Allelopathy is usually regarded as a terrestrial science and little attention has been paid to the allelochemicals involved in the interactions between aquatic autotrophs. Also, chemicals involved in the interaction between algae (macro or microscopic) and their environment are not often acknowledged as allelopathic. Allelopathy is a key factor for explaining microalgae assemblages, community structure and the dynamics of the populations within the ecosystem. Herein we present an overview of what is known in this field, describing the chemicals yet identified and which kind of interactions are they involved in.

Allelopathy is a biological process including interactions between two plants through the production of chemical compounds (allelochemicals) that are released by leaching, volatilization, decomposition, or root exudation. Hence, allelopathy together with competition is a promising environment-friendly tool especially for weed management. However, detailed knowledge of this phenomenon is necessary for its successful application due to very still limited available knowledge. Suitable use of allelopathic crops in agriculture could reduce the pesticide application and thereby reduce the environmental and food pollution, decrease costs in agriculture, improve food security in poor regions and soil productivity, and increase biodiversity and sustainability in the agro-ecosystem. Weed management in organic agriculture is one of the most difficult aspects of organic farming and uses especially preventive methods that include ways such as cover crops, mulches, green manure, and intercropping in which allelopathy could play an important role. Therefore, this review focuses on the possibilities of the allelopathy application especially in organic agriculture. Roots of allelopathic plants as cover crops, intercrops, green manure, or so-called smothering crops or decomposing residues release compounds in the soil that are toxic to weeds. The weed-suppressive effect is influenced by species, planting date, seeding rate and method, weather, and other factors. Decomposition time of plant residues and amounts of biomass are important factors of weed control by mulching. Annual, biennial, or perennial herbaceous plants in a pure or mixed stand can be grown for these purposes. Biofumigation is the name for one type of allelopathy that includes the effects of the chemicals, i.e., highly toxic isothiocyanates, produced by Brassica green manure. The balance in the crop rotation is necessary due to possible autotoxicity. These days, allelopathic plants as catch crops or trap crops found utilization in plant protection of tropical regions against parasite weeds, because they can reduce the parasite seed bank by 72%. Other applications of allelopathy for weed control include the use of plant residues as an herbicide agent, e.g., water extracts, pellets, flours, by-products of crop processing, etc. Sorgaab, an extract of sorghum, is produced commercially as a natural herbicide. Allelopathic compounds act as repellents for herbivorous pests, so the same strategy used in weed control could be effective against pests and pathogens, e.g., push–pull strategy. All possible applications of allelopathy need to combine with other methods of plant protection. Newly investigated pollen allelopathy could reduce reproductive ability of wind pollination annual weeds. Pollen of allelopathic species would be artificially dusted on the stigmatic surface of other plants. This phenomenon is yet to be studied and field tested. The new crop varieties with elevated allelopathic activity could be a great chance not only for organic farming. Hybridization could be the promising method. However, allelopathic activity was identified as a quantitative trait and therefore this characteristic is affected by both genetic effects and environmental conditions.

Summary

A possible method of manipulating allelopathy would be to develop crop varieties showing an increased tolerance to allelopathic chemicals. We therefore examined four flax (Linum usitatissimum) varieties and two wild Linum species in the presence of p-coumaric acid and four barley (Hordeum vulgare) varieties in the presence of p-coumaric acid, scopoletin and wild oat (Avena fatua) extract. Analysis of variance indicates significant interaction between variety and treatment for shoot and root growth for seedling flax, shoot growth for older flax, and root growth for seedling barley. These differences in tolerance between varieties could be exploited to develop-varieties with greater tolerances to the allelochemicals produced by weeds or in crop residues and therefore potentially more tolerant of the presence of weeds.

Allelopathy plays an major role in agricultural management such as weed control, crop protection, and crop re-establishment. Compositae plants have potent allelopathic activity, and the activity is confirmed through (a) bioassays with aqueous or various solvent extracts and residues, (b) fractionation, identification, and quantification of causative allelochemicals, and (c) mechanism studies on the allelochemicals. Most assessments of allelopathy involve bioassays of plant or soil extracts, leachates, fractions, and residues based on seed germination and seedling growth in laboratory and greenhouse experiments. Plant growth may be stimulated below the allelopathic threshold, but severe growth reductions may be observed above the threshold concentration depending upon the sensitivity of the receiving species. Generally germination is less sensitive than is seedling growth, especially root growth. Some approaches showed that field soil collected under donor plants significantly reduced or somewhat promoted growth of the test plants. Petri-dish bioassays with methanol extracts or fractions and causative phenolic allelochemicals showed significant phytotoxic activities in concentration- dependent manner. Delayed seed germination and slow root growth due to the extracts could be confounded with osmotic effects on rate of imbibition, delayed initiation of germination, and especially cell elongation; the main factor that affects root growth before and after the tip penetrates the seed coat. Light and electron microscopic approaches extract evaluation at the ultrastructural level have been precisely investigated. Many Compositae plants have allelopathic potentials, and the activities and types and amount of causative compounds differ depending on the plant species. The incorporation of allelopathic substances into agricultural management may reduce the use of pesticides and lessen environmental deterioration.

