Abstract: Summary 1We investigated the role of a native generalist soil pathogen through which a non-native invasive plant species may suppress naturalized/native plant species. 2We found that rhizosphere soils of Chromolaena odorata, one of the world's most destructive tropical invasive weeds, accumulate high concentrations of the generalist soil borne fungi, Fusarium (tentatively identified as F. semitectum), thus creating a negative feedback for native plant species. 3Soils collected beneath Chromolaena in the Western Ghats of India inhibited naturalized/native species and contained over 25 times more spores of the pathogenic fungi Fusarium semitectum than soils collected at the same locations beneath neighbouring native species that were at least 20 m from any Chromolaena plant. Sterilization of these soils eliminated their inhibitory effect. Chromolaena root leachate experimentally added to uninvaded soils increased Fusarium spore density by over an order of magnitude, and increased the inhibitory effect of the soils. 4The positive effect of Chromolaena root leachates on Fusarium spores was attenuated by activated carbon, suggesting a biochemical basis for how the invader stimulated the pathogen. 5Synthesis. Invasive plants have been shown to escape inhibitory soil biota in their native range and to inhibit soil biota in their invaded range, but our results indicate that the impacts of Chromolaena are due to the exacerbation of biotic interactions among native plants and native soil biota, which is to our knowledge a new invasive pathway.

Abstract: Since varietal differences in allelopathy of crops against weeds were discovered in the 1970s, much research has documented the potential that allelopathic crops offer for integrated weed management with substantially reduced herbicide rates. Research groups worldwide have identified several crop species possessing potent allelopathic interference mediated by root exudation of allelochemicals. Rice, wheat, barley and sorghum have attracted most attention. Past research focused on germplasm screening for elite allelopathic cultivars and the identification of the allelochemicals involved. Based on this, traditional breeding efforts were initiated in rice and wheat to breed agronomically acceptable, weed-suppressive cultivars with improved allelopathic interference. Promising suppressive crosses are under investigation. Molecular approaches have elucidated the genetics of allelopathy by QTL mapping which associated the trait in rice and wheat with several chromosomes and suggested the involvement of several allelochemicals. Potentially important compounds that are constitutively secreted from roots have been identified in all crop species under investigation. Biosynthesis and exudation of these metabolites follow a distinct temporal pattern and can be induced by biotic and abiotic factors. The current state of knowledge suggests that allelopathy involves fluctuating mixtures of allelochemicals and their metabolites as regulated by genotype and developmental stage of the producing plant, environment, cultivation and signalling effects, as well as the chemical or microbial turnover of compounds in the rhizosphere. Functional genomics is being applied to identify genes involved in biosynthesis of several identified allelochemicals, providing the potential to improve allelopathy by molecular breeding. The dynamics of crop allelopathy, inducible processes and plant signalling is gaining growing attention; however, future research should also consider allelochemical release mechanisms, persistence, selectivity and modes of action, as well as consequences of improved crop allelopathy on plant physiology, the environment and management strategies. Creation of weed-suppressive cultivars with improved allelopathic interference is still a challenge, but traditional breeding or biotechnology should pave the way.

Abstract: Introduction Reality and Future of Allelopathy, G.R. Waller Chemistry Studies Ecophysiology and Potential Modes of Action for Selected Lichen Secondary Metabolites, J. Romagni, R.C. Rosell, N.P.D. Nanayakkara, and F.E. Dayan Bioactive Compounds from Potomagetonaceae on Aquatic Organisms, M. della Greca, A. Fiorentino, and M. Isidori Fate of Phenolic Allelochemicals in Soils: the Role of Soil and Rhizosphere Microorganisms, U. Blum Benzoxazolin-2(3H)-ones: Generation, Effects, and Detoxification in the Competition Among Plants, D. Sicker, H. Hao, and M. Schulz Heliannanes: A Structure-Activity Relationship (SAR) Study, F.A. Macias, J. C. G. Galindo, and J.M.G. Molinillo Chemistry of the Host-Parasite Interactions, J.C.G. Galindo, F.A. Macias, M.D. Diaz, and J. Jorrin Application of Analytical Techniques to the Determination of Allelopathic Agents in Wheat Root Exudates: A Practical Case Study, T. Haig The Importance of Alkaloidal Functions, M.S. Blum Allelochemical Properties of Quinolizidine Alkaloids, M. Wink Mode of Action Studies Mode of Action of Phytotoxic Terpenoids, S.O. Duke and A. Oliva Mode of Allelochemical Actions of Phenolic Compounds, F. Einhellig Mode of Action of Hydroxamic Acid BOA and Other Related Compounds, A.M. Sanchez-Moreiras, T. Coba de la Pena, A. Martinez, L. Gonzalez, F. Pellisier, and M.J. Reigosa Mode of Action of Phytotoxic Fungal Metabolites, H.G. Cutler, S.J. Cutler, and D. Matesic Proteomic Techniques for the Study of Allelopathic Stress Produced by Some Mexican Plants on Protein Patterns of Bean and Tomato, R. Cruz-Ortega, T. Romero-Romero, G. Ayala-Cordero, and A.L. Anaya Application of Microscopic Techniques of the Study of Seeds and Microalgae Under Olive Oil Wastewater Stress, G. Alliota, R. Ligrone, C. Ciniglia, A. Pollio, M. Stanzione, and G. Pinto Bioassays: Valuable Tools for the Study of Allelopathy, R. Hoagland and R.D. Williams

