Search

Background and aims: Nitrogen (N) deposition and native allelopathic plants may affect seed germination and growth of species through their effects on soil microbes and soil nutrient availability. However, our understanding of the interactions between N addition and allelopathic plants on the regeneration of alpine grasslands remains limited. Methods: Here, we investigated the effects of N addition and the presence of the allelopathic plant Ligularia virgaurea (Maxim.) Mattf. on seed germination, survival, and growth of native herbaceous species (Elymus nutans, Delphinium kamaonense and Tibetia himalaica) on the Qinghai-Tibet Plateau. We used piecewise structural equation modelling to assess both the direct effects of N addition, allelopathic plants, and their interactions and indirect effects mediated by soil properties, soil microbial richness and diversity, and soil enzyme activity. Results: We found that (1) L. virgaurea directly increased seed germination and early plant survival, and reduced plant root-to-shoot ratio; N addition directly increased early plant survival and biomass, (2) L. virgaurea indirectly increased plant biomass via bacterial richness, (3) N addition offsets the increase in seed germination promoted by the presence of L. virgaurea via soil acid phosphatase. Conclusion: Our study suggests the importance of direct and indirect roles of allelopathic plants, N addition and their interaction on seed germination, survival and plant growth. Our results highlight the need to consider the interactions between environmental and biological factors as well as their direct and indirect effects to obtain reliable predictions and mechanistic understanding of the response of alpine plants to future climate change. [ABSTRACT FROM AUTHOR]

Background and aims: Biological nitrification inhibition (BNI) is a trait that could improve nitrogen-use efficiency of a crop. We studied varietal differences in BNI potential of rice at the vegetative phase when plants were subjected to water stress. Methods: We obtained water-soluble root exudates (RE) and water-extracts of crushed root tissues (RT) from two-week-old seedlings of five rice cultivars grown continuously under adequate water or under water stress and assessed their ability to suppress the activity of Nitrosomonas europaea and the growth of lettuce seedlings. We also investigated how growing rice under continuously saturated (TC) and unsaturated (TU) soil moisture conditions affected potential nitrification rate (PNR) and community structure of ammonia-oxidizing bacteria (AOB) in the rhizosphere compared with bulk soil. Results: We observed that only RT at ≥ 0.50 mg ml−1 suppressed the activity of N. europaea with significant (p < 0.05) differences among rice cultivars. Subjecting rice cultivars PI312777 and PI338046 to water stress significantly (p < 0.05) increased BNI of RT. Inhibition of lettuce root growth by RT was strongly correlated with its BNI (r = 0.83, p < 0.05). In T-RFLP profiles of AOB, the relative abundance of T-RF R205 was lower in TU than in TC in all cultivars except Rexmont. Conclusions: The rice cultivars studied produced compounds that could inhibit ammonia oxidizers, but the degree of BNI differed based on the moisture stress experienced by the crop. Lineages of rice cultivars with known allelopathic potential would be promising candidates to consider for BNI potential. [ABSTRACT FROM AUTHOR]

Progress in understanding allelopathic interactions among plants has long been hampered by the complexity of the many direct and indirect interactions involved. Plant processes and growth are not only affected by allelochemicals but by resource limitations, pathogens, herbivores, and microbial interactions. Interference mechanisms frequently interact, and the magnitude of effects can depend on the plant's biotic and abiotic environment. The rhizosphere is chemically complex, with thousands of potentially bioactive allelochemicals produced by plants, microorganisms and soil invertebrates. The rhizosphere is also dynamic, in that concentrations of these metabolites vary as pulses of allelochemicals are released by plant roots and other organisms, as they leach from decaying plant material, as microorganisms degrade and sometimes transform allelochemicals, and as allelochemicals are taken up by plants, bind to soil components, or leach from the root zone. Recent advancements in instrumentation and technologies for the analysis of trace levels of chemical substances in soil, as well as the development of genomics, proteomics, and transcriptomics approaches allow researchers to probe both the biosynthesis of allelochemicals and plant responses to allelochemical exposures. These new technologies will provide much more detailed information on rhizosphere chemistry and about the production and response to metabolites by individual cells. This review describes case studies and current examples that illustrate how these new approaches and tools can enhance our understanding of allelopathic interactions, and argues that to truly advance our understanding of allelopathic interactions, these must be applied in ecologically rigorous and meaningful ways. [ABSTRACT FROM AUTHOR]

Background and aims: Host-specific Epichloë spp. are endophytic fungal symbionts of pooid grasses that produce herbivore-deterring alkaloids and alter the host's metabolite and protein profiles. Early observations suggested that Epichloë may have negative allelopathic effects on neighbouring plant species, particularly Trifolium spp. clovers, but subsequent allelopathy experiments produced variable results. We examined two hypotheses: (1) Epichloë strains differ in allelopathic effect, and (2) Epichloë allelopathy negatively affects other plant species. Methods: We performed a greenhouse experiment using root exudates from Lolium perenne L. hosting different E. festucae var. lolii (Latch, M.J. Chr. & Samuels) C.W. Bacon & Schardl strains to compare their allelopathic effects on native legumes and forbs. We then used meta-analysis to examine the evidence to date for allelopathic effects of Epichloë endophytes. Results: We found little difference in effect among E. festucae var. lolii strains and very little evidence for negative allelopathic effects of Epichloë in cool-season grasses across a range of methodologies, target plant species, and response measures. Negative allelopathic effects were detected only for root hair measures, which were from a single study. Positive effects on biomass were found for some experimental subgroups, including legumes. However, the majority of response variables showed no evidence for Epichloë allelopathy. Conclusions: Although there is currently little evidence for negative Epichloë allelopathic effects, our meta-analysis identified several research gaps. Experiments testing the functional belowground effects of Epichloë presence may help to determine its effects on non-host plant performance via plant-soil feedbacks. [ABSTRACT FROM AUTHOR]

