WO2005039292A1

WO2005039292A1 - Herbicidal compositions of a. repens derived flavones and benzoflavones

Info

Publication number: WO2005039292A1
Application number: PCT/US2004/035602
Authority: WO
Inventors: Jorge M. Vivanco; Frank R. Stermitz; Harsh Pal Bais
Original assignee: Colorado State University Research Foundation
Priority date: 2003-10-24
Filing date: 2004-10-25
Publication date: 2005-05-06
Also published as: CA2485781A1

Abstract

The present invention provides herbicidal compositions based on exudates of the invasive Acroptilon repens (Russian knapweed) plant. An active component from the exudate is 7,8-benzoflavone (α-naphthoflavone) and the isomer 5,6-benzoflavone (β-naphthoflavone) as well as flavone are shown to have useful phytotoxic activity. Methods of isolating the 7,8-benzoflavone from the exudate and methods of using flavone-containing herbicidal compositions are also provided.

Description

HERBICIDAL COMPOSITIONS OF A. REPENS DERIVED FLAVONES AND BENZOFLAVONES

FIELD OF THE INVENTION The present invention resides in the field of herbicidal compositions and methods of making and using the same.

BACKGROUND OF THE INVENTION Many herbicides and pesticides in current use have human and animal toxicities requiring controlled application as well as clean up of contaminated land and ground water. These requirements greatly increase the actual cost of using these agricultural products beyond the costs associated with purchasing and applying the chemicals. One proposed solution to these problems has been the search for naturally-occurring chemicals with herbicidal or insecticidal properties without significant animal or human toxicities. This has led to the study of invasive plants which have the ability to spread over large areas by the secretion of allelochemicals to kill or inhibit the growth of surrounding plants. The few millimeters of soil immediately surrounding a plant root constitute a unique physical, biochemical and ecological environment. The rhizosphere is largely controlled by the root system through chemicals secreted into the surrounding soil. Root exudates include low molecular weight compounds such as amino acids, organic acids, sugars, phenolics and various secondary metabolites and high molecular weight compounds like mucilage and proteins. Through the exudation of a wide variety of compounds, roots may regulate the soil microbial community in their immediate vicinity, cope with herbivores, encourage beneficial symbioses, change the chemical and physical properties of the soil, and inhibit the growth of competing plant species. Countering a challenge, roots may respond by secreting certain chemicals such as secondary metabolites, proteins and even volatiles. Root secretions may play symbiotic or defensive roles as a plant ultimately develops a positive or negative communication, depending on the other elements of its rhizosphere. An example of negative communication is provided by Acroptilon repens (Russian knapweed, A. repens). This noxious weed displaces other weeds and crops through root exudates. The past five decades of research in the field of knapweed allelopathy has witnessed minimal success in characterizing the responsible allelochemical(s). Thus, there is a long felt need for the identification of the chemicals responsible for the allelopathic chemical(s) from Acroptilon repens that could be used as an effective, naturally-occurring herbicidal product.