Abstract: Phragmites australis is considered the most invasive plant in marsh and wetland communities in the eastern United States. Although allelopathy has been considered as a possible displacing mechanism in P. australis, there has been minimal success in characterizing the responsible allelochemical. We tested the occurrence of root-derived allelopathy in the invasiveness of P. australis. To this end, root exudates of two P. australis genotypes, BB (native) and P38 (an exotic) were tested for phytotoxicity on different plant species. The treatment of the susceptible plants with P. australis root exudates resulted in acute rhizotoxicity. It is interesting to note that the root exudates of P38 were more effective in causing root death in susceptible plants compared to the native BB exudates. The active ingredient in the P. australis exudates was identified as 3,4,5-trihydroxybenzoic acid (gallic acid). We tested the phytotoxic efficacy of gallic acid on various plant systems, including the model plant Arabidopsis thaliana. Most tested plants succumbed to the gallic acid treatment with the exception of P. australis itself. Mechanistically, gallic acid treatment generated elevated levels of reactive oxygen species (ROS) in the treated plant roots. Furthermore, the triggered ROS mediated the disruption of the root architecture of the susceptible plants by damaging the microtubule assembly. The study also highlights the persistence of the exuded gallic acid in P. australis's rhizosphere and its inhibitory effects against A. thaliana in the soil. In addition, gallic acid demonstrated an inhibitory effect on Spartina alterniflora, one of the salt marsh species it successfully invades.

Abstract: The present study was conducted to investigate the effect of the residue of Chenopodium murale L. on growth, nodulation and macromolecule content of two legume crops, viz., Cicer arietinum L. (chickpea) and Pisum sativum L. (pea). A significant reduction in root and shoot length as well as dry matter accumulation occurred when both the legumes were grown in the soil amended with 5, 10, 20 and 40 g residue kg−1 soil. In general, a gradual decline in growth was associated with an increasing amount of residues in the soil. There was also a significant reduction in total chlorophyll content and the amounts of protein and carbohydrates (macromolecules) in plants growing in the residue-amended soil. The nodulation was completely absent in chickpea and pea when the plants were grown in the soil amended with 10 and 20 g residue kg−1 soil, respectively. At a lower rate of residue amendment (5 g kg−1 soil), a significant decline in nodule number and weight, and leghaemoglobin content was recorded. Root oxidizability, an indirect measure of tissue viability and cellular respiration, was adversely affected in both the legumes under various treatments of residue amendment. The observed growth reduction concomitant with increased proline accumulation indicated the presence of some inhibitory compounds in the residue-amended soil. It was rich in phenolics identified as protocatechuic, ferulic, p-coumaric and syringic acid with 12.8, 30.4, 20.2 and 33.6% relative content, respectively. The results suggest that the residue of C. murale releases phenolic allelochemicals, which deleteriously affect the growth, nodulation and macromolecule content of chickpea and pea.