Correlation between the distributional frequency of undergrowth species of red pine forest and their germination and growth effected by pine extracts and leachates was found. It was made clear by germination and growth tests that pine toxic substances inhibit the germination and growth of low frequency species more than high frequency species in a red pine forest and that these substances are contained in descending concentration in fresh and fallen leaves, roots, pine forest soil, and pine rain. The concentration of pine toxic substances in extracts or leachates was affected by extracting or leaching within a given period of time, requiring a few hours for extracts or a few days for leachates. The amount of dry weight inhibition of the undergrowth treated by pine leachates was expressed as a growth inhibition index (GII) for the comparison of tolerance in various species. GII is a relative value (%) of the test groups against the control and it is an exponential function of the amount of pine toxic substances affecting the dry weight of the undergrowth. The substances were analyzed by paper and gas chromatography. Benzoic acid and 11 phenolic acids were identifed by gas chromatography. Benzoic acid was considered to be a key factor of allelopathy in the red pine forest.

The toxic effects of the allelopathic nonsubstituted monoterpenes β-pinene and limonene on yeast,Saccharomyces cerevisiae, were proportional to the size of the monoterpene droplets in suspension. Both the toxic effects and the size of the droplets in suspension were decreased by adding different solvents with the monoterpene as follows: dimethylsulfoxide – dimethylformamide ≫ ethanol > dioxane. Oxygen consumption was inhibited about 80% by 1 mM β-pinene added in dimethylsulfoxide but less than 10% when β-pinene was added in dioxane. Parallel decreases in droplet size and toxic effects of either monoterpene were also induced by hydrating the monoterpene-dimethylformamide or monoterpene-dimethylsulfoxide before addition to yeast. Molecular aggregation may be a mechanism to potentiate the allelopathic properties of monoterpenes when these associate with diverse soil components.

We used the substrate-induced growth-response (SIGR) method to quantify salicylate-mineralizing microbes and total microbial biomass in soils from under willows (Salix brachycarpa) and in surrounding meadows dominated by the sedge Kobresia myosuroides. Willows had a strong effect on the biomass of salicylate-mineralizing microbes in both years of this study. There were always higher biomass levels of salicylate mineralizers in soils from under Salix (4.6–10.1 μg C/g) than under Kobresia (0.23–0.76 μg/g). In contrast, total microbial biomass was not significantly different under these plant species in 1996 and was only higher under Salix on one date in 1997. These results show that the standing biomass and activity of salicylate-mineralizing microbes can be greatly enhanced by salicylate-producing plants in the field. Given this finding, it is unlikely that simple phenolic compounds like salicylate would persist for very long in soil beneath the plants that produce them.

Barnyard grass is a problematic weed worldwide. It competes with crops and causes reduction in crop yields. In this study, barnyard grass suppressed rice emergence, and the degree of rice inhibition was proportional to the density of barnyard grass. Root exudates of barnyard grass reduced germination and growth of lettuce, rice, and monochoria. Fifteen compounds potentially involved in the phytotoxic activities of barnyard grass were isolated and identified, including phenolics, long-chain fatty acids, lactones, diethyl phthalate, acenaphthene, and derivatives of phthalic acids, benzoic acid, and decane. Quantities of diethyl phthalate, decanoic acid, myristic acid, stearic acid, 7,8-dihydro-5,6-dehydrokavain, and 7,8-dihydrokavain were 2.7, 11.1, 19.6, 35.5, 10.3, and 15.5 μg/ml of barnyard grass root exudates, respectively. The two lactones exhibited the greatest inhibition, followed by the phenolics and the derivatives of phthalic acids. Fatty acids had stronger suppression than diethyl phthalate and ethyl ester-4-ethoxy-benzoic acid. The acenaphthene and decane derivatives were the least phytotoxic. The phytotoxins released by barnyard grass roots showed strong inhibition on growth of broadleaf indicator plants and paddy weeds, but were less effective on barnyard grass itself and rice. Our study revealed that in addition to competition, barnyard grass also interferes with rice and other plants in its surroundings by chemical means.