SUMMARY OF THE INVENTION The present invention provides herbidical compounds and compositions and methods of using these chemicals to effectively control unwanted plants. The herbicidal compositions are based on exudates of A. repens and chemicals identified in those exudates. Specifically, herbicidal compositions are based on 7,8-benzoflavone which has been identified as the active allopathic ingredient of A. repens exudates. Preferably, the herbicidal compositions of the present invention contain a flavone or flavone derivative such as flavone, 7,8-benzoflavone or 5,6-benzoflavone or combinations of these ingredients and additional ingredients such as extenders, thickners, dispersants, tackifiers, colorants, fertilizers, growth regulators, stabilizers, antifoams, preservatives, viscosity regulators, binders, surfactants and combinations thereof. These additional ingredients may represent between about 5% and about 95% of the composition. The herbicidal compositions of the present invention may contain a flavone or flavone derivative in a concentration of between about lOμg/ml and about 500μg/ml. However, the herbicidal compositions may be prepared as a concentrate for dilution. In this instance, the concentration of the flavone compound in the composition may be greater than 500 μg/ml. The herbicidal composition may be an exudate of A. repens. The invention provides methods of controlling undesirable plant growth by contacting undesirable plants with a composition containing a chemical having herbicidal activity such as flavone, 7,8-benzoflavone or 5,6-benzoflavone or combinations of these chemicals. The method may include applying the composition to the media in which the undesirable plant is growing. Alternatively, the method may include mixing the composition into media in which the undesirable plant is growing. The application of the herbicidal composition may include preemergence application, postemergence application and/or seed dressing. The method may also include applying the composition to a media targeted for crop plant growth. Similarly, the invention provides a method of inhibiting germination of a seed by contacting a seed with a composition containing 7,8- benzoflavone, 5,6-benzoflavone and/or flavone. In this method, the application may include contacting the media containing the seed such as soil. This may also include contacting or mixing of the composition with the media before the seed is contacted with the media. The present invention also provides a method of making the herbicidal compositions of the present invention by isolating a root extract of A. repens and combining the root exudate with a an ingredient such as extenders, thickners, dispersants, tackifiers, colorants, fertilizers, growth regulators, stabilizers, antifoams, preservatives, viscosity regulators, binders, surfactants or combinations thereof. In this method, the root extract may be extracted into a solvent such as methanol or hexane. Additionally, the solvent may be freeze dryed after the extraction. The present invention also provides a herbicidal composition made by the method of isolating a root extract of A. repens and extracting 7,8-benzoflavone from the A. repens extract.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows the structure of the flavones for use in the herbicidal compositions and methods of the present invention; 7,8-benzoflavone (1), 5,6-benzoflavone (2), and flavone (3). Figure 2 shows the effect of 7,8-benzoflavone, 5,6-benzoflavone, and flavone on (A) shoot and (B) root differentiation of different weeds on the seventh day after treatment. Compounds were dissolved in CH₃OH and administered (<250 μg/ml) to the media containing 10-day-old seedlings. Twelve- well aseptic plates were subsequently incubated under a 16 hour light and 8 hour dark photoperiod in an incubator. The data represent the percent inhibition relative to the untreated control in shooting and rooting efficiency response in various tested seedlings. (Values are Mean ± S.D, n=10). Figure 3 shows the effect of 7,8-benzoflavone (1, 100 μgmlml) over a 7-day time course on roots of (A) Arάbidopsis thaliana, (B) Centaurea diffusa, (C) Centaurea maculosa and (D) Linaria dalmatica depicting cytoplasmic precipitation, a clear signature of cell death and rhizotoxicity (inset). DETAILED DESCRIPTION OF THE INVENTION The present invention is drawn to herbicidal compositions based on exudates and extracts of Acroptilon repens and to methods of manufacturing and using these compositions. The herbicidal compositions preferably provide effective pre- and post- emergent herbicidal activity through the use of a naturally-occurring allelopathic chemical. Acroptilon repens (Russian knapweed, A. repens) is an Eurasian-derived noxious invader of the western United States. It was formerly classified as part of Centaurea, a complex genus now known to be polyphyletic. Russian knapweed has long been suspected to produce allelochemicals and early work to identify the phytotoxic compounds focussed on sesquiterpene lactones, which were not potent enough as phytotoxins to be considered ecologically important. Polyacetylenes and thiophene derivatives are also known A. repens root chemicals and one of the polyacetylenes, among several isolated from A. repens roots, proved to be phytotoxic. This compound could also be isolated from soil around the