Showing papers on "Allelopathy published in 2012"

Microbes as Targets and Mediators of Allelopathy in Plants

Abstract: Studies of allelopathy in terrestrial systems have experienced tremendous growth as interest has risen in describing biochemical mechanisms responsible for structuring plant communities, determining agricultural and forest productivity, and explaining invasive behaviors in introduced organisms. While early criticisms of allelopathy involved issues with allelochemical production, stability, and degradation in soils, an understanding of the chemical ecology of soils and its microbial inhabitants has been increasingly incorporated in studies of allelopathy, and recognized as an essential predictor of the outcome of allelopathic interactions between plants. Microbes can mediate interactions in a number of ways with both positive and negative outcomes for surrounding plants and plant communities. In this review, we examine cases where soil microbes are the target of allelopathic plants leading to indirect effects on competing plants, provide examples where microbes play either a protective effect on plants against allelopathic competitors or enhance allelopathic effects, and we provide examples where soil microbial communities have changed through time in response to allelopathic plants with known or potential effects on plant communities. We focus primarily on interactions involving wild plants in natural systems, using case studies of some of the world’s most notorious invasive plants, but we also provide selected examples from agriculturally managed systems. Allelopathic interactions between plants cannot be fully understood without considering microbial participants, and we conclude with suggestions for future research.

Mechanisms for cellular transport and release of allelochemicals from plant roots into the rhizosphere

Abstract: Allelochemicals and other metabolites released by plant roots play important roles in rhizosphere signalling, plant defence and responses to abiotic stresses. Plants use a variety of sequestration and transport mechanisms to move and export bioactive products safely into the rhizosphere. The use of mutants and molecular tools to study gene expression has revealed new information regarding the diverse group of transport proteins and conjugation processes employed by higher plants. Transport systems used for moving secondary products into and out of root cells are similar to those used elsewhere in the plant but are closely linked to soil environmental conditions and local root health. Root cells can rapidly generate and release large quantities of allelochemicals in response to stress or local rhizosphere conditions, so the production and transport of these compounds in cells are often closely linked. Plants need to manage the potentially toxic allelochemicals and metabolites they produce by sequestering them to the vacuole or other membrane-bound vesicles. These compartments provide secure storage areas and systems for safely moving bioactive chemicals throughout the cytosol. Release into the apoplast occurs either by exocytosis or through membrane-bound transport proteins. This review discusses the possible transport mechanisms involved in releasing specific root-produced allelochemicals by combining microscopic observations of the specialized root cells with the physical and chemical properties of the exudates.

Genetic evidence for natural product-mediated plant–plant allelopathy in rice (Oryza sativa)

Abstract: • There is controversy as to whether specific natural products play a role in directly mediating antagonistic plant-plant interactions - that is, allelopathy. If proved to exist, such phenomena would hold considerable promise for agronomic improvement of staple food crops such as rice (Oryza sativa). • However, while substantiated by the presence of phytotoxic compounds at potentially relevant concentrations, demonstrating a direct role for specific natural products in allelopathy has been difficult because of the chemical complexity of root and plant litter exudates. This complexity can be bypassed via selective genetic manipulation to ablate production of putative allelopathic compounds, but such an approach previously has not been applied. • The rice diterpenoid momilactones provide an example of natural products for which correlative biochemical evidence has been obtained for a role in allelopathy. Here, we apply reverse genetics, using knock-outs of the relevant diterpene synthases (copalyl diphosphate synthase 4 (OsCPS4) and kaurene synthase-like 4 (OsKSL4)), to demonstrate that rice momilactones are involved in allelopathy, including suppressing growth of the widespread rice paddy weed, barnyard grass (Echinochloa crus-galli). • Thus, our results not only provide novel genetic evidence for natural product-mediated allelopathy, but also furnish a molecular target for breeding and metabolic engineering of this important crop plant.

