lepidium sativum L.) seed bioassay were 140 and 113 g/ml for the acid and aldehyde, respectively. Sudex tissue collected at 7 days of age possessed a greater percentage of these phytoinhibitors on a per gram basis than did older sudex tissue. As sudex tissue age increased, the percentage ofp-hydroxybenzaldehyde in ether extracts of tissue also increased, while the percentage ofp-hydroxybenzoic acid decreased.Journal article No. 88-10-163 of the Kentucky Agricultural Experiment Station.     

Synergistic inhibitory effects of vanillic andp-hydroxybenzoic acids on radish and grain sorghum

Radish and grain sorghum germination and sorghum growth were inhibited in a synergistic manner by combinations of vanillic andp-hydroxybenzoic acids. At threshold inhibition levels, 2.5 × 10^–3 M vanillic acid-treated radish seeds had 71 % of control germination after 24 hr and 2.5 × 10^–3 Mp-hydroxybenzoic acid-treated radish yielded 95% germination. A mixture of 2.5 × 10^–3 M of each of these two phytotoxins showed 52% germination after 24 hr. Equimolar mixtures of 5 × 10^–3 M vanillic andp-hydroxybenzoic acids allowed sorghum germination of 60% of untreated seeds after 24 hr, whereas separate treatments of individual phenols had 93% and 96% of control seed germination. Sorghum root and shoot elongation and total seedling growth were more sensitive than germination to vanillic andp-hydroxybenzoic acid treatments, and synergistic effects also were apparent. A combination of 5 × 10^–3 M vanillic with 5 × 10^–3 Mp-hydroxybenzoic reduced root length more than either did individually, and a mixture of 5 × 10^–4 M vanillic with 5 × 10^–4 Mp-hydroxybenzoic acid reduced sorghum seedling growth to approximately that resulting from a 10^–3 M concentration of either phenol alone. Phytotoxin levels inhibitory to sorghum growth caused small increases in lower leaf surface diffusive resistance, but did not close stomates, and this effect was not judged to be the cause of reduced sorghum growth.     

Allelopathic potential ofnuphar lutea (L.) Sibth. & Sm. (Nymphaeaceae)

Aqueous extracts ofNuphar lutea (L.) Sibth. & Sm. leaves (blades plus petioles) and roots plus rhizomes were tested for allelopathic activity using lettuce seedling andLemna minor L. assay systems. The 12.5, 25, 125, and 250 parts per thousand (ppt) treatments of both extracts killed the lettuce seedlings. At 2.5 ppt of extract, radicle growth of lettuce was 29% of the control for leaves and 31% of the control for roots plus rhizomes.Lemna minor frond number was reduced to 34% of the control by the 25 ppt leaf extract and to 43% of the control by the 25 ppt roots plus rhizomes extract.L. minor was killed by concentrations of 125 ppt and above of both plant part extracts. As expected, the frond number and total chlorophyll content measured by theL. minor assay were highly correlated. Osmotic potentials below 143 mOsmol/kg had no influence onL. minor growth. Neither the osmotic potential nor the pH of the undiluted extracts ofN. lutea were in the range known to influence the growth of either lettuce seedlings orL. minor. Nuphar lutea extracts were many times more inhibitory than 16 other hydrophytes we previously examined.     