plant roots in concentrations sufficient to have a significant effect on the surrounding plant community. Surprisingly, 7,8-benzoflavone (α-naphthoflavone), a compound not previously known to be naturally-occurring, has been identified as a phytotoxic component of the root exudate of A. repens. Further testing revealed that 5,6-benzoflavone (β- naphthoflavone), an isomer of 7,8-benzoflavone, as well as flavone were also phytotoxic, with flavone having the highest phytotoxic potency of these three chemicals. These flavones have been found to inhibit seed germination and root growth, and thereby displace other plants, after secretion from the roots of A. repens. Typical of the knapweed plants, A. repens is itself resistant to the effects of 7,8-benzoflavone and 5,6- benzoflavone. However, flavone is also active against A. repens. Thus, one embodiment of the present invention is a flavone-containing herbicidal composition. This composition may contain flavone, 7,8-benzoflavone, 5,6- benzoflavone, an exudate from the roots of A. repens, agriculturally-acceptable salts of these flavones or combinations of these herbicidal ingredients. The flavones may be isolated from A. repens, synthesized, or purchased commercially. The herbicidal composition can be used to control, kill, suppress or inhibit the growth of susceptible plants. As used herein, the term "control" is inclusive of the actions of killing, inhibiting growth, reproduction or proliferation, and removing, destroying or otherwise diminishing the occurrence and activity of plants and is applicable to any of the stated actions, or any combination thereof. In accordance with this invention it has been found that the growth of germinating seeds, emerging seedlings, maturing and established woody and herbaceous vegetation and aquatic plants can be controlled by exposing the emerging seedlings or above- or below-ground portions of maturing and established vegetation, or the aquatic plants of the action of an effective amount of the flavone-containing herbicidal compositions of the present invention. The compounds can be used individually, as admixtures of two or more compounds, or in admixture with an adjuvant. These compositions are effective as post- emergent phytotoxicants or herbicides, e.g., the selective control of the growth of one or more monocotyledonous species and/or one or more dicotyledonous species in the presence of other monocotyledons and/or dicotyledons. Furthermore, these compounds are characterized by broad spectrum activity, i.e., they control the growth of a wide variety of plants including, but not limited to, fems, conifer (pine fir and the like), aquatic, monocotyledons and dicotyledons. The flavone-containing herbicidal compositions of the present invention is preferably applied to the target plant as a liquid or a solid. The term "plant" as used herein means terrestrial plants and aquatic plants. The compositions of this invention are suitable for all methods of application commonly used in agriculture, including pre- emergence application, post-emergence application and seed dressing. For example, suitable application means include watering, spraying, atomizing, dusting and scattering. The flavone-containing herbicidal compositions according to the invention can be applied before and after the plants have emerged, that is to say pre-emergence and post- emergence. They can also be incorporated into the soil before or during sowing. The herbicidally-active flavone compounds or A. repens extracts can be incorporated with formulations customarily used in the agricultural industry such as solutions, emulsions, wettable powders, suspensions, powders, dusts, pastes, soluble powders, granules, suspension-emulsion concentrates, natural and synthetic materials impregnated with active components and microencapsulations in polymeric substances. These formulations are produced in a known manner, for example by mixing the active compounds with extenders, that is, liquid solvents, and/or solid carriers, optionally with the use of surfactants, that is emulsifϊers and/or dispersants, and/or foam-formers. If the extender used is water, it is also possible to employ, for example, organic solvents as auxiliary solvents. Essentially, suitable liquid solvents are: aromatics such as xylene, toluene or alkylnaphthalenes, chlorinated aromatics or chlorinated aliphatic hydrocarbons such as chlorobenzenes, chloroethylenes or methylene chloride, aliphatic hydrocarbons such as cyclohexane or paraffins, for example petroleum fractions, mineral and vegetable oils, alcohols such as butanol or glycol and also their ethers and esters, ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone or cyclohexanone, strongly polar solvents such as dimethylformamide and dimethyl sulphoxide, and also water. Suitable solid carriers include, for example, ammonium salts and ground natural minerals such as kaolins, clays, talc, chalk, quartz, attapulgite, montmorillonite or diatomaceous earth, and ground synthetic minerals, such as highly disperse silica, alumina and silicates. Suitable solid carriers for granules include, for example, crushed and fractionated natural rocks such as calcite, marble, pumice, sepiolite and dolomite, and also synthetic granules of inorganic and organic meals, and granules of organic material such as sawdust, coconut shells, maize cobs and tobacco