Direct and indirect effects of allelopathy in the soil legacy of an exotic plant invasion

Abstract: Invasive species may leave behind legacies that persist even after removal, inhibiting subsequent restoration efforts. We examined the soil legacy of Cytisus scoparius, a nitrogen-fixing, putatively allelopathic shrub invading the western US. We tested the hypothesis that allelopathy plays a critical role in the depressive effect of Cytisus on the key native Douglas-fir, both directly on tree growth and indirectly via effects on its ectomycorrhizal fungi (EMF). In a greenhouse factorial experiment, we used activated carbon to inhibit Cytisus-produced allelochemicals and sucrose to reduce elevated nitrogen (N). We found that: (1) Cytisus-invaded soils depressed Douglas-fir growth compared to uninvaded forest soils. The effect of adding Cytisus litter was positive (possibly reflecting an N fertilization effect) only in the presence of activated carbon, providing evidence for a role of allelopathic compounds. Activated carbon did not increase growth in the absence of Cytisus litter. Finally, sucrose addition provided weak support for a nitrogen effect of Cytisus litter. (2) Seedlings grown in Cytisus soils had lower EMF abundance compared to those in uninvaded forest soils. In forest soil from one site, adding Cytisus litter also decreased EMF abundance. Douglas-fir growth increased significantly with EMF across sites and soils suggesting that changes in EMF were linked to tree growth. The fungal taxon Cenococcum geophilum was significantly depressed in Cytisus soils compared to forest soils, while Rhizopogon rogersii abundance was similar across soil types. These results together suggest an overall negative effect of Cytisus on the growth of a dominant native tree and its fungal symbionts. Our study suggests how the role of allelopathy in ecological restoration may play out on two time scales: through immediate, direct impacts on native plants as well as through long-term, persistent impacts mediated by the collapse or transformation of microbial communities.

Allelopathy: a tool for weed management in forest restoration

Abstract: Forest restoration uses active management to re-establish natural forest habitat after disturbance. However, competition from early successional species, often aggressively invasive exotic plant species, can inhibit tree establishment and forest regeneration. Ideally, restoration ecologists can plant native tree species that not only establish and grow rapidly, but also suppress exotic competitors. Allelopathy may be a key mechanism by which some native trees could reduce the abundance and impact of exotic species. Allelopathy is a recognized tool for weed management in agriculture and agroforestry, but few studies have considered how allelopathic interactions may aid restoration. Here we introduce the “Homeland Security” hypothesis, which posits that some naive exotic species may be particularly sensitive to allelochemicals produced by native species, providing a tool to reduce the growth and impacts of invasive exotic species on reforestation. This article explores how exploiting allelopathy in native species could improve restoration success and the re-establishment of natural successional dynamics. We review the evidence for allelopathy in agroforestry systems, and consider its relevance for reforestation. We then illustrate the potential for this approach with a case study of tropical forest restoration in Panama. C4 grasses heavily invade deforested areas in the Panama Canal watershed, especially Saccharum spontaneum L. We measured the effect of leaf litter from 17 potential restoration tree species on the growth of invasive C4 grasses. We found that leaf litter from legume trees had a greater inhibitory effect on performance of S. spontaneum than did litter from non-legume trees. However, allelopathic effects varied greatly among species within tree functional groups. Further evaluation of intra- and inter-specific interactions will help to improve our selection of restoration species.

Intercropping with garlic alleviated continuous cropping obstacle of cucumber in plastic tunnel

Abstract: Continuous cropping obstacle is prevalent for protected cultivated cucumber in China. Intercropping garlic may effectively relieve this obstacle due to its allelopathic and antimicrobial effects. A two-growing season investigation was carried out during autumn 2009 to spring 2010 in plastic tunnel to determine the effects of intercropped garlic and green garlic on the overall growth of cucumber and soil biological properties. Results showed that green garlic exhibited the inhibitory effect on the growth of cucumber in spring cultivation 2010. Garlic–cucumber intercropping system increased yield of cucumber differently depending on garlic cultivars, with only cv. G005 showing significant increase (13.4%). Intercropping systems were evaluated as a greater net benefit system as compared to monoculture cultivation. Populations of soil bacteria and actinomyces were stimulated, while fungi were inhibited under intercropping system. Activities of soil invertase, urease, and alkaline phosphatase were enc...

