Showing papers on "Allelopathy published in 2001"

The relative importance of allelopathy in interference: the effects of an invasive weed on a native bunchgrass.

Abstract: The relative importance of allelopathy and resource competition in plant-plant interactions has been vigorously debated but seldom tested. We used activated carbon to manipulate the effects of root exudates of Centaurea maculosa, a noxious weed in much of western North America, on root elongation rates and growth of the native bunchgrass Festuca idahoensis in order to investigate the relative importance of allelopathy in the total interference of Centaurea. In root observation chambers, Festuca root elongation rates decreased to ≈50% of the control, beginning 4 days before contacting Centaurea roots in silica sand. However, when activated carbon, which has a high affinity for adsorbing to organic compounds, was added to the sand the effects of Centaurea roots on Festuca root elongation were reduced. In other experiments, Festuca plants were 50% smaller when grown with Centaurea than with conspecifics in pure silica sand. However, Festuca grown with Centaurea in mixtures of sand and activated carbon were 85% larger than Festuca grown with Centaurea in silica sand without carbon. These results suggest that allelopathy accounts for a substantial proportion of the total interference of Centaurea on Festuca, shifting the balance of competition in favor of Centaurea. However, Centaurea outperformed Festuca even in the presence of activated carbon, demonstrating the importance of the combined roles of resource competition and allelopathy.

Weed allelopathy, its ecological impacts and future prospects: a review

Abstract: Summary The importance, characteristics, positive and negative impacts, and future role of weeds as an integral part of the natural and agroecosystems are evaluated and discussed. Interference between plants in nature and the importance of differentiating between competition and allelopathy are interpreted. Allelopathy as one component of weed/crop interference, allelochemicals from weed species and their possible mechanism of action are listed and discussed. Weed species with inhibitory action against cultivated crops, other weed species, and plant pathogens, as well as self-inhibitory (autopathic) species are reviewed. Stimulatory or inhibitory allelopathic effects of different crop plants, trapping and catching species, and the potential of allelopathic weeds in inhibiting or stimulating certain parasitic weed species are discussed and evaluated. Allelopathy as a mechanism and future strategy for agricultural pest control and farm management and the potential use and development of some allelochemicals...

Plant allelochemical interference or soil chemical ecology

Abstract: While allelopathy has been defined as plant-plant chemical interference, there has been much confusion about what the concept encompasses and how important it is in nature. We distinguish between (1) direct plant-plant interference mediated by allelochemicals, and (2) the effects of secondary compounds released by plants on abiotic and biotic soil processes that affect other plants. It very difficult to demonstrate direct effects of chemicals released by a plant on nearby plants. Although soil ecology-mediated effects of secondary plant compounds do not fit the classical concept of allelopathy, we find support in the literature for the hypothesis that the most important effects of compounds released into the soil environment by plants on other plants occur through such indirect effects. The emphasis on, and skepticism of, direct plant-plant allelopathic interference has led some researchers to demand unreasonably high standards of evidence for establishing even the existence of allelopathic interactions, standards that are not demanded for other plant-plant interactions such as resource competition. While the complete elucidation of the mechanisms by which allelochemicals function in the field is many years away, such elucidation is not necessary to establish the existence of allelopathic interactions. We propose that most of the phenomena broadly referred to as allelopathic interference are better conceptualized and investigated in terms of soil chemical ecology. Even when direct plant-plant allelochemical interference occur, the levels of allelochemicals in the environment and their effects on plants are heavily influenced by abiotic and biotic components of the soil ecosystem. Putting allelopathy in the context of soil ecology can further research and reduce some of the less fruitful controversy surrounding the phenomenon.

Allelopathy in agroecosystems: an overview.

Abstract: Summary Allelopathy plays an important role in the agroecosys-tems leading to a wide array of interactions between crop-crop, crop-weed and tree-crops. Generally, these interactions are harmful to the receiver plants but provide a selective benefit to the donor. Soil microbes play a key role in determining such interactions as they not only alter the nature of allelopathic interactions but also modify the expression of allelochemicals. Soil sickness problem in the croplands could also be attributed to the allelopathic property or even the autotoxicity. The allelochemicals released largely by the plant residues that are left in the fields after the harvest of crops add to the multifarious problems. If properly managed, these residues could be used for controlling weeds and pests. As is true for any chemical based response, allelopathic interactions are also concentration specific. The promotory functions that are inbuilt need to be worked out and exploited. Now a days allelopathic interactions, in general,...