Aqueous methanol extracts of Bangladesh rice (Oryza sativa L. cv. BR17) inhibited the growth of roots and shoots of cress (Lepidium sativum), lettuce (Lactuca sativa), alfalfa (Medicago sativa), timothy (Phleum pratense), Digitaria sanguinalis, Echinochloa crus-galli and Echinochloa colonum. Increasing the extract concentration increased the inhibition, suggesting that the BR17 may have growth inhibitory substances and possess allelopathic potential. The aqueous methanol extract of the BR17 was purified and a main inhibitory substance was isolated and determined by spectral data as 2,9-dihydroxy-4-megastigmen-3-one. This substance inhibited root and shoot growth of cress and E. crus-galli seedlings at concentrations greater than 0.03 and 3 μM, respectively. The concentrations required for 50% growth inhibition on cress roots and shoots were 0.22 and 0.47 μM, respectively, and on E. crus-galli roots and shoots were 36 and 133 μM, respectively. These results suggest that 2,9-dihydroxy-4-megastigmen-3-one may contribute to the growth inhibitory effect of BR17 and may play an important role in the allelopathy of BR17. Thus, Bangladesh rice BR17 may be potentially useful for weed management in a field setting.

Low-elevation ponderosa pine (Pinus ponderosa Doug. ex. laws) forests of western Montana are naturally fire maintained ecosystems. However, 80–140 years of fire-exclusion has led to the formation of dense, mixed stands of ponderosa pine and Douglas-fir (Pseudotsuga menzesii (Mirbel) Franco), an understory co-dominated by graminoids and ericaceous shrubs, and low N availability. Ericaceous shrubs in particular have been found to influence soil processes in boreal ecosystems and potentially exacerbate N limiting conditions. In this set of studies, we investigated the influence of graminoid and ericoid litter chemistry on soil processes and evaluated the influence of charcoal as a sorbant of C compounds and depositional product of fire. A series of experiments were performed with two common understory plants of this ecosystem, elk sedge (Carex geyeri Boott) and kinnikinnick (Arctostaphylos uva-ursi (L.) Spreng.), an ericaceous shrub. Charcoal (100 g m⁻²) and glycine (5 g m⁻²) were applied in factorial combination to intact litter microcosms of these species. Non-ionic resin capsules were used to monitor mobile C compounds and ionic resins were used to monitor net N mineralization and nitrification in-situ. Greenhouse studies revealed that the addition of glycine and charcoal leads to a significant increase in net nitrification in shrub litter microcosms, but not sedge litter microcosms, as measured by NO ₃⁻ sorption to ionic resin capsules. Charcoal and glycine also resulted in a significant increase of anthrone reactive C (soluble hexose sugars, an index of bioavailable C) in shrub litter microcosms. Analysis of leaf litter leachate from these two plant communities indicated similar nutrient concentrations, but almost 20 times more phenolic compounds in shrub leaf leachates. Charcoal was shown to be extremely effective at sorbing phenols, removing over 80% of phenolic compounds from solution. These results suggest that charcoal deposition after fire may modify a nitrification interference mechanism by sorbing plant secondary metabolites. After time, charcoal loses its ability to sorb C compounds and ericaceous litter decomposition, and subsequent release of phenolics, may interfere with nitrification once again.

In laboratory tests the allelopathic potential ofErica vagans, Calluna vulgaris, andDaboecia cantabrica was determined. Aqueous extracts of flowers ofD. cantabrica and leaves ofC. vulgaris inhibit root and hypocotyl growth of red clover, the former causing 51% inhibition of germination. Intact aerial parts of the Ericaceae here studied drastically reduced the growth of red clover and 100% inhibition of germination was caused by flowers ofD. cantabrica. Inhibition of aqueous extracts remains after Chromatographic separation, and two well-defined inhibition zones may be observed. Hydrosoluble organic compounds (phenol-like compounds) could probably be responsible for the inhibitions detected.

As candidate(s) for allelopathic substance(s), two plant growthinhibitors were isolated from aqueous leachate of leaves of mesquite, whichshowa strong allelopathy, and which were identified as syringin and(–)-lariciresinol by their spectral analyses. Syringin inhibited root andshoot growth of lettuce seedlings at concentrations greater than 0.8μM, and root and shoot growth of barnyard grass seedlings atconcentrations greater than 2.7 and 26.9 μM, respectively. Onthe other hand, (–)-lariciresinol inhibited root and shoot growth oflettuce seedlings at concentrations greater than 2.8 and 0.8μM,and root and shoot growth of barnyard grass seedling at concentrations greaterthan 0.8 and 2.8 μM, respectively. The contents of syringin and(–)-lariciresinol in the rhizosphere soil of mesquite were 0.34 and 0.38μg/g soil, respectively. These results indicate that syringinand (–)-lariciresinol are allelopathic substance(s), and may play rolesinthe allelopathy of mesquite.