Allelopathic assessment of fresh aqueous extracts of chenopodium album l. for growth and yield of wheat (triticum aestivum l.)

Abstract: Chenopodium album L. is an annual weed of cultivated fields. In order to determine its allelopathic potentials, water soluble leaf extracts at different concentrations (25, 50 and 75 %) were tested on growth and yield related attributes of wheat (Triticum aestivum L.) in a pot culture experiment. Results showed that application of concentrated extracts had detrimental effects on plant height, number of tillers and spike length which corresponded to significantly lower grain yield. However, lower concentration (25 %) of the extract promoted these parameters. Results of this study reported significantly suppressive effects of higher concentrations (50 and 75 %) on plant height of T. aestivum L. corresponding to lower grain yield.

Seaweed allelopathy against coral: surface distribution of a seaweed secondary metabolite by imaging mass spectrometry.

Abstract: Coral reefs are in global decline, with seaweeds increasing as corals decrease. Although seaweeds inhibit coral growth, recruitment, and survivorship, the mechanism of these interactions is poorly understood. Here, we used field experiments to show that contact with four common seaweeds induces bleaching on natural colonies of Porites rus. Controls in contact with inert, plastic mimics of seaweeds did not bleach, suggesting seaweed effects resulted from allelopathy rather than shading, abrasion, or physical contact. Bioassay-guided fractionation of the hydrophobic extract from the red alga Phacelocarpus neurymenioides revealed a previously characterized antibacterial metabolite, neurymenolide A, as the main allelopathic agent. For allelopathy of lipid-soluble metabolites to be effective, the compounds would need to be deployed on algal surfaces where they could transfer to corals on contact. We used desorption electrospray ionization mass spectrometry (DESI-MS) to visualize and quantify neurymenolide A on the surface of P. neurymenioides, and we found the molecule on all surfaces analyzed, with highest concentrations on basal portions of blades.

Perspectives on allelopathic disruption of plant mutualisms: a framework for individual- and population-level fitness consequences

Abstract: Mutualisms with mycorrhizal fungi, pollinators, and seed dispersers are critical for plant survival and reproduction. However, mutualism effectiveness is highly sensitive to disturbance by environmental stressors. Allelopathy is often overlooked, yet likely important, as a potential stress on plant mutualism function. Allelochemicals can affect plant mutualisms by either directly interfering with the plant’s ability to produce resources and rewards for its mutualistic partners or by directly or indirectly altering the non-plant mutualist’s behavior. Here we explore the potential effects of allelochemicals on plant mutualisms. Since allelochemicals can reduce plant growth and carbon acquisition, we suggest that allelopathy could directly diminish: (1) carbon provisioning to mycorrhizal fungi, (2) flower, pollen, and nectar production for pollinators, and (3) fruit attractiveness to seed dispersers. Similarly, allelochemicals that directly affect mycorrhizal fungi functioning can reduce the flow of soil resources to their plant partner. Further, volatile allelochemicals or uptake of allelochemicals from the soil by the plant could alter pollen/nectar or fruit attractiveness and indirectly influence pollinator and seed disperser behavior. Finally, we explore the extent to which plant-produced chemicals could have a direct or indirect positive effect on plant mutualisms. We end using these questions to frame future avenues of research that could help to move studies of allelopathy into the broader ecological context of mutualisms.