Searching for Rice Allelochemicals

Abstract: agement strategies for rice, which would be less dependent on synthetic herbicides. A bioactivity-guided isolation method was developed with the obThe search for allelochemicals in rice necessitates jective of isolating the allelochemicals in rice (Oryza sativa L.). Roots evaluating the activity in a laboratory set-up to distinof the allelopathic rice cultivar Taichung Native 1, grown hydroponiguish between competition and allelopathy, which cancally, were extracted and fractionated, with the activity of the fractions not be distinctly separated in field studies (Olofsdotter followed using a 24-well culture plate microbioassay. Some of the fractions obtained consisted of pure compounds, but none inhibited et al., 1997). Depending on one’s objectives, different the growth of barnyardgrass [Echinochloa crusgalli (L.) Beauv.] at methods could be followed in searching for active conthe lower concentration at which they were tested. Identified com- stituents from plants. These include bioassay-guided isopounds were azelaic acid; r-coumaric acid; 1H-indole-3-carboxalde- lation, fractionation-driven bioassay, isolate and assay, hyde; 1H-indole-3-carboxylic acid; 1H-indole-5-carboxylic acid; and and biochemical combinatorial chemistry approaches. 1,2-benzenedicarboxylic acid bis(2-ethylhexyl)ester. r-Coumaric acid, The advantages and disadvantages of each of these a known allelochemical, inhibited the germination of lettuce (Lactuca methods are discussed in more detail by Duke et al. sativa L.) seedlings at 1 mM. However, r-coumaric acid was active (2000a). We chose bioassay-guided isolation as the best against barnyardgrass only at concentrations higher than 3 mM. The way to proceed because the active component is not two most active fractions obtained from the bioassay-guided isolation known. were still a mixture of compounds as analyzed by gas chromatography‐ Bioassay-guided isolation integrates the processes of mass spectrometry (GC-MS). Further fractionation is being done to isolate and identify the allelochemical(s) in these active fractions. separation of compounds in a mixture, using various This work has demonstrated the use of bioassay-guided isolation in analytical methods, with results obtained from biologiidentifying allelochemicals in rice and has correlated observed field cal testing. The process begins with testing an extract activity with laboratory experiments. to confirm its activity, followed by crude separation of the compounds in the matrix and testing the crude fractions (Fig. 1). Further fractionation is carried out

Rice—A Step Toward Use of Allelopathy

Abstract: Rice (Oryza sativa L.) allelopathy has been on the research agenda for a decade. Now it is important to step back and look at its progress to enable priority setting for future research. This paper aims to do so primarily using the following five-step protocol for allelopathy research: (i) carrying out laboratory, greenhouse, and field studies to illustrate the effect of released allelochemicals; (ii) isolating, identifying, and characterizing allelochemicals; (iii) establishing a correlation between growth inhibition and allelochemicals; (iv) performing genetic mapping of quantitative trait loci (QTLs) correlated with allelopathy; and (v) breeding for allelopathic cultivars tested for competitive ability in greenhouse and field experiments. Recent research on rice allelopathy has resulted in the following research milestones: ○ There is large variation in allelopathy among rice cultivars. ○ Allelopathy plays a role under field conditions. ○ Allelopathic rice can suppress both mono- and dicot weed species. ○ Progress has been made in identifying rice allelochemicals. ○ Quantitative trait loci correlated with allelopathy have been identified. This paper discusses the progress made in recent years and suggests some direction for future research.