Abstract: Methyl jasmonate (MeJA) and methyl salicylate (MeSA) are important signaling molecules that induce plant defense against insect herbivores and microbial pathogens. We tested the hypothesis that allelopathy is an inducible defense mechanism, and that the JA and SA signaling pathways may activate allelochemicals release. Exogenous application of MeJA and MeSA to rice (Oryza sativa L.) enhanced rice allelopathic potential and led to accumulation of phenolics, an increase in enzymatic activities, and gene transcription of phenylalanine ammonia-lyase (PAL) and cinnamate 4-hydroxylase (C4H), two key enzymes in the phenylpropanoid pathway. Aqueous extracts of the leaves of rice IAC165, a putative allelopathic variety, treated with MeSA (5 mM) or MeJA (0.05 mM), showed increased inhibitory effects (25 and 21%, respectively) on root growth of barnyardgrass (Echinochloa crus-galli L.), and increased inhibitory effects (18 and 23%, respectively) on shoot growth. Aqueous extracts from leaves of Huajingxian 1 rice, a putative nonallelopathic variety treated with MeJA and MeSA, caused 63 and 24% inhibition of root growth in barnyardgrass seedlings. The root exudates of both IAC165 and Huajingxian 1 plants treated with MeJA and MeSA for 48 hr also showed significant increases in their inhibitory effects on root growth of barnyardgrass seedlings. At the four-leaf stage, levels of 3,4-hydroxybenzoic acid, vanillic acid, coumaric acid, and ferulic acid that accumulated in the leaves were 5.3-, 31.3-, 2.2-, and 1.7-fold higher in response to MeJA exposure, and 3.3-, 13.1-, 2.0-, and 2.2-fold higher in response to MeSA. Treatments of MeSA and MeJA enhanced the PAL activity in the rice leaves up to 52.3 and 80.1%, respectively, whereas C4H activity was increased by 40.2 and 67%. Gene transcription of PAL and C4H in rice leaves significantly increased after the plants were subjected to treatment with MeJA and MeSA. These results suggest that allelopathy may be an active defense mechanism, and that plant signaling compounds are potentially valuable in its regulation.

Abstract: Two flavone O-glycosides were isolated from allelopathic rice seedlings and have been identified as 5,4'-dihydroxy-3',5'-dimethoxy-7-O-beta-glucopyranosylflavone and 7,4'-dihydroxy-3',5'-dimethoxy-5-O-beta-glucopyranosylflavone. Considerable levels of these glycosides could be found in allelopathic rice tissues. They could not be detected in the soils growing these allelopathic rice seedlings. Only their aglycone, 5,7,4'-trihydroxy-3',5'-dimethoxyflavone, could be found in the soil. Further experiments showed that two flavone O-glycosides were exuded from allelopathic rice roots to the rihzosphere and then transformed into their aglycone form, that is, 5,7,4'-trihydroxy-3',5'-dimethoxyflavone, with a great diversity of biological activities on associated weeds and microbes by soil interactions once released. The glycosides degraded rapidly (t1/2 < 2 h), whereas their aglycone was more resistant toward degradation in paddy soils, in which the half-life (t1/2) at low (25 mug/g) and high (200 mug/g) doses reached 19.86 +/- 3.64 h (r 2 = 0.97) and 28.78 +/- 3.72 h (r 2 = 0.98), respectively. Furthermore, the mobility of both glycosides and their aglycone in paddy soil was evaluated by soil TLC with bioassay. The mobility of the glycosides (Rf = 0.418 +/- 0.069, n = 18) is higher than that of the aglycone (Rf = 0.361 +/- 0.048, n = 18). The results suggested that two flavone O-glycosides are formed in rice biosynthesis and that storage of the allelochemicals and their aglycone 5,7,4'-trihydroxy-3',5'-dimethoxyflavone is the agent of alleloapthic rice which interferes with weeds or microbes in paddy soil.

Abstract: A study was conducted to determine the potential and nature of root-mediated allelopathic interference of Chenopodium murale on wheat. Early growth of wheat reduced significantly in agar medium where C. murale seedlings were previously growing as well as in rhizosphere soil of C. murale. The reduction in wheat growth was due to the presence of inhibitory metabolites released by roots of C. murale in the growth media. Even the soil incorporation of root residues also reduced the wheat growth in terms of seedling length and seedling dry weight. Only a partial amelioration in growth inhibition occurred upon charcoal supplementation or nitrogen fertilization in these amended soils. Root residues did not reduce the available nutrients in the soil, which was rather nutrient rich. These results indicated the definite role of allelopathy of C. murale roots in retarding wheat growth. Root amended soils contained significantly higher amount of phytotoxic phenolics as the putative allelochemicals, which were ferulic acid, vanillic acid, p-coumaric acid and benzoic acid. The study concluded that C. murale roots and their exudates exerted allelopathic effects on wheat by releasing water-soluble phenolic acids as putative allelochemicals in soil.