Chemical Defenses of Freshwater Macrophytes Against Crayfish Herbivory

We measured feeding preferences of the crayfish Procambarus clarkii for fresh tissue from four species of freshwater macrophytes (Habenaria repens, Saururus cernuus, Ceratophyllum demersum and Typha angustifolia). We then determined the role of plant chemical defenses in generating these preferences by incorporating crude aqueous and organic extracts from each species into palatable foods and comparing feeding on these foods to feeding on control foods lacking these extracts. Tissue toughness, dry mass and ash-free mass per volume, and percentages of carbon, nitrogen, and phenolics were also measured for each of the four macrophytes. Although it had a low nutritional value, Ceratophyllum was the preferred food when it was offered as fresh tissue; it did not produce a chemically deterrent extract. The lipophilic crude extract from Typha significantly deterred crayfish feeding, but this highly nutritious plant was preferred when offered in an agar-based diet lacking structural defenses. Habenaria and Saururus were low preference foods that did not appear to be structurally defended; each species contained both lipophilic and water-soluble extracts that significantly deterred feeding. Fractionation of the lipophilic crude extract from Saururus indicated the presence of at least three deterrent compounds. From the orchid Habenaria, we isolated and identified a novel bis-p-hydroxybenzyl-2-alkyl-2-hydroxysuccinoate metabolite, habenariol, that appeared to explain most of the feeding deterrent activity present in the lipophilic extract of this species. The concentration of the metabolite in frozen collections of this plant doubled if we allowed the material to thaw before placing it in extraction solvents.     

Effects of mixtures of four phenolic acids on leaf area expansion of cucumber seedlings grown in Portsmouth B1 soil materials

Cucumber seedlings growing in a 12 mixture of soil (Portsmouth B₁) and sand adjusted to pH 5.2 were treated every other day five times with 0, 0.0625, 0.125, 0.25, or 0.5 mol/g soil of ferulic, caffeic,p-coumaric,p-hydroxybenzoic, protocatechuic, sinapic, syringic, or vanillic acids. Treatments began when seedlings were 8 days old. The effects on mean absolute rates of leaf expansion were used to estimate the relative potencies of these phenolic acids to ferulic acid. Based on the results of this experiment, ferulic,p-coumaric,p-hydroxybenzoic, and vanillic acids were chosen for further study. Materials and procedures were identical in the second study, but treatments consisted of mixtures of the four phenolic acids at concentration combinations designed to achieve 40 % or 60 % inhibition of absolute rates of leaf expansion. Using joint action analysis, a model describing the action of the phenolic acid mixtures was developed. A model involving only two factor terms was sufficient to describe the observed responses of cucumber leaf area to the phenolic acid mixtures. The action ofp-hydroxybenzoic acid on absolute rates of leaf expansion was inhibited by the presence of the other three phenolic acids. No other antagonisms or synergisms existed among the four compounds.This research was partially supported by the North Carolina Agricultural Research Service, Raleigh, North Carolina 27695-7643 and by the US-Spain Joint Committee for Scientific and Technological Cooperation project CCA-8309/166.     

Phytotoxicity of Vulpia Residues: II. Separation, Identification, and Quantitation of Allelochemicals from Vulpia myuros

Phytotoxicity-based extraction and fractionation were employed to separate and purify the allelochemicals from an aqueous extract of vulpia (Vulpia myuros) residues. Further analyses, identification, and quantitation of these allelochemicals were conducted by gas chromatography–mass spectrometry on the most toxic fraction. Twenty-one compounds were identified in the ether fraction from the vulpia residue aqueous extract. They were catechol, hydroquinone, 3,4-dimethoxyphenol, pyrogallol, coniferyl alcohol, and benzoic, succinic, hydrocinnamic, salicylic, protocatechuic, vanillic, gentisic, syringic, p-hydroxybenzoic, -hydroxybenzenepropanoic, p-hydroxyphenylacetic, p-hydroxybenzenepropanoic, hydroferulic, p-coumaric, hydrocaffeic, and ferulic acids. A chromatographic internal standard method with multiple-point calibration graphs was used to quantify the identified compounds. Quantities in the vulpia residues ranged from 1.36 to 81.0 g/g dry residue, in total accounting for 0.05% of the dry weight residue. The combined syringic and hydroferulic acids were present in the largest amount, 140.11 g/g residue. Vanillic, succinic, p-hydroxybenzenepropanoic, and salicylic acids were next in amount, ranging from 37.24 to 81.24 g/g residue. Catechol, hydrocinnamic acid, and hydroquinone were present in the smallest quantities, ranging from 1.36 to 1.82 g/g residue. The remainder of the compounds were intermediate in amount, ranging from 2.33 to 18.1 g/g residue.     