Expression of allelopathy in the soil environment: soil concentration and activity of benzoxazinoid compounds released by rye cover crop residue

Abstract: Rye (Secale cereale L.) residue is known to suppress weeds in agronomic environments and to produce benzoxazinoid (BX) compounds which are phytotoxic. Experiments were conducted to determine the duration of indicator plant inhibition and BX soil concentrations in response to field incorporated or surface rye residue. Surface rye residue was highly inhibitory to lettuce (Lactuca sativa L.) and smooth pigweed (Amaranthus hybridus L.) throughout an experimental period of 4 weeks, whereas soil removed from beneath residue and assayed in pots had little phytotoxicity, suggesting that physical rather than chemical mechanisms were involved. Incorporated residue inhibited lettuce and pigweed for approximately 2 weeks after incorporation, which corresponded to the period when elevated BX levels were detected in soil, suggesting potential allelopathy. The most toxic BX compounds, APO, DIBOA, and DIMBOA, were present at relatively low levels, whereas the less toxic compounds, BOA and MBOA, and the non-toxic compounds, HBOA and HMBOA, were the predominant BX species in amended soils. When the benzoxazolinones BOA and MBOA were exogenously added to soils to maintain extractable levels of up to 10 μg g−1 soil (100–500 times higher than measured BX in field soils), no significant inhibition of pigweed plants was observed. This result indicated that the observed association between the duration of plant inhibition and BX from incorporated rye was not causal, and that other compounds released with similar dynamics were likely responsible. This approach provides a sound basis for demonstrating the presence of allelopathy in natural or managed ecosystems.

Context dependency of the allelopathic effects of Lonicera maackii on seed germination

Abstract: Allelopathic effects of invasive plants on native flora may be mitigated by the abiotic and biotic environment into which the allelochemicals are released. Lonicera maackii (Amur honeysuckle), an invasive plant of the eastern deciduous forest, suppresses seed germination in laboratory assays. We investigated how L. maackii leachate interacts with abiotic conditions and with the soil microbial community. First, we tested the effects of leaf extract from L. maackii on germination of the native woodland herb, Blephilia hirsuta, under different light and soil conditions. We found that germination of Blephilia hirsuta was reduced by L. maackii extract, but abiotic conditions did not interact with this effect. We also tested the effects of leaf extract on germination of five native woodland species and L. maackii placed in sterile or live soil. There was an overall suppressive effect of L. maackii extract on itself and the other five native species tested. However, L. maackii extract interacted with live soil in ways that differed with the species being tested and, in some cases, changed over time. Our results indicate that allelopathic potential of L. maackii shows context dependency with respect to soil microorganisms and native species identity but not to light conditions or soil type. Our results imply that restoration of invaded areas may require active reintroduction of species sensitive to allelopathy in live soil. Further, laboratory assays of allelopathy should consider the interaction of allelochemicals with biotic and abiotic conditions to more accurately predict the impacts of allelopathy on plant communities.

Allelopathic effects of the invasive Prosopis juliflora (Sw.) DC. on selected native plant species in Middle Awash, Southern Afar Rift of Ethiopia

Abstract: The allelopathic effects of the invasive Prosopis juliflora (Sw.) DC. was studied on seed germination and seedling growth of Acacia nilotica (L.) Willd. ex Del., Acacia tortilis (Forssk.) Hayne, Cenchrus ciliaris L. and Enteropogon rupestris (J.A. Schmidt) A. Chev. Vegetation sampling in different habitat types in the area was made to identify the target plant species. Comparison of canopy characteristics among P. juliflora, A. nilotica and A. tortilis was also made to observe differences if any in canopy closure. P. juliflora was recorded in all habitat types in highest density and observed affecting the plant diversity there in. Its growth characteristics and dense thicket formation restrict light to the ground flora and hence diminishes plant diversity. Leaf, bark and root aqueous extract of P. juliflora at 0, 0.5, 0.8, 1, 2 and 6% were prepared and their effect studied on germination percentage and seedling growth of the study plant species. Germination of A. nilotica and A. tortilis was not affected by all aqueous extracts of different organ parts of P. juliflora while leaf and root extracts at higher concentrations inhibited germination of C. ciliaris and E. rupestris. Shoot and root growth of the study species were inhibited by leaf and root at higher concentrations. Seed germination of all species except A. nilotica was inhibited by soil amended with decaying plant parts and under canopy soil. The effect is species specific and annuals (grasses and herbs) were affected more than perennials. Leaf seems to contain greater number/ amount of inhibitors than does root and bark. Bark seems to contain the least. Heavy accumulation of toxic substances at under canopy soil of P. juliflora may be one of the reasons for its invasiveness and low plant diversity.