stalks; as emulsifiers and/or foam- formers; for example, nonionic and anionic emulsifiers, such as polyoxyethylene fatty acid esters, polyoxyethylene fatty alcohol ethers, for example alkylaryl polyglycol ethers, alkylsulphonates, alkyl sulphates, arylsulphonates and also protein hydrolysates. Suitable dispersants include, for example, lignin-sulphite waste liquors and methylcellulose. Tackifiers such as carboxymethylcellulose and natural and synthetic polymers in the form of powders, granules or latices, such as gum arabic, polyvinyl alcohol and polyvinyl acetate, as well as natural phospholipids such as cephalins and lecithins, and synthetic phospholipids, can be used in the formulations. Other additives can be mineral and vegetable oils. It is also possible to use colorants such as inorganic pigments, for example iron oxide, titanium oxide and Prussian Blue, and organic dyestuffs, such as alizarin dyestuffs, azo dyestuffs and metal phthalocyanine dyestuffs, and trace nutrients such as salts of iron, manganese, boron, copper, cobalt, molybdenum and zinc. The formulations generally comprise between about 0.1% and about 95% by weight of the active compound or combination of compounds, preferably between about 0.5% and about 90%. The active compounds according to the invention can be present in commercially available formulations and in forms prepared from these formulations such as in a composition with other active compounds, such as insecticides, attractants, sterilizing agents, bactericides, acaricides, nematicides, fungicides, growth-regulating substances or other herbicides. The insecticides include, for example, phosphoric acid esters, carbamates, carboxylates, chlorinated hydrocarbons, phenylureas and substances produced by microorganisms. It is also possible to admix other known active compounds such as fertilizers and growth regulators with the herbicidal compositions of the present invention. The compositions may also contain inactive ingredients effecting the composition without imparting herbicidal activity on their own such as stabilizers, e.g. where appropriate epoxidised vegetable oils (epoxidised coconut oil, rapeseed oil, or soybean oil), antifoams, typically silicone oil, preservatives, viscosity regulators, binders, as well as other chemical agents including other herbidices such as imazamethabenz-methyl, sulfosulfuron, tribenuron-methyl, amidosulfuron, metosulam, flurtamone, 2,4-D, bromoxynil, dichlorprop-P, tribenuron(-methyl), diflufenican, glyphosate(-isopropyl- ammonium), metsulfuron-methyl, fluroxypyr, isoproturon, imazamox, diclofop-methyl, carfentrazone-ethyl, clodinafop-propargyl, thifensul uron-methyl and mixtures thereof. The herbidical compositions of the present invention may contain any herbicidally-effective amounts of flavone, 7,8-benzoflavone, 5,6-benzoflavone or exudates of A. repens or salts thereof. Preferably, the compositions contain between about lOμg/ml and about 500 μg/ml of one of the flavone-containing compounds of the present invention, more preferably the compositions contain between about 50 μg/ml and about 250 μg/ml of the flavone-containing compounds, more preferably the compositions contain between about 50 μg/ml and about 100 μg/ml of the flavone-containing compounds. Flavone was found to be a stronger herbicide than either 7,8-benzoflavone or 5,6-benzoflavone and is therefore preferably used in smaller concentrations within the herbicidal compositions. Preferably, flavone is used in the concentration range of about 25 μg/ml to about 75 μg/ml. Most preferably, flavone is used in the concentration of about 50 μg/ml. Additionally, the flavone-containing herbicidal compositions may be supplied in a concentrated form for dilution prior to application. In these forms, the flavone-containing herbicidal compounds can be supplied in concentrations exceeding 500 μg/ml up to the solubility of the flavone compound in the desired solvent. In this embodiment of the present invention, the concentrate is preferably supplied with instructions for further dilution of the concentrate prior to use. Any of the additional components that may be added to the herbicidal preparations of the present invention may occupy between about 1% and about 99% of the composition. Preferably the additional components occupy no more than between about 5% and about 95% of the composition. Further embodiments of the present invention include methods of controlling undesired plants by the application of one or more of the flavone-containing herbicidal compositions to the undesired plant. The flavone-containing herbicidal compositions described above may be applied directly to the plant targeted for control or applied to the area surrounding the plant including the habitat of the plant or the media in which the plant is growing. As used herein, the term media means any medium capable of sustaining plant growth including, but not limited to, soils, aqueous solutions, hydroponic systems, sterilized media, and nutrient-enriched or enhanced media. The herbicidal compositions may be applied to the target vegetation prior to visible growth of the plant or after the plant has begun to grow. In a further embodiment of the present invention, the flavone-containing herbicidal compositions described above are used to inhibit germination of a seed. In this embodiment, the compositions may be applied