Locating Genes Controlling Allelopathic Effects against Barnyardgrass in Upland Rice

Abstract: To understand the genetic control of allelopathy in rice (Oryza sativa L.), quantitative trait loci (QTL) mapping was performed using a population of 142 recombinant inbred lines derived from a cross between cultivar IAC 165 (japonica upland variety) and cultivar CO 39 (indica irrigated variety). The map contained 140 DNA markers. The relay seeding technique, which is a laboratory bioassay measuring the inhibition in weed root growth due to the presence of rice seedlings, was used to evaluate the allelopathic effect of the rice lines. Cultivar IAC 165 showed strong and consistent allelopathic activity against barnyardgrass [Echinochloa crus-galli (L.) Beauv.], whereas CO 39 was weakly allelopathic. Transgressive segregation for allelopathic activity in both directions was observed in the population. No significant correlation was found between root morphology of the lines and their allelopathic potential, suggesting that allelopathy in rice was under genetic control independent from root morphology. Four maineffect QTLs located on three chromosomes were identified, which collectively explained 35% of the total phenotypic variation of the allelopathic activity in the population. One pair of digenic epistatic loci, not involving any of the main-effect loci, was also detected. Once confirmed, these QTLs may be useful for genetic improvement of allelopathy in rice using marker-assisted selection. (Resume d'auteur)

Allelopathy in wheat (Triticum aestivum)

Abstract: Summary Wheat (Triticum aestivum) allelopathy has potential for the management of weeds, pests and diseases. Both wheat residue allelopathy and wheat seedling allelopathy can be exploited for managing weeds, including resistant biotypes. Wheat varieties differ in allelopathic potential against weeds, indicating that selection of allelopathic varieties might be a useful strategy in integrated weed management. Several categories of allelochemicals for wheat allelopathy have been identified, namely, phenolic acids, hydroxamic acids and short-chain fatty acids. Wheat allelopathic activity is genetically controlled and a multigenic model has been proposed. Research is underway to identify genetic markers associated with wheat allelopathy. Once allelopathic genes have been located, a breeding programme could be initiated to transfer the genes into modern varieties for weed suppression. The negative impacts of wheat autotoxicity on agricultural production systems have also been identified when wheat straws are retained on the soil surface for conservation farming purposes. A management package to avoid such deleterious effects is discussed. Wheat allelopathy requires further study in order to maximise its allelopathic potential for the control of weeds, pests and diseases, and to minimise its detrimental effects on the growth of wheat and other crops.

Identification of allelopathic compounds from rice (Oryza sativa L.) straw and their biological activity

Abstract: Experiments were conducted to identify allelochemicals from rice (Oryza sativa L.) straw extracts of four rice cultivars (Gin shun, Kasawala mundara, Philippine 2 and Juma 10), and to test their bi...

Crop Allelopathy and Its Role in Ecological Agriculture

Abstract: Summary A number of crops exhibit allelopathic interactions that play a significant role in the complex environment of agroecosystems. Several studies have shown that allelopathic crops reduce growth, development and yield of other crops growing simultaneously or subsequently in the fields. Another aspect of interest regarding crop allelopathy is that allelochemicals may exhibit inhibitory effect on the same crop which is commonly called as crop autotoxicity. It is predominantly common in fields where sole cropping under reduced or no-tillage system is practiced. Though any crop part can be allelopathic, including even the pollens, but decomposing crop residues exhibit more influence on other plants. Furthermore, the extent of allelopathy by a crop plant varies with age, part and type of cultivar being used. Nowadays allelo-pathic crops are being used as an important tool in managing weeds and harmful pests under sustainable pest management programs. In this context several cover/smother and green manure ...

Screening and Future Exploitation of Allelopathic Plants as Alternative Herbicides with Special Reference to Hairy Vetch

Abstract: Summary Allelopathic cover crops are one of the most promising application of allelopathy for weed control and reduce the synthetic herbicides input. Fifty three cover crop plant species (26 leguminous, 19 graminaceous and 8 others) were assessed for the allelopathic activity using Plant Box Method. It was found that leguminous cover crops such as hairy vetch (Vicia villosa) and velvetbean (Mucuna pruriens), graminaceous cover crops, such as oat (Avena sativa) and rye (Secale cereale), certain cultivars of wheat (Triticum aestivum) and barley (Hordeum vulgare) were promising. By bioassay screening, 23 species were selected for field tests. Fall-sown cover crops such as hairy vetch, rye, wheat, oat, grass pea, and mustard are more effective when compared to spring-sown cover crops. Hairy vetch was most promising for the weed control in abandoned fields because of its ability to die off during summer season to make a thick straw-like mulch.