Solid-phase microextraction (SPME) was used to measure allelochemical uptake by tomato plants in vivo. Exogenously applied 1,8-cineole was rapidly taken up by tomato, with the first traces of cineole being detected in the tomato stem just 1–2 h after a single application of 0.5 mM cineole to soil. The pulse of cineole persisted in the tomato stem for 72 h. When cineole concentrations were measured 24 h after a single application, trace amounts of cineole could be detected in tomato stem fluid at application concentrations as low as 50 μM. Tomato was also found to take up camphor, menthol, and coumarin, but not carveol. In preliminary tests with common ragweed (Artemisia annuifolia L.) and purslane (Portulaca oleraceae L.) plants growing in garden beds, both ragweed and purslane took up 1,8-cineole, and purslane also took up camphor. The quantitation of allelochemical uptake by plants is considered to be a crucial test of hypotheses of allelopathic effects, but demonstration of allelochemical uptake has had to be inferred based on observed toxicity due to the lack of methods to measure uptake in vivo. This new technique now provides a means of tracking compounds within target plants. Furthermore, the demonstrated rapid uptake of 1,8-cineole by plants suggests a potential mechanism whereby brief pulses of allelochemicals over an extended period of time might be able to exert an allelopathic effect on plants.

On many hillsides of Taiwan there is a unique pattern of weed exclusion byPhyllostachys edulis (bamboo) andCryptomeria japonica (conifer) in which the density, diversity, and dominance of understory species are very different. Although the physical conditions of light, soil moisture, and soil nutrients strongly favor the growth of understory in a bamboo community, the biomass of its undergrowth is significantly low, indicating that physical competition among the understory species in the bamboo and conifer communities does not cause the observed differences. However, the biochemical inhibition revealed by these two plants appeared to be an important factor. The growth ofPellionia scabra seedlings, transplanted from the study site into greenhouse pots, was evidently suppressed by the aqueous leachate of bamboo leaves but was stimulated by that of conifer leaves. The radicle growth of lettuce, rye grass, and rice plants was also clearly inhibited by the leachate and aqueous extracts of bamboo leaves but not by those of conifer leaves. Six phytotoxins,o-hydroxyphenylacetic,p-hydroxybenzoic,p-coumaric, vanillic, ferulic, and syringic acids were found in the aqueous leachate and extracts of leaves and alcoholic soil extracts ofP. edulis, while the first three compounds were absent in the extracts ofC. japonica. The phytotoxicities of extracts were correlated with the phytotoxins present in both leaves and soils. The understory species might be variously tolerant to the allelopathic compounds produced by the two plants, resulting in a differential selection of species underneath. Therefore, comparative allelopathic effects ofPhyllostachys edulis andCryptomeria japonica may play significant roles in regulating the populations of the understories.

Changes in soluble and cell wall bound peroxidase (POD, EC 1.11.1.7) activity, phenylalanine ammonia-lyase (PAL, EC 4.3.1.5) activity, and lignin content in roots of ferulic acid-stressed soybean (Glycine max (L.) Merr.) seedlings and their relationships with root growth were investigated. Three-day-old soybean seedlings were cultivated in half-strength Hoagland nutrient solution containing 1.0 mM ferulic acid for 24–72 hr. Length, fresh weight, and dry weight of roots decreased, while soluble and cell wall bound POD activity, PAL activity, and lignin content increased after ferulic acid treatment. These enzymes probably participate in root growth reduction in association with cell wall stiffening related to the formation of cross-linking among cell wall polymers and lignin production.

Allelopathy (the chemical based communication among plants) is gaining more and more attention thanks to its likely outcomes in several fields, but it is, in the meantime, a questionable topic, due to a difficult determination. Despite the broad literature, allelopathic interactions are far from being clarified. Conclusive evidence in this field is not a simple find, as this biological activity is governed by a number of different factors. Such an issue can be solved only provided that suitable methods are used. Understanding allelopathy in natural ecosystems may be crucial in order to think about the exploitation of this resource. Mediterranean plants, in particular, have been proven to be a source of natural active compounds. This paper reviews the most outstanding methods used for plant extract or pure metabolite phytotoxic or allelopathic properties determination. Furthermore, it points out the bioactive metabolites reported from plants characteristics of Mediterranean ecosystems.