Separation of allelopathy from resource competition using rice/barnyardgrass mixed-cultures.

Abstract: Plant-plant interference is the combined effect of allelopathy, resource competition, and many other factors Separating allelopathy from resource competition is almost impossible in natural systems but it is important to evaluate the relative contribution of each of the two mechanisms on plant interference Research on allelopathy in natural and cultivated plant communities has been hindered in the absence of a reliable method that can separate allelopathic effect from resource competition In this paper, the interactions between allelopathic rice accession PI312777, non-allelopathic rice accession Lemont and barnyardgrass were explored respectively by using a target (rice)-neighbor (barnyardgrass) mixed-culture in hydroponic system The relative competitive intensity (RCI), the relative neighbor effect (RNE) and the competitive ratio (CR) were used to quantify the intensity of competition between each of the two different potentially allelopathic rice accessions and barnyardgrass Use of hydroponic culture system enabled us to exclude any uncontrolled factors that might operate in the soil and we were able to separate allelopathy from resource competition between each rice accession and barnyardgrass The RCI and RNE values showed that the plant-plant interaction was positive (facilitation) for PI312777 but that was negative (competition) for Lemont and barnyardgrass in rice/barnyardgrass mixed-cultures The CR values showed that one PI312777 plant was more competitive than 2 barnyardgrass plants The allelopathic effects of PI312777 were much more intense than the resource competition in rice/barnyardgrass mixed cultures The reverse was true for Lemont These results demonstrate that the allelopathic effect of PI312777 was predominant in rice/barnyardgrass mixed-cultures The most significant result of our study is the discovery of an experimental design, target-neighbor mixed-culture in combination with competition indices, can successfully separate allelopathic effects from competition

Allelopathic effects of invasive species (Alliaria petiolata, Lonicera maackii, Ranunculus ficaria) in the Midwestern United States

Abstract: Garlic mustard (Alliaria petiolata), Amur honeysuckle (Lonicera maackii) and lesser celandine (Ranunculus ficaria) are three invasive species in US Midwestern forests. The comparative allelopathic effects of leaf extracts of these species on germination and reproduction of Arabidopsis thaliana were investigated in a growth room. Highest extract concentrations (0.3 and 0.2 g fresh leaf tissue/mL distilled water) of L. maackii delayed germination in potting soil compared to the control. Extracts of L. maackii also decreased the number of siliques in potting soil compared to the control and to A. petiolata extracts, with extracts of R. ficaria having intermediate effects. In field soil, extracts of L. maackii and R. ficaria significantly decreased the number of siliques compared to extracts of A. petiolata. In a third experiment, effects on germination of three agricultural species (Brassica oleracea, Lactuca sativa and Ocimum basilicum), were studied. Ranunculus ficaria and L. maackii extracts were least harmful to germination, while A. petiolata extracts were most harmful. Germination of L. sativa and O. basilicum was more sensitive to A. petiolata and R. ficaria extracts, while germination of B. oleracea was more sensitive to L. maackii extracts. These results showed differential allelopathic effects of these invasive species, which varied with test species and experimental conditions.

Allelopathic Potentiality of Medicinal Plant Leucas aspera

Abstract: Excessive use of herbicide in the crop field threatens the environment by destroying flora and fauna. Allelopathic substances could be act as substitutes of chemical herbicides to suppress the target plants. Leucas aspera (Willd.) Linn., a herbaceous plant has already provided tangible evidence of its potential with remedies for different sorts of medical problems, but still now, not a single work is reported related to it s allelopathic activity. Hence, to explore the allelopathy of L. aspera, the aqueous methanol extracts of this plant at four different concentrations (3, 10, 30 and 100 mg dry weight equivalent extract mL ) were tested 1 against seven test plant species, namely cress (Lepidum sativum L.), alfalfa (Medicago sativa L.), lettuce (Lactuca sativa L.), Italian ryegrass (Lolium multiflorum Lam.), barnyard grass (Echinochloa crusgalli L.), jungle rice (Echinochloa colonum L. Link) and timothy (Phleum pratense L.). The aqueous methanol extract of L. aspera significantly inhibited the seedlings growth of all the test plant species at different inhibition levels. The root growth was more sensitive than the shoot growth to the plant extract and the inhibitory activity was concentration dependent. Comparing the extract concentrations required for 50% inhibition, the seedlings growth of timothy was most sensitive to the extract, whereas jungle rice was less sensitive. These results indicated that L. aspera may contain growth inhibitory substances and possess allelopathic activity. Therefore, L. aspera could be used as a potential candidate for isolation and identification of allelochemicals, which can lead to the development of ecologically acceptable bio-herbicides used for sustainable agriculture.