Abstract: An experiment was conducted to understand the growth inhibitory effects of aqueous extracts derived from Lantana camara L. (a globally recognized invasive alien weed) on six popular agricultural crops of Bangladesh. The test was conducted in sterilized petridishes with a photoperiod of 24 hours and an average temperature of 29°C. The effect of different concentrations of L. camara leaf extracts were recorded and compared with control (i.e., distil water). Result showed different concentrations of aqueous leaf extracts caused significant inhibitory effect on germination, root and shoot elongation and development of lateral roots of receptor crops. Bioassays also indicated that the inhibitory effect was proportional to the concentrations of the extracts and higher concentration had the stronger inhibitory effect whereas the lower concentration showed stimulatory effect in some cases. The inhibitory effect was much pronounced in root and lateral root development rather than shoot and germination.

Abstract: Allelopathic studies investigate the positive and negative effects of secondary metabolites of plants, microorganisms and fungi on the development of neighboring individuals. In this work, the allelopathic effects of aqueous extracts of leaves, bark and root bark of Brazilian sassafras (Ocotea odorifera (Vell.) Rowher) on seed germination, root and shoot growth, chlorophyll content and respiratory activity of the root cells of sorghum seedlings (Sorghum bicolor (L) Moench cv. Embrapa BR 303) were analyzed. The extracts were prepared by adding dried, powdered plant material to distilled non-ionized water at 1:10 (w/v). After continuous agitation for 24 h, these solutions were decanted and vacuum-filtered through filter paper, thus constituting the tested extracts. Bark and root bark aqueous extracts caused root growth inhibition in sorghum seedlings while shoot growth was stimulated by the root bark extract. Leaf and bark extracts induced an increase in wet root biomass and a decrease in chlorophyll content. All extracts caused negative effects on sorghum seedling root cell respiration. We concluded that secondary metabolites from Brazilian sassafras caused metabolic and morphological alterations in the sorghum seedlings and the presence of these plants in the environment can cause similar effects on other neighboring plant species.

Abstract: Plants can interact with other plants through the release of chemical compounds or allelochemicals. These compounds released by donor plants influence germination, growth, development, and establishment of receptor plants; having an important role on the pattern of vegetation, i.e as invasive strategy, and on crop productivity. This phytotoxic or negative effect of the released allelochemicals (allelochemical stress) is caused by modifying or altering diverse metabolic processes, having many molecular targets in the receptor plants. Recently, using an aggressive and allelopathic plant Sicyos deppei as the donor plant, and Lycopersicon esculentum as the receptor plant, we showed that the allelochemicals released by S. deppei caused oxidative damage through an increase in reactive oxygen species (ROS) and activation or modification of antioxidant enzymes. Based on this study, we proposed that oxidative stress is one of the mechanisms, among others, by which an allelopathic plant causes phytotoxicity to other plants.

Abstract: Polyphagous herbivores encounter allelochemicals as complex mixtures in their host plants, and the toxicity of an individual compound may be influenced by the chemical matrix in which it is encountered. Certain plant constituents may reduce toxicity of cooccurring compounds by inducing detoxification systems, including cytochrome P450s, which can metabolize a broad range of substances. The polyphagous corn earworm Helicoverpa zea encounters a diversity of plant allelochemicals in its many host plants and, as well, can encounter aflatoxins, mycotoxins produced by Aspergillus flavus and Aspergillus parasiticus that infect damaged grains. Dietary supplementation of each of three plant allelochemicals that are frequently (coumarin, COU), occasionally (indole-3-carbinol, 13C), or rarely (xanthotoxin, XAN) encountered by H. zea larvae substantially reduced the toxicity of aflatoxin B1 (AFB1) to H. zea. Compared to fourth instars on diets containing 1 microg/g AFB1 that failed to develop and pupate, fourth instars on diets containing I3C and XAN increased in mass by 216.1 and 700% after 6 days, and pupated at rates of 40 and 88%, respectively. Diets containing COU or XAN also significantly reduced the mortality rates of caterpillars exposed to the insecticides, diazinon and carbaryl. Diets containing COU and XAN increased CYP6B8 transcripts 2.6-fold; CYP321A1 transcripts increased 20.7, 8.3, and 10.6-fold in response to COU, I3C, and XAN, respectively. These results indicate that consumption of plant allelochemicals can ameliorate toxicity of natural and synthetic toxins encountered by insects, and they suggest that P450s induced by these allelochemicals contribute to detoxification of these chemicals in H. zea.