Allelopathic effects of barley extracts on germination and seedlings growth of bread and durum wheats

Abstract: Phytotoxicity of barley extracts ( Hordeum vulgare L.) on durum wheat (Triticum durum L.) and bread wheat (Triticum aestivum L.) was investigated. Water extracts of barley, variety Rihane were bioassayed on germina- tion and seedling growth of both wheat species to: (i) test the heterotoxicity of barley on wheat, (ii) study the dynamics of allelopathic potential over four growth stages and (iii) identify the most allelopathic plant part of barley. Whole bar- ley plants were extracted at growth stage 4 (stems not developed enough), whilst for the following growth stages roots, stems, and leaves were extracted separately. Seedling growth bioassays demonstrated that the two wheat species responded differently to the allelopathic potential of barley with a greater sensitivity shown by the bread wheats. For both wheat species, radicle growth was more depressed than coleoptile growth, though stimulation of seedling growth was observed for durum wheat. The allelopathic potential of barley plant parts was not stable over its life cycle for either bread or durum wheat. It appeared that potential increased near physiological maturity. Leaves and roots were the most phytotoxic barley plant parts for durum and bread wheats, respectively. Results suggested that the response by durum wheat and bread wheat varied depending on the source of allelochemicals (plant part) and the growth stage of the barley plant. Consequently, barley should be considered a depressive prior crop for both durum wheat and bread wheat in a field cropping sequence.

Variation in the Allelopathic Effect of Rice with Water Soluble Extracts

Abstract: The allelopathic effect of rice (Oryza saliva L.) on lettuce (Lactuca saliva L.) and ducksalad [Heteranthera limosa (Sw.) Willd.] was investigated with water soluble extracts. Ducksalad is a major weed in southern USA rice fields. P1312777' and Rexmont', which are suppressive and nonsuppressive to ducksalad in the field, were used to establish the bioassay of allelopathic activity in rice. Water soluble compounds, which were extracted from rice seedlings and adult plants, were applied to lettuce seeds. Leaf extract of P1312777 inhibited the root growth of lettuce-more strongly than those of Rexmont. Extracts from the leaves of rice seedlings at the six-leaf stage inhibited the growth of ducksalad and lettuce, and a dose relationship existed between the inhibitory effect and the two test plants. A wide range of variation in allelopathic activity among rice cultivars was assessed using water soluble extracts from their leaves and lettuce as a test plant.

Allelopathic effects of walnut leaf extracts and juglone on seed germination and seedling growth

Abstract: SummaryThe allelopathic effects of juglone and leaf extracts of walnut (Juglans regia L.) on seed germination and seedling growth of 11 species were investigated. Seed germination was less affected than root and shoot growth in all species. Both seed germination and seedling growth of tomato, cucumber, garden cress and alfalfa were inhibited strongly by the treatments. However, seed germination of wheat, barley, corn, watermelon, radish and bean was not affected but their seedling growth was inhibited slightly. In muskmelon, interestingly, seedling growth was increased by both walnut leaf extracts and juglone. Positive correlation was found between the effects of juglone and the extracts.

Strategies for Using Transgenes to Produce Allelopathic Crops1

Abstract: Crop allelopathy has seldom been used effectively by farmers in weed management. Traditional breeding methods have not been successful in producing highly allelopathic crops with good yields. Genetic engineering may have the potential for overcoming this impasse. Crops have been made resistant to insects, pathogens, and herbicides with transgenes, but biotechnology has not produced crops that control weeds with allelochemicals. The strategies for producing allelopathic crops by biotechnology are relatively complex, usually involving multiple genes. One can choose to enhance production of allelochemicals already present in a crop or to impart the production of new compounds. The first strategy involves identification of the allelochemical(s), determination of the enzymes and genes encoding them, and the use of genetic engineering to enhance their production. The latter strategy employs altering existing biochemical pathways by insertions of transgenes to produce new allelochemicals. With either strategy, t...

Phytotoxicity of vulpia residues: III. Biological activity of identified allelochemicals from Vulpia myuros.