Reducing the allelopathic effect of Parthenium hysterophorus L. on wheat (Triticum aestivum L.) by Pseudomonas putida

Abstract: The present study evaluates the role of Pseudomonas putida NBRIC19 in alleviating biotic stress of Parthenium hysterophorus (Parthenium) in Triticum aestivum. Due to presence of Parthenium there was 43.76, 53.08 and 78.65% inhibition in root length, shoot length and dry weight of wheat respectively. This inhibition was recovered when P. putida NBRIC19 treatment resulted in 52.29, 28.73 and 76.31% increase in root length, shoot length and dry weight respectively as compared to control. P. putida NBRIC19 was able to form more biofilm under toxic environment of allelochemicals and enhanced expression of stress responsive genes in wheat. Inoculated wheat plants showed lower activity of catalase and ascorbate peroxidase under biotic stress of Parthenium indicating that inoculated plants felt less stress as compared to uninoculated plants. Microbial community structure in bacterized and nonbacterized wheat rhizosphere in presence and absence of Parthenium, was investigated using Biolog. There was significant increase in microbial diversity in P. putida NBRIC19 bacterized wheat rhizosphere. Functional microbial diversity revealed that P. putida NBRIC19 had shifted the microflora in such a manner that utilization of phytotoxic allelochemicals increased to lessen its toxic effect and finally it resulted in better growth of wheat in presence of Parthenium. Principal component analysis showed that microbial community function in nonbacterized wheat rhizosphere in presence (WPC) and absence (WC) of Parthenium is totally different from each other but due to P. putida NBRIC19 treatment there was close clustering of WPT and WT indicating a total shift in microbial community structure.

Differential allelopathic potential of sunflower (Helianthus annuus L.) genotypes on weeds and wheat (Triticum aestivum L.) crop

Abstract: Studies were conducted to screen eight sunflower (Helianthus annuus L.) genotypes for their allelopathic potential against weeds and wheat crop, which customarily follows sunflower in Iraq. All sunflower genotypes significantly inhibited the total number and biomass of companion weeds and the magnitude of inhibition was genotype dependent. Among the eight genotypes tested, Sin-Altheeb and Coupon were the most weed-suppressing cultivars, and Euroflor and Shumoos were the least. A subsequent field experiment indicated that sunflower residues incorporated into the field soil significantly inhibited the total number and biomass of weeds growing in the wheat field. Sunflower genotypes Sin-Altheeb and Coupon appeared to inhibit total weed number and biomass more and significantly increased wheat yield compared with the least-suppressive genotypes (Euroflor and Shumoos). Chromatographic analyses by HPLC revealed the presence of 13 secondary metabolites in residues of the tested sunflower genotypes. All the isola...

Allelopathic potential of Triticum spp., Secale spp. and Triticosecale spp. and use of chromosome substitutions and translocations to improve weed suppression ability in winter wheat

Abstract: With 3 figures and 3 tables Abstract A search was carried out for allelopathic sources among accessions of Triticum, Secale, Triticosecale and wheat–rye substitution and translocation lines to be used in breeding programmes to improve weed suppression ability of wheat. A bioassay with mustard as target plants was used for the screening. Mustard was chosen among seven tested target plants because it showed a particular high root growth inhibition when grown together with rye compared with wheat. None of the Triticum accessions studied showed potential allelopathic activity of interest for breeding, but most Triticosecale did. Several of the wheat–rye substitution and translocation lines also showed high allelopathic activity. The highest activity was found in lines with a substitution of 1R or 2R. Some multiple substitution lines and lines with only rye chromatin also showed high allelopathic activity. It is suggested that in vitro selection of wheat–rye substitution lines with high allelopathic potential with a bioassay with mustard as target plants could be used to improve weed suppression ability of wheat.