Abstract: Current views of plant communities emphasize the importance of competition for resources and colonization ability in determining seedling establishment and plant distributions. Many desert shrubs are surrounded by bare zones that lack other plants or have different suites of species beneath them compared with the open desert surrounding them. Releases of biochemicals as volatiles from leaves, leachates from litter, or exudates from roots have been proposed as mechanisms for this pattern, but such phytotoxicity has been controversial. I tested the hypothesis that experimental clipping of sagebrush foliage enhances its effect as a germination inhibitor. Germination of native forbs and grasses was reduced in association with clipped, compared with unclipped, sagebrush foliage in lath house and field experiments. Sagebrush seeds were not significantly affected. Air contact was required for this inhibition of germination. Soil contact and leaf litter were not required and added little inhibition of germination. These results suggest a potentially large, indirect, and previously overlooked role for interactions between herbivory and germination that could affect plant community structure.

Abstract: Section 1: New Methodology and Approach (Dose Response, Bioassay): Dose/Response Relationships in Allelopathy Research Can Data Derived from Field and Laboratory Bioassays Establish the Existence of Allelopathic Interactions in Nature? Plant-box Method: A Specific Bioassay to Evaluate Allelopathy through Root Exudates Section 2: New Allelochemicals (Pharmaceuticals, Degradation, Promotion, Ion Dissolution): Isolation, Structural Elucidation and Synthesis of Biologically Active Allelochemicals for Potential Use as Pharmaceuticals Recent Chemical Aspects of Wheat Allelopathy Ecological Relevance of the Degradation Processes of Allelochemicals Iron Dissolution Reaction of Mugineic Acids for Iron Acquisition of Graminaceous Plants Chemical and Biological Analysis of Novel Allelopathic Substances, Lepidimoide and Lepidimoic Acid Section 3: Allelopathy in Potential Invasive Weeds: Allelopathic Activity of White Rocket [Diplotaxis erucoides (L.) DC.] Weed-crop Interferences in Hungary Section 4: Allelopathic Cover Crops to Suppress Weeds: Allelopathic Activity of Buckwheat: A Ground Cover Crop for Weed Control Sunflower-desired Allelopathic Crop for Sustainable and Organic Agriculture? The Potential for Allelopathy During Decomposition of Hairy Vetch Residue Allelopathic Effect of Astragalus Adsurgens Pall Root Culture Evaluation of Weed Suppressive Effect of Allelochemicals of Ornamental Marigold Species Section 5: Rice Allelopathy: Rice Allelopathy Allelochemicals Involved in Rice Allelopathy Section 6: New Approach in Tree Allelopathy: Variation in Allelopathic Influence Among Wide Range of Tree Species Monitoring Allelopathic Expression and Functional Performance of Tamarind (Tamarindus indica L.): A Case Study Influence of Water Extract from Uncaria Tomentosa Bark on Ultrastructure of Capsicum Section 7: New Field in Allelopathy (Aquatic Plants, Mushrooms, Insects, Animals): Production of Allelochemicals by an Aquatic Plant, Myriophyllum spicatum L. Fruiting Bodies of Mushrooms as Allelopathic Plants Allelopathic Action of Triticale Allelochemicals Towards Grain Aphid Rat Sexual Behavior and Volatile Substance from Plants

Abstract: Elemental allelopathy suggests that nickel (Ni)-rich leaves shed by hyperaccumulators inhibit the germination and growth of nearby plant species. Here, the germination of eight herbaceous species following addition of Alyssum murale biomass or Ni(NO3)2, with the same Ni level added to soil, was assessed. The distribution of Ni in soil was tested by determining Ni phytoavailability and speciation over time. Phytoavailable Ni in soil amended with biomass declined rapidly over time due to Ni binding to iron (Fe)/manganese (Mn) oxides in the soil. No significant effects on seed germination were observed. Unlike the Ni complex in Alyssum biomass, more Ni remained soluble and phytoavailable in soil amended with Ni(NO3)2, thus significantly inhibiting seed germination. High-Ni leaves shed by hyperaccumulators did not appear to create a 'toxic zone' around the plants and inhibit germination or growth of competing plants. The lack of an allelopathic effect was probably related to low Ni availability.