to the seed or to media or containers in which the seeds are located. Thus, in this embodiment, the flavone-containing herbicidal composition may be preventively applied to the media or containers in which the seeds are located prior to the seeds becoming present in order to effectively inhibit the germination of the seeds should they later become present. Another embodiment of the present invention is a method for the selective control of weeds in crops of cultivated plants which includes treating the cultivated plants, the seeds or seedlings or the crop area thereof with a herbicidally effective amount of flavone-containing herbicidal compositions of the present invention. The method is applicable to crop plants that are not affected by the flavone-containing herbicidal compositions of the present invention or can tolerate higher concentrations of the flavone- containing herbicidal compositions of the present invention than the vegetation targeted for elimination within the crop area. Additional objects, advantages, and novel features of this invention will become apparent to those skilled in the art upon examination of the following examples thereof, which are not intended to be limiting. EXAMPLES Example 1. The Phytotoxic Effects of Exudates from A. repens on Other Plant Species. Seeds of plant species were obtained from the collections of Professor R. Huf auer, Department of Bioagricultual Sciences & Pest Management, who also identified these species in the field. Seeds of Acroptilon repens (L.) DC, Centaurea maculosa Lam., Centaurea diffusa Lam. Gaillardia aristata Pursh (Asteraceae) and Linaria dalmatica (L.) Mill. (Scrophulariaceae) were obtained from natural populations in Larimer or Routt Counties, CO. Seeds of Arabidopsis thaliana were purchased from Lehle Seed Company, Texas. The 5,6-benzoflavone, 7,8-benzoflavone and flavone were obtained from Indofine Chemical Co., Hillsborough, NJ. Seeds were washed in running tap water and surface sterilized using sodium hypochlorite (0.3 % v v^"1) for 10-15 min, followed by 3-4 washes in sterile distilled water. Seeds were inoculated on static MS (Murashige and Skoog, 1962) basal media in petri dishes and allowed to germinate for 10 days until roots and shoots emerged. These were then incubated under 16 hr Light/8 hr dark cycles at 25 ± 2°C. The light intensity in the growth chamber was 24 μmol/m²/s. Ten-day-old seedlings were transferred to 5-ml, 12-well aseptic plates with 2 ml of liquid MS basal media. Plant cultures were maintained on an orbital platform shaker set at 90 rpm. Root exudates were collected from the Acroptilon repens plant cultures, filter sterilized to avoid collecting root border cells and cellular debris, autoclaved (120° C for 30 min at 15 lb pressure), and administered in different concentrations (1-3 ml v/v) over the test plant seedlings as were collected HPLC-eluants. Ten-day-old seedlings of A. repens, C. maculosa, C. diffusa, Arabidopsis thaliana, L. dalmatica and G. aristata were placed on MS basal medium in petri dishes after initial surface sterilization. Petri dishes were kept under a 16 hr light/8 hr dark photoperiod in an incubator. Length of shoots, number of shoots and length of the primary root of the treated and untreated plants were measured and were denoted as units for shoot and root differentiation. Similar procedures were used to test phytotoxicity of 5,6-benzoflavone, 7,8-benzoflavone and flavone. All the plant species showed mortality on the 7 day after addition of root exudates from A. repens. Plants showed wilting symptoms prior to senescence with reduced shoot and root differentiation after administration of the root exudates. Hexane extracts of freeze-dried medium in which A. repens had been grown were subjected to HPLC analysis and collected fractions were used for bioassay. Essentially all the phytotoxicity was confined to a single compound and no activity remained in the extracted medium. The active component was identified as 7,8-benzoflavone (α- naphthoflavone). Phytotoxicity of 7,8-benzoflavone, along with structural analogs 5,6- benzoflavone and flavone, were assayed in more detail for their effect on test plant phenotypic responses in terms of shoot and root differentiation inhibition. A concentration of 250 μg/ml for all the three tested flavones showed a distinct plant inhibitory profile (Fig. 1). Generally, the phytotoxic activities were in the range of 50-250 μg/ml except for flavone, which was active at 50 μg/ml. All plants except A. repens showed mortality on the 7^th day after the addition of 7,8-benzoflavone and 5,6- benzoflavone but flavone was also active against A. repens (Figure 2). The addition of 100 μg/ml of 7,8-benzoflavone to the roots of Arabidopsis thaliana, C. diffusa, C. maculosa and L. dalmatica led to a condensation of the cytoplasm characteristic of cell death, which showed 7,8-benzoflavone to be a potent rhizotoxin (Figure 3). The phytotoxic potency of 7,8-benzoflavone is a little less than that observed for (-)-catechin, but flavone appears to be approximately as potent. One of the North American native species {G. aristata) was found to be resistant to 7,8-benzoflavone and 5,6-benzoflavone, although not to flavone (Figure 2). In the field, populations of G. aristata are also resistant to knapweed invasion, one of the few native species to exhibit such resistance.