Allelopathic Potentialities of Leachates of Leaf Litter of Some Selected Tree Species on Gram Seeds under Laboratory Conditions

Abstract: The present study investigates that physiological response on gram seeds (Cicer arietinum L.) under treatment of different leaf leachate in laboratory conditions. The aqueous leaf leachate of seven variety of selected tree species viz. Acacia auriculiformis A. Cunn ex.Benth, Anacardium occidentale L., Albizia lebbeck(L)Benth, Eucalyptus citriodora Hook, Emblica officinalis Gaertn, Shorea robusta Gaertn.f., Tectona grandis Linn f was studied on physiological changes during gram seed germination. Aqueous extracts of tree species significantly reduced the frequency of seed germination, vigour index; shoot length, root length, fresh and dry weight. Plant pigment viz., chlorophyll, carotenoids reduced significantly. The soluble sugar content reduced with 100 %( v/v) of leachate treatment but proline and phenol content increased on the test plant. The result revealed that Eucalyptus citriodora. and Shorea robusta extract had greater inhibitory effect on germination and vigour index over the control due to the presence of allelochemicals in the leachate. Leachate biochemical analysis also supports the inhibitory effect of seed germination and biochemical effects of seedlings. Allelopathy, allelochemicals, proline, vigour index, carotenoids.

Allelopathy an Environmentally Friendly Method for Weed Control

Abstract: Biological weed management is a system that incorporates the use of diverse biological organisms and biologically-based approaches including allelopathy, crop competition, and other cultural practices to significantly reduce weed densities in a manner that is similar to use of chemical herbicides alone. Interest in developing effective biological weed management systems continues to increase because of a growing awareness of problems associated with the constant and intensive use of chemical herbicides, which include surface- and groundwater contamination, detrimental impacts on nontarget organisms, development of weeds resistant to herbicides, and consumer concerns for residues on food. Among different biological methods of weed control, allelopathy could lead to reduced labour costs and increased efficiency, without any adverse effects on the environment. Many of the compounds produced by green plants that are not involved in primary plant metabolism are observed to function as chemical warfare agents against competing plants and pests. Many such natural compounds have the potential to be exploited as herbicides or as leads for discovery of new herbicides. The paper highlights the different concepts of using allelopathy for eco-friendly control of weeds.

Microbial Community Changes Elicited by Exposure to Cyanobacterial Allelochemicals

Abstract: An increasing body of evidence points out that allelopathy may be an important process shaping microbial communities in aquatic ecosystems. Cyanobacteria have well-documented allelopathic properties, mainly derived from the evaluation of the activity of allelopathic extracts or pure compounds towards monocultures of selected target microorganisms. Consequently, little is known regarding the community dynamics of microorganisms associated with allelopathic interactions. In this laboratory-based study, a Microcystis spp.-dominated microbial community from a freshwater lake was exposed, for 15 days, to exudates from the cyanobacterium Oscillatoria sp. strain LEGE 05292 in laboratory conditions. This cyanobacterium is known to produce the allelochemicals portoamides, which were among the exuded compounds. The community composition was followed (by means of polymerase chain reaction followed by denaturing gradient gel electrophoresis and microscopic analyses) and compared to that of a non-exposed situation. Following exposure, clear differences in the community structure were observed, in particular for cyanobacteria and unicellular eukaryotic taxa. Interestingly, distinct Microcystis genotypes present in the community were differentially impacted by the exposure, highlighting the fine-scale dynamics elicited by the exudates. These results support a role for cyanobacterial allelochemicals in the structuring of aquatic microbial communities.