Constituents of Amomum tsao-ko and their radical scavenging and antioxidant activities

Constituents of the fruit of Amomum tsao-ko were investigated following a preliminary screening of the antioxidant activity of several extracts of the fruit of this plant that showed that the dichloromethane extract and the ethyl acetatesoluble and water-soluble fractions of the 70% aqueous acetone extract had higher activity than α-tocopherol and butylated hydroxytoluene (BHT). Eleven compounds were isolated from the ethyl acetate-soluble fraction, and their structures were elucidated as (+)-hannokinol (1), meso-hannokinol (2), (+)-epicatechin (3), (−)-catechin (4), β-sitosterol (5), β-sitosterol 3-O-glucoside (6), 2,6-dimethoxyphenol (7), protocatechualdehyde (8), protocatechuic acid (9), vanillic acid (10), and p-hydroxybenzoic acid (11) based on mass and various nuclear magnetic resonance (NMR) spectroscopic techniques. This is the first isolation of epicatechin and catechin from the genus Amomum. The radical scavenging activity of the isolated compounds was evaluated using 1,1-diphenyl-2-picrylhydrazyl (DPPH) and colorimetric and electron spin resonance (ESR) analyses. The antioxidant activity of the compounds was also determined based on the oxidative stability index (OSI). The catechins and catechol derivatives showed strong activities in both the DPPH radical scavenging activity and antioxidant activity assays.     

Phytotoxicity of <Emphasis Type="Italic">Phytolacca americana</Emphasis> Leaf Extracts on the Growth,and Physiological Response of <Emphasis Type="Italic">Cassia mimosoides</Emphasis>

We examined the allelochemical effects of control soil, native soil (treated soil), and leaf extracts of Phytolacca americana (pokeweed) on the germination rate and seedling growth of Cassia mimosoides var. nomame. We also studied the resulting changes in root-tip ultrastructure and peroxidase isozyme biochemistry. P. americana leaf extract inhibited seed germination, seedling growth, and biomass when compared to control and treated soil. Root and shoot growth in treated soil was stimulated relative to control soil, but root growth was inhibited by 50% in the leaf extract treatment. Biomass of C. mimosoides seedlings grown on leaf extract was reduced sevenfold when compared to the control seedlings. The amounts of total phenolic compounds in the leaf extract, treated soil, and control soil were 0.77, 0.14, and 0.03 mg l⁻¹, respectively. The root tips of C. mimsoides treated with leaf extracts of P. americana showed amyloplasts and large central vacuoles with electron-dense deposits inside them when compared to control root tips. The activity of guaiacol peroxidase (GuPOX) in whole plant, roots, and shoots of C. mimosoides increased as leaf extract increased; maximum activity was observed in extract concentrations of 75% and higher. Root GuPOX activity was three times higher than in shoots. Therefore, we conclude that inhibition of C. mimosoides growth is related to the phenolic compounds in the P. americana leaf extract and the ultrastructure changes in root-tip cells and increased GuPOX activity is a response to these allelochemicals.     

Components of Honeybee Royal Jelly as Deterrents of the Parasitic <Emphasis Type="Italic">Varroa</Emphasis> Mite, <Emphasis Type="Italic">Varroa destructor</Emphasis>

The parasitic mite Varroa destructor Anderson & Trueman reproduces on the immature stage of the honeybee, Apis mellifera L. Mites are found more often on drone brood than worker brood and only infrequently on queen brood. We investigated the chemical basis for the low incidence of mites on queen brood. V. destructor mites were deterred by a crude extract of royal jelly, a glandular secretion produced by nurse bees and fed to queen larvae. Bioassay-driven fractionation of the crude extract via column chromatography resulted in one active fraction that was as active as the crude extract. Compounds in the active fraction were identified using gas chromatography (GC) and coupled gas chromatography/mass spectrometry (GC-MS). Before injection, compounds were esterified with MeOH/sulfuric acid, followed by silylation of any hydroxyl groups present. The active fraction contained at least 22 compounds, all fatty acids, several of which contained an additional hydroxyl group on the alkyl chain. Synthesis of some of these compounds that are not commercially available is described. A synthetic mixture containing most of the compounds in the active fraction was as active as the active fraction in the bioassay.     