Example 2. Isolation and Identification of a Phytotoxic Compound from the Root Exudates of A. repens. This example demonstrates the isolation and identification of a phytotoxic compound from the root exudates of A. repens. Filter-sterilized growth media exudates (2 ml) were extracted with 5 ml hexane, vortexed and stored for 24 h at 4°C. The supernatant was transferred with a Pasteur pipette to a separate test tube, and 1ml of hexane was added. The supernatant was further concentrated by freeze-drying, and the weighed powder was re-dissolved in 500 μl of absolute MeOH for HPLC analyses. Compounds in the root exudates were chromatographed by gradient elution on a reverse phase 5 μm, C₁₈ column (25 cm x 4.6 mm) (Supelco™ Co.) using a Summit Dionex pump and injection system with detection at 210 nm. Mobile phase Solution A was double distilled water and Solution B was MeOH. A multi-step gradient was used for all separations with an initial injection volume of 50 μL and a flow rate of 1 ml/min: 0-10 min 5% B, 10-15 min 20% B, 15-25 min 20% B, 25-40 min 80% B, 40-60 min 100% B, 60-70 min 100 % B, 70-80 min 5% B. Major peaks were collected and bioassayed for phytotoxicity, with toxicity showing only for a 44 min eluant. This eluant was collected from several injections, concentrated under vacuum at 30° C and repurified by reinjection and collection. The 1H and C¹³ NMR spectra (Varian INOVA™ 500 MHz in CDC1₃ and CD₃OD) and EIMS of the active compound were identical to those of commercial 7,8- benzoflavone and nonidentical to those of 5,6-benzoflavone. EIMS m/z (rel. int.) 272 (95), 244 (15), 170 (100), 114 (55); 1H NMR (CDC1₃): δ 6.93 (IH, s), 7.56 (3H, m), 7.68 (2H, m), 7.74 (IH, d, J= 8.5), 7.90 (IH, m), 7.99 (2H, m), 8.14 (IH, d, J= 8.5), 8.55 (IH, m); C¹³ NMR (75 MHz in CD₃OD): δ 108.1, 121.0, 121.1, 123.6, 125.4, 127.1, 127.8, 129.0, 129.6, 130.6, 131.1, 132.9, 133. 3,137.8, 155.3, 165.3, 180.5. The foregoing examples have been presented for purposes of illustration and description. The foregoing is not intended to limit the invention to the form or forms disclosed herein. Although the description of the invention has included description of one or more embodiments and certain variations and modifications, other variations and modifications are within the scope of the invention, e.g., as may be within the skill and knowledge of those in the art, after understanding the present disclosure. It is intended to obtain rights which include alternative embodiments to the extent permitted, including alternate, interchangeable and/or equivalent structures, functions, ranges or steps to those claimed, whether or not such alternate, interchangeable and/or equivalent structures, functions, ranges or steps are disclosed herein, and without intending to publicly dedicate any patentable subject matter.