Abstract: Twenty compounds identified in vulpia (Vulpia myuros) residues as allelochemicals were individually and collectively tested for biological activity. Each exhibited characteristic allelochemical behavior toward the test plant, i.e., inhibition at high concentrations and stimulation or no effect at low concentrations, but individual activities varied. Allelopathins present in large quantities, such as syringic, vanillic, and succinic acids, possessed low activity, while those present in small quantities, such as catechol and hydrocinnamic acid, possessed strong inhibitory activity. The concept of a phytotoxic strength index was developed for quantifying the biological properties of each individual allelopathin in a concise, comprehensive, and meaningful format. The individual contribution of each allelopathin, assessed by comparing the phytotoxic strength index to the overall toxicity of vulpia residues, was variable according to structure and was influenced by its relative proportion in the residue. The majority of compounds possessed low or medium biological activity and contributed most of the vulpia phytotoxicity, while compounds with high biological activity were in the minority and only present at low concentration. Artificial mixtures of these pure allelochemicals also produced phytotoxicity. There were additive/synergistic effects evident in the properties of these mixtures. One such mixture, formulated from allelochemicals found in the same proportions as occur in vulpia extract, produced stronger activity than another formulated from the same set of compounds but in equal proportions. These results suggest that the exploration of the relative composition of a cluster of allelopathins may be more important than simply focusing on the identification of one or two compounds with strong biological activity and that synergism is fundamental to the understanding of allelopathy.

Microbial Allelochemicals and Pathogens as Bioherbicidal Agents1

Abstract: The initiative to use plant pathogens and allelochemicals from pathogens and other microorganisms as biological weed control agents (bioherbicides) began about 30 yr ago. Since then, numerous plant pathogens (bacteria and fungi) and microbial allelochemicals have been isolated, identified, and tested for their bioherbicidal potential. Pathogens (and in some cases microbial phytotoxins) may be used directly on target weed species, or such allelochemicals may provide unique chemical templates for the synthesis of new herbicide classes with novel molecular modes of action. To date, the most successful microbial products that have led to the development of commercial herbicides are bialaphos (commercially available in Japan) and glufosinate (marketed worldwide). Glufosinate is the ammonium salt of phosphinothricin, which is the active ingredient of bialaphos derived from a nonphytopathogenic Streptomyces species. This overview will examine selected advances in the isolation and identification of novel plant p...

Allelopathic Effects of Tree Species on Some Soil Microbial Populations and Herbaceous Plants

Abstract: The allelopathic potential of four tree species on soil microbial populations and some herbaceous plants (two understory species and one general biotest species) was investigated. Effects of three nonindigenous tree species, Eucalyptus globulus Labill, Pinus radiata D.Don and Acacia melanoxylon R.Br., on microorganisms participating in the cycle of nitrogen were evaluated, comparing them with those produced by the autochthonous Quercus robur L. Influence of the trees on Lactuca sativa L., Dactylis glomerata L. and Trifolium repens L. was also checked in bioassays. Cell numbers of Nitrosomonas sp. were negatively affected by Acacia and Eucalyptus stands, mainly during spring, when flowers are especially abundant on the ground. Proteolytic microorganisms were also negatively affected by Eucalyptus and Pinus stands, whilst Quercus stand did not show any toxicity. Soil bioassays showed clear inhibitory effects on germination and growth of understory plants, particularly soils from Eucalyptus and Acacia stands. The greatest effects had the soil from Acacia stand, which was phytotoxic during the whole period of germination and growth of understory plants. Allelopathic phenomena could be, at least partially, responsible of the low species diversity in the understory of the nonindigenous tree stands.

Allelochemicals in wheat (Triticum aestivum L.): cultivar difference in the exudation of phenolic acids.

Abstract: Analysis by GC-MS/MS showed that a worldwide collection of 58 wheat accessions differed significantly in the amounts of 7 known phenolic acids exuded by the living roots of 17-day-old wheat seedlings. The quantities of exuded allelochemicals varied with the specific compound and ranged from 2.3 to 18.6, from 0.6 to 17.5, from 0.1 to 4.9, from 0.0 to 52.7, from 0.33 to 12.7, from 1.5 to 20.5, and from 1.6 to 23.4 microg/L of water/agar for p-hydroxybenzoic, vanillic, cis-p-coumaric, syringic, cis-ferulic, trans-p-coumaric, and trans-ferulic acids, respectively. The concentrations of p-hydroxybenzoic and vanillic acids exuded by wheat seedlings were normally distributed in the 58 accessions. The level of each phenolic acid in root exudates did not correlate well to that previously observed in wheat. In comparison with weakly allelopathic accessions, strongly allelopathic accessions exuded larger quantities of allelochemicals into the growth medium. The chemical basis for wheat seedling allelopathy is an area for further investigation.