Identification of Volatile Compounds Released by Leaves of the Invasive Plant Croftonweed (Ageratina adenophora, Compositae), and their Inhibition of Rice Seedling Growth

Abstract: Several volatile allelochemicals were identified and characterized from fresh leaf tissues of the invasive croftonweed. A simple bioassay was used to demonstrate the release of volatile allelochemicals from leaf tissues. The bioassays revealed that foliar volatile components of croftonweed exhibited significant effects on the seedling growth of upland rice. Peroxidase (POD) activity, superoxide dismutase (SOD) activity, and root oxidizability rose as the concentration of volatiles increased. Activity for both POD and SOD significantly increased with exposure to 15 g and 20 g of croftonweed leaf tissue for 5 d. Root activity was significant at 10 g compared to the control. The volatile components also stimulated the development of the aerenchyma tissue and inhibited lateral root formation. Leaf volatiles of croftonweed were identified by gas chromatography coupled with mass spectrometry (GC–MS). Some of the compounds identified included α-phellandrene, camphene, ρ-cymene, 2-carene, α-pinene, limon...

Potent endogenous allelopathic compounds in Lepidium sativum seed exudate: effects on epidermal cell growth in Amaranthus caudatus seedlings

Abstract: Many plants exude allelochemicals – compounds that affect the growth of neighbouring plants. This study reports further studies of the reported effect of cress (Lepidium sativum) seed(ling) exudates on seedling growth in Amaranthus caudatus and Lactuca sativa. In the presence of live cress seedlings, both species grew longer hypocotyls and shorter roots than cress-free controls. The effects of cress seedlings were allelopathic and not due to competition for resources. Amaranthus seedlings grown in the presence of cress allelochemical(s) had longer, thinner hypocotyls and shorter, thicker roots – effects previously attributed to lepidimoide. The active principle was more abundant in cress seed exudate than in seedling (root) exudates. It was present in non-imbibed seeds and releasable from heatkilled seeds. Release from live seeds was biphasic, starting rapidly but then continuing gradually for 24 h. The active principle was generated by aseptic cress tissue and was not a microbial digestion product or seed-treatment chemical. Crude seed exudate affected hypocotyl and root growth at ;25 and ;450 m gm l 21 respectively. The exudate slightly (28%) increased epidermal cell number along the length of the Amaranthus hypocotyl but increased total hypocotyl elongation by 129%; it resulted in a 26% smaller hypocotyl circumference but a 55% greater epidermal cell number counted round the circumference. Therefore, the effect of the allelochemical(s) on organ morphology was imposed primarily by regulation of cell expansion, not cell division. It is concluded that cress seeds exude endogenous substances, probably including lepidimoide, that principally regulate cell expansion in receiver plants.

Allelopathic effect of Cassia tora on seed germination and growth of mustard

Abstract: Cassia tora L. (CT) is a widely growing weed in India with medicinal value and has been reported to show allelopathic potential causing biological suppression on crops. The present study was conducted to observe its suppression on Brassica campestris L. (mustard). The rate of seed germination and the root length, shoot length, chlorophyll content, fresh weight (FW), dry weight (DW), and relative water content (RWC) of 10-day-old mustard seedlings were observed after treatment with aqueous extracts of the roots, leaves, and seeds of CT and compared with control sets treated with distilled water. A confirmatory chemical test was conducted to determine the presence of phenols, ketones, acids, and alcohols in the CT plant extracts. Different concentrations of the extracts showed inhibition (up to 100%) or deterioration in all of the parameters. Roots were more affected than shoots and the extract taken from leaves was more suppressive than that taken from other plant parts. The FW, DW, and RWC were considerably reduced upon treatment with the tested extracts. The extracts (>25%) also caused a reduction in the chlorophyll content. The overall results indicate the possible suppressive effect of allelochemicals (phenols, ketones, acids, and alcohols) present in CT on mustard. The overall results with regard to mustard plants followed the same trend of concentration-dependent inhibition in the order of leaf > seed > root.