Claims

What is claimed is: 1. A herbicidal composition comprising an exudate of A. repens.

2. The herbicidal composition of Claim 1, wherein the exudate comprises 7,8-benzoflavone.

3. A herbicidal composition comprising a compound selected from the group consisting of 7,8-benzoflavone, 5,6-benzoflavone and flavone.

4. The herbicidal composition of Claim 3 further comprising an ingredient selected from the group consisting of extenders, thickners, dispersants, tackifiers, colorants, fertilizers, growth regulators, stabilizers, antifoams, preservatives, viscosity regulators, binders, surfactants and combinations thereof.

5. The herbicidal composition of Claim 3, wherein the compound is present in a concentration range of between about lOμg/ml and about 500μg/ml.

6. The herbicidal composition of Claim 3, prepared for further dilution wherein the compound is present in a concentration of greater than 500 μg/ml.

7. The herbicidal composition of Claim 3, wherein the at least one ingredient comprises between about 5% and about 95% of the composition. 8. A method of controlling undesirable plant growth comprising the step of contacting an undesirable plant with a composition comprising a compound selected from the group consisting of 7,

8-benzoflavone, 5,6-benzoflavone and flavone.

9. The method of Claim 8, wherein the composition is an exudate of A. repens.

10. The method of Claim 8, wherein the contacting comprises applying the composition to the media in which the undesirable plant is growing.

11. The method of Claim 10, wherein the applying comprises mixing the composition into media in which the undesirable plant is growing.

12. The method of Claim 10, wherein the contacting comprises a treatment selected from the group consisting of preemergence application, postemergence application and seed dressing.

13. The method of Claim 10, wherein the contacting step comprises applying the composition to a media targeted for crop plant growth.

14. A method of inhibiting germination of a seed comprising the step of contacting a seed with a composition comprising a compound selected from the group consisting of 7,8-benzoflavone, 5,6-benzoflavone and flavone.

15. The method of Claim 14, wherein the contacting comprises applying the composition to a media containing the seed.

16. The method of Claim 15, wherein the media containing the seed is soil.

17. The method of Claim 15, wherein the contacting comprises applying the composition to a media prior to contacting the media with the seed.

18. A method of making a herbicidal composition comprising the step of isolating a root extract of A. repens.

19. The method of Claim 18, wherein the isolating step comprises extracting 7,8-benzoflavone from the root extract into a solvent.

20. The method of Claim 19, wherein the solvent is selected from the group consisting of methanol and hexane.

21. The method of Claim 19, comprising the additional step of freeze drying the solvent after the extracting step.

22. A 7,8-benzoflavone-containing preparation produced by the process comprising: a. isolating a root extract of A. repens; b. extracting 7,8-benzoflavone from the A. repens extract into a solvent; and, c. removing the solvent from the 7,8-benzoflavone.

23. The method of Claim 22, wherein the solvent is selected from the group consisting of methanol and hexane.

24. The method of Claim 22, comprising the additional step of freeze-drying the solvent after the extracting step.