Allelopathic bacteria and their impact on higher plants.

Abstract: The impact of allelopathic, nonpathogenic bacteria on plant growth in natural and agricultural ecosystems is discussed. In some natural ecosystems, evidence supports the view that in the vicinity of some allelopathically active perennials (e.g., Adenostoma fasciculatum, California), in addition to allelochemicals leached from the shrub's canopy, accumulation of phytotoxic bacteria or other allelopathic microorganisms amplify retardation of annuals. In agricultural ecosystems allelopathic bacteria may evolve in areas where a single crop is grown successively, and the resulting yield decline cannot be restored by application of minerals. Transfer of soils from areas where crop suppression had been recorded into an unaffected area induced crop retardation without readily apparent symptoms of plant disease. Susceptibility of higher plants to deleterious rhizobacteria is often manifested in sandy or so-called skeletal soils. Evaluation of phytotoxic activity under controlled conditions, as well as ways to apply allelopathic bacteria in the field, is approached. The allelopathic effect may occur directly through the release of allelochemicals by a bacterium that affects susceptible plant(s) or indirectly through the suppression of an essential symbiont. The process is affected by nutritional and other environmental conditions, some may control bacterial density and the rate of production of allelochemicals. Allelopathic nonpathogenic bacteria include a wide range of genera and secrete a diverse group of plant growth-mediating allelochemicals. Although a limited number of plant growth-promoting bacterial allelochemicals have been identified, a considerable number of highly diversified growth-inhibiting allelochemicals have been isolated and characterized. Some species may produce more than one allelochemical; for example, three different phyotoxins, geldanamycin, nigericin, and hydanthocidin, were isolated from Streptomyces hygroscopicus. Efforts to introduce naturally produced allelochemicals as plant growth-regulating agents in agriculture have yielded two commercial herbicides, phosphinothricin, a product of Streptomyces viridochromogenes, and bialaphos from S. hygroscopicus. Many species of allelopathic bacteria that affect growth of higher plants are not plant specific, but some do exhibit specificity; for example, dicotyledonous plants were more susceptible to Pseudomonas putida than were monocotyledons. Differential susceptibility of higher plants to allelopathic bacteria was noted also in much lower taxonomical categories, at the subspecies level, in different cultivars of wheat, or of lettuce. Therefore, when test plants are employed to evaluate bacterial allelopathy, final evaluation must include those species that are assumed to be suppressed in nature. The release of allelochemicals from plant residues in plots of 'continuous crop cultivation' or from allelopathic living plants may induce the development of specific allelopathic bacteria. Both the rate by which a bacterium gains from its allelopathic activity through utilizing plant excretions, and the reasons for the developing of allelopathic bacteria in such habitats, are important goals for further research.

Screening methods for the evaluation of crop allelopathic potential

Abstract: There is increasing interest in the development of allelopathic crop varieties for weed suppression. Allelopathic varieties are likely to be able to suppress weeds by natural exudation of bioactive allelochemicals, thereby reducing dependence upon synthetic herbicides. Screening bioassays are essential tools in identifying crop accessions with allelopathic potential. A number of crops have been screened for this allelopathic trait, and key issues in selecting and designing screening bioassays are reviewed. It is recommended that a combination of different bioassays be used in the evaluation of crop allelopathic potential. Laboratory bioassays, field testing, and chemical screening are important steps, and none of them can be precluded if conclusive evidence of crop allelopathy is to be established. More concerted efforts are needed in screening crop germplasm before the development of allelopathic varieties occurs.