The aqueous extract of the root ofSolidago altissima was lethal to nematodes. The major compound isolated from the toxic fraction wascis-dehydromatricaria ester (methyl 2-decene-4,6,8-triynoate), which is already known as an allelopathic compound of a composite showing growth-inhibition to a rice seedling. This compound showed high toxicity to plant-parasite nematodes. A composite exudes one defending substance against both competitive plants and plant-parasite nematodes.

This study was conducted to evaluate the inhibitory potential of P. harmala leaf, stem and root extract on germination and growth of Avena fatua L. and Convolvulus arvensis L., as well as identification of the phytotoxic substances responsible for this activity. According to our results, the degree of toxicity of different P. harmala plant parts can be arranged in the following order: leaves > stems > roots. The two test species differed in their sensitivity to P. harmala extracts. Inhibitory effect on shoot length and seedling dry weight was more pronounced in C. arvensis, whereas higher reduction in germination, root length and total chlorophyll content occurred in A. fatua. A significant amount of water-soluble phenolic acids were found in P. harmala plant extracts. Total phenolic acids content was higher in leaf extracts when compared to that of stem or root extracts. Seven phenolic acids including gallic acid, vanillic acid, 4-hydroxybenzoic acid, 3,4-dihydroxybenzoic acid, caffeic acid, syringic acid and ferulic acid were found in P. harmala leaf extracts. On the other hand, we identified four phenolic acids from stem (galllic acid, vanillic acid, 3,4-dihydroxybenzoic acid and caffeic acid) and root (galllic acid, 4-hydroxybenzoic acid, syringic acid and cinnamic acid) extracts. The greater number of growth inhibitors detected in the leaves might explain the stronger inhibitory activity. Overall, our results suggest that P. harmala might be used as a natural herbicide for weed control and consequently reduce dependence on synthetic herbicides.

This study evaluates the relative contributions of leaves and roots to the belowground allelopathic effects of Mikania micrantha. The hypothesis that leaves contribute more to the allelopathic effect than roots was experimentally tested. We assessed the allelopathic effects of aqueous extracts from Mikania leaves and roots on the seed germination and seedling growth of two co-occurring woody plants in southern China, Lagerstroemia indica L. and Robinia pseudoacacia L. The results showed that the aqueous extracts from Mikania leaves and roots had inhibitory effects on the woody species. Allelopathic activity depended on the concentration of the extracts, target species, and the extract sources (i.e., leaves vs. roots of Mikania). Leaf extract showed stronger allelopathic effects than root extract on germination percentage, initial germination time, speed of germination, and shoot height; while root extract had greater allelopathic effects on roots than leaf extract. The latter phenomenon might greatly promote the invasion success of Mikania due to more direct and effective allelopathy of root. Our results suggest that allelopathy of root extract on belowground biomass might be greater than that of leaf extract for some species in contrast allelopathy of leaf extract on belowground biomass might also be greater than that of root extract for other species, at least for their effects on root growth of the target species.

3-(3′,4′-Dihydroxyphenyl)-l-alanine (l-DOPA), which is synthesized in velvet bean (Mucuna pruriens), inhibits plant growth. The concentration of l-DOPA in soil is reduced by adsorption and transformation reactions, which can result in the reduction of its plant-growth-inhibitory activity. To determine which part of the l-DOPA structure is involved in the adsorption and soil transformation reactions, we compared the kinetics of l-DOPA disappearance in a volcanic ash soil with that of l-phenylalanine (3-phenyl-l-alanine) and l-tyrosine (3-(4′-hydroxyphenyl)-l-alanine), compounds that are similar in structure to l-DOPA but do not have a catechol (o-dihydroxybenzene) moiety. l-Phenylalanine and l-tyrosine were not adsorbed and transformed in the soil at equilibrium pH values between 4 and 7. These results suggest that the adsorption and transformation reactions of l-DOPA in the soil involve the catechol moiety and not the amino and carboxylic acid groups, which are common to all three compounds. Like l-DOPA, (+)-catechin, another allelochemical that contains a catechol moiety, underwent adsorption and soil transformation reactions. Thus, we concluded that the concentrations of allelochemicals bearing a catechol moiety in soils will decrease rapidly owing to adsorption and transformation reactions, and this decrease will be faster in soils with a high pH value or high adsorption ability. Owing to this decrease in concentration, allelopathic phenomena may not occur.