Distribution and Exudation of Allelochemicals in Wheat Triticum aestivum

Wheat allelopathy has potential for weed suppression. Allelochemicals were identified in wheat seedlings, and they were exuded from seedlings into agar growth medium. p-Hydroxybenzoic, trans-p-coumaric, cis-p-coumaric, syringic, vanillic, trans-ferulic, and cis-ferulic acids and 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA) were identified in both the shoots and roots of 17-day-old wheat seedlings and their associated agar growth medium. Wheat accessions with previously identified allelopathic activity tended to contain higher levels of allelochemicals than poorly allelopathic ones. The allelopathic compounds present in the shoots generally also were identified in the roots and in the agar medium. Allelochemicals were distributed differentially in wheat, with roots normally containing higher levels of allelochemicals than the shoots. When the eight allelochemicals were grouped into benzoic acid and cinnamic acid derivatives, DIMBOA, total coumaric, and total ferulic acids, the amount of each group of allelochemicals was correlated between the roots and the shoots. Most of the allelochemicals identified in the shoots and roots could be exuded by the living roots of wheat seedling into the agar growth medium. However, the amounts of allelochemicals in the agar growth medium were not proportional to those in the roots. Results suggest that wheat plants may retain allelochemicals once synthesized. The presence of allelochemicals in the agar growth medium demonstrated that wheat seedlings were able to synthesize and to exude phytotoxic compounds through their root system that could inhibit the root growth of annual ryegrass.     

Isolation,characterization and activity of phytotoxic compounds from quackgrass [Agropyron repens (L.)Beauv.]

Previous experiments showed that legumes grown in the presence of living or herbicidally treated quackgrass residues or extracts exhibited reduced seedling root and shoot growth and decreased nodulation and nitrogen fixation. Aqueous extracts of quackgrass shoots were most inhibitory to plant growth. Upon sequential partitioning of an aqueous extract of quackgrass shoots, the ether extract possessed the most activity and caused 50% reductions in radicle elongation of eight crop and weed species at concentrations of less than 240 g/ml (small-seeded species) and 1000 g/ml (large-seeded species). Snapbeans (Phaseolus vulgaris L. Bush Blue Lake) grown aseptically in agar containing an ether extract at 100 and 200 g/ml exhibited severe root browning, lack of root hair formation, and a two- to three-fold reduction in root and shoot dry weights. The ether extract of quackgrass shoots had no inhibitory effect on the growth of fourRhizobium species in Petri dishes or two species in broth culture. Inhibitors present in the ether extract may influence the legumeRhizobium symbiosis indirectly by reducing legume root growth and root hair formation. The ether extract of quackgrass shoots was separated using high-pressure liquid, thin-layer, and liquid column chromatography in an attempt to isolate and identify the inhibitors responsible for the inhibition of seedling growth. Two closely related flavonoid inhibitors were isolated from the ether extract. One was identified as 5,7,4-trihydroxy-3,5-dimethoxyflavone (tricin). Both flavonoids caused 50% inhibition of radicle elongation in cress (Lepidium sativum L. Burpee curly) seeds at concentrations of less than 125 g/ml. Both flavonoids were found in ether extracts of quackgrass shoots and rhizomes, but the largest amounts of both compounds occurred in quackgrass shoots collected from the field.Journal article No. 11887 of the Michigan Agricultural Experiment Station.