Allelopathic Potential in Rice Germplasm Against Ducksalad, Redstem and Barnyard Grass

Abstract: Summary Of 17,279 rice accessions or varieties from 110 countries in the USDA-ARS rice working collection, about 12,000 accessions in the rice collection have been evaluated for allelopathy to ducksalad (Heteranthera limosa[Sw.] Willd.) and about 5,000 have been evaluated for allelopathy to redstem (Ammannia coccinea Rottb.) and barnyard grass (Echinochloa crus-galli[L.] Beauv.). A total of 412 rice accessions were identified in field tests in 1988, 1989 and 1990 that had an area of allelopathic activity > 10 cm to ducksalad and 145 accessions that had the same area of activity to redstem. A total of 94 accessions demonstrated apparent allelopathic activity to barnyard grass. The rice accessions that showed allelopathic activity in the field had 2-3 times more root biomass than cultivars that did not show allelopathic activity. In barnyard grass infested plots the grain yield reduction of rice accessions that demonstrated allelopathy activity was about 37% as compared to a 60 to 68% reduction in grain yie...

Phytotoxicity of vulpia residues: IV. Dynamics of allelochemicals during decomposition of vulpia residues and their corresponding phytotoxicity.

Abstract: The behavior and dynamics of 20 identified allelochemicals in vulpia residues were both collectively and individually monitored, and their kinetic phytotoxicity was assessed The total content of the identified allelochemicals in decaying vulpia residues increased from 031 to 124 mg/g dry residue over a 21-day decomposition period, while the total phenolic content increased from 186 to 216 mg/g dry residue This corresponded to a phytotoxicity increase from 42% to 82% of radical inhibition Allelochemicals changed in composition and quantity over the duration of the residue decomposition Addition of soil to the residues reduced the total allelochemical contents extracted and altered the dynamic pattern In the same period, the total content of allelochemicals declined from 0061 to 0046 mg/g residue + soil, with the total phenolics reduced from 020 to 011 mg/g residue + soil, corresponding to a radical length increase from 53% to 109% of control Only 14 of the identified allelochemicals were detected in the mix of soil and residues, in contrast to 20 present in the residues alone The implications of these findings are discussed

Allelopathic activity of Mexican sunflower [Tithonia diversifolia (Hemsl.) A. Gray] in soil under natural field conditions and different moisture conditions

Abstract: The allelopathic activity of Mexican sunflower [Tithonia diversifolia (Hemsl.) A. Gray] in soil under natural field conditions and the effect of water stress on the growth and allelopathic activity of this plant were investigated. Seed germination, shoot growth and root growth of tested plant species in soil collected from a field where Mexican sunflower had been grown for 5 years were less than those in soil from an area without the plant. Growth of young leaves, mature leaves, senescent leaves, stem and roots of Mexican sunflower was reduced with decrease in soil moisture level. The allelopathic activity of water extracts (per dry weight of starting material) from each part of the plants grown at low soil moisture levels was greater than that of the water extracts from the same part of the plants grown at high soil moisture levels. The allelopathic activity was found in the soil from the pots previously planted with Mexican sunflower to a similar extent (per plant) at different soil moisture levels. These results suggested that, in the field, under water stress conditions, the growth of Mexican sunflower was reduced but the plants contained a greater amount of allelopathic substance(s) per dry weight than in the absence of water stress. It was considered that allelopathic activity of Mexican sunflower in soil was maintained to a similar extent under various soil moisture conditions in natural fields.

Allelopathic Effect of Glucosinolate- containing Plant Green Manure on Pythium sp. and Total Fungal Population in Soil

Abstract: Two Brassicaceae (Iberis amara L. selection ISCI14 and Rapistrum rugosum All. selection ISCI4) and a Capparidacea (Cleome hassleriana L. selection ISCI2) possessing glucosinolates whose degradation products exhibit high fungitoxic activity in vitro were assayed as biocidal plants in a green manure simulation. The trials were carried out in pots, using aboveground fresh plant tissues incorporated at a realistic field rate into soil naturally infected by Pythium sp. The effect of these plant tissues on total fungal populations and Pythium sp. were compared with Crambe abyssinica H. cv. Mario, a Brassicacea containing glucosinolates whose degradation products exhibit low fungitoxic activity in vitro, and a plant ( Helianthus annuus L.) not containing glucosinolates. All green manure treatments induced increases in total fungi over a 10-week period, showing an enhanced microflora level compared with untreated soil. Pythium sp. was strongly suppressed by the C. hassleriana, I. amara, and R. rugosum selections, while sunflower and crambe treatments increased Pythium sp. in a manner similar to that observed for total fungal population. These findings indicate that the green manures assayed suppress Pythium sp. and also induced an increase in total soil microbial activity.