Abstract: The indiscriminate use of synthetic chemical compounds for weed control has been often responsible of damage to both the environment and the human health. To challenge these problems, in the last years research has increased its effort to find out alternative farming strategies. A feasible alternative could be the identification of natural substances with allelopathic effects for the realization of natural herbicides. Some research has already highlighted the possibility of using essential oils, extracted from aromatic plants, for weed control. The advantage in the utilization of such natural compounds is the quickly breaking down process into the environment and so the possible application in sustainable agriculture like organic farming. Objective of this research was the evaluation of the inhibition effect exerted by the essential oils of cinnamon, peppermint and lavender on seeds germination of some of the most common weeds species of the Mediterranean environment (pigweed, wild mustard and ryegrass). The results have highlighted a control in the weeds germination. Among the essential oils tested, cinnamon oil has exerted the highest inhibition effect compared with lavender and peppermint ones. The dicotyledonous species have been more susceptible compared with the monocotyledonous, even if it has been recorded only for redroot pigweed a dose able to inhibit totally the seed germination.

Abstract: Mycorrhiza is a symbiotic association between the plant roots and certain soil fungi: Arbuscular Mycorrhizae (AM) and Ectomycorrhizae (ECM) are two most common types of these fungi. AM are common in agricultural crops and weeds, while ECM are associated with trees. In mycorrhizal relationships, there is bidirectional movement of nutrients. In allelopathy, the living or dead plants release allelochemicals, which mostly exert negative effects on the neighbour plants and. adversely affect the mycorrhizal colonization. The plants of Brassicaceae and Chenopodiaceae are non-mycorrhizal, because of exudation of specific allelochemicals from their roots. These non-host species exhibit negative effect on mycorrhizal colonization of plants growing in their vicinity and also on plant species cultivated after their harvest. AM inoculations enhance the crop growth by alleviating the allelopathic stress possibly through increased nutrient uptake. The ECM often experience the chemical stress from forest floor allelochemicals. ECM fungi are able to alleviate the toxic effects of allelochemicals in host plants through detoxification of allelochemicals and enhanced nutrient uptake. The allelopathic stress alleviating potential of mycorrhizal fungi may be best exploited by selecting well-adopted mycorrhizal species in different habitats.

Abstract: TOPAL S., KOCACALISKAN I., ARSLAN 0. & TEL A. Z. 2007. Herbicidal effects of juglone as an allelochemical. - Phyton (Horn, Austria) 46 (2): 259-269, 1 figure. -English with German summary. Juglone is a natural constituent of walnut with phytotoxic and allelopathic properties. Its herbicidal effects on the weed species wild mustard (Sinapis arvensis L.), creeping thistle (Cirsium arvense L.), field poppy (Papaver rhoeas L.) and henbit (Lamium amplexicaule L.) were investigated using wheat (Triticum vulgare Vill. cv. Gerek 79) and barley (Hordeum vulgare L. cv. Kishk) varieties as control plants. A juglone concentration of 5.74 mM had a lethal effect on field poppy. Juglone concentrations of 1.15 and 2.30 mM decreased the elongation and fresh weight of the weeds, but not wheat and barley. The elongation of the shoot was more significantly decreased than that of the root in all weeds. Chlorophyll content of the weeds was also decreased by juglone. The study reveals that juglone is a potent inhibitor of growth of the weeds and therefore it can be evaluated as a herbicide for future weed management strategies.

Abstract: The allelopathic potential of the foliage tissues of eight barley cultivars was investigated using seedlings bioassays of Indian brown mustard (Brassica juncea) and green foxtail (Setaria viridis) as typical broadleaf and grass weeds of the croplands. The correlations of allelochemical compounds with seed germination of weeds were determined. The barley cultivars used were Jackson, Bronco, CDC Dolly, B1602, Harrington, UNA80, Stander, and TR251. Three dilutions of the extracts of each barley cultivar (20, 10 and 5 g/L) plus deionized distilled water, as a control, were assayed on the target weeds. The effects of barley cultivars and extract concentrations on seed germination and radicle and shoot lengths of the target weed seeds were highly significant (P<0.001). By increasing the concentration of the allelopathic extracts, the percent germination, radicle and shoot lengths of both target plants seedlings decreased. Seed germination and seedling growth of B. juncea were more sensitive than those of S. viridis. Relative to the germination inhibition of target weed seeds in comparison to the untreated control, the selected barley cultivars were categorized as being highly allelopathic, moderately allelopathic and of low allelopathic potential. Among the phenolic compounds detected, the cumulative effect of four small-quantity-compounds (vanillic, chlorogenic, p-coumaric, and ferulic acids) on average weed germination was very high (R 2 = 0.83), with B. juncea it was high (R 2 = 0.70) and with S. viridis it was very low (R 2 = 0.15). The two high concentration phenolic compounds of protocatechuic and p-hydroxybenzoic acid, when accumulated with the small-quantity phenolic compounds of barley cultivars, reduced the weed seeds germination correlation to r=- 0.11.