Background: Plants condition the soil in which they grow, thereby altering the performance of subsequent plants growing in this soil. This phenomenon, known as plant-soil feedback (PSF), has garnered increasing interest. Experiments are moving from single species soil pairings in the glasshouse to community-level field trials. Consequently, our knowledge of the role PSF plays in shaping ecosystem functions has advanced. However, knowledge gaps remain. Scope: Here, we explore intrinsic and extrinsic abiotic and biotic drivers of PSF such as maternal effects, plant functional traits, self-DNA, plant-plant competition, herbivory, interactions between soil organisms, temperature, drought, flooding, greenhouse gases, (micro)nutrients, plant-litter-soil feedback and priority effects. These drivers have begun to feature in experiments, thereby increasing our mechanistic understanding of PSF. Nonetheless, many of these topics have received insufficient coverage to determine general principles across larger temporal and spatial scales. Further, conflicting terminology has excluded PSF studies from reviews and meta-analyses. We review terms such as soil sickness, Janzen-Connell hypothesis, soil-related invasive species work, soil legacies, allelopathy and soil-related succession that overlap with PSF but are generally not named as such. Conclusion: Holistic experimental designs that consider the continual reciprocal feedback between the extrinsic environment, plants and soil, as well as the unification of terminologies are necessary if we are to realise the full potential of PSF for understanding and steering ecosystem processes. Here, we compile outstanding questions related to PSF research that emphasis the aforementioned topics and suggest ways to incorporate them into future research in order to advance plant-soil ecology. [ABSTRACT FROM AUTHOR]

Aims: Soil legacies mediate interactions between native and introduced plants, contributing to both invasion and biotic resistance to invasion. Given that nitrogen deposition can promote allelochemical release, reduce the benefits of soil microbes, and affect trait plasticity, nitrogen deposition likely alters soil legacies as well. However, it is not clear how mechanisms that facilitate adaptation to soil legacies are altered by nitrogen deposition. Methods: In a greenhouse setting, we investigated how an invasive and a native plant in northern China (Rhus typhina and Ailanthus altissima, respectively) acclimate to soil legacies and how these dynamics change with nitrogen availability. We measured plant functional traits, soil microbial abundance, microbial enzyme activities, and soil allelopathic effects to characterize plant responses to soil legacies from plants of the same and of the other species. Results: Rhus typhina had a stronger growth response to soil legacies than did A. altissima. Rhus typhina established a novel plant-soil feedback by increasing fungi and bacteria, changing the composition of the microbial community, and effectively transforming negative effects of soil allelopathy to positive effects. Nitrogen deposition promoted the growth of R. typhina and alleviated the negative effects of heterospecific soil legacies on the performance of R. typhina. Conclusions: Invasive plants can acclimate to the soil legacies of native species through a combination of high trait plasticity, manipulating soil microbes, and establishing novel plant-soil feedbacks. Nitrogen deposition can facilitate invasive species acclimating to soil legacies by monopolizing nitrogen absorption, though this may diminish the benefit of soil microbes. [ABSTRACT FROM AUTHOR]

Background and aims: Weeds are a major biotic stressor impacting crop production. Improving the competitiveness of wheat (Triticum aestivum L.) could provide a useful tool in integrated weed management. While wheat typically exhibits conservative early growth, early vigour has been increased through long-term recurrent selection for greater early biomass and leaf area. However, the influence of integrating such vigour into breeding lines for improving competitive ability remains to be explored.In replicated controlled environment experiments, the effect of breeding early shoot vigour on root development and below-ground competitiveness was carefully examined. Physical and chemical characteristics of wheat vigour lines were assessed and compared with commercial cultivars in hydroponics and field soil experiments. Measurements included early root growth, rhizosheath size and growth responses in the presence of annual ryegrass, a major weed in wheat production.The vigorous lines exhibited larger leaf widths, increased cell file number, increased total root length and larger rhizosheaths compared with commercial parents. Numerous secondary metabolites with known allelopathic effects on weeds were detected in the roots and the rhizosphere, and significant allelochemical level differences observed between distilled water and soil water extract-treated plants. Although the vigour lines were significantly more competitive than the commercial cultivars against ryegrass, they produced similar levels of phytotoxic secondary metabolites.Competition below-ground was strongly suppressive of ryegrass for the more vigorous genotypes suggesting that breeding with shoot vigour had pleiotropic effects on key root traits for below-ground wheat competitiveness.Methods: Weeds are a major biotic stressor impacting crop production. Improving the competitiveness of wheat (Triticum aestivum L.) could provide a useful tool in integrated weed management. While wheat typically exhibits conservative early growth, early vigour has been increased through long-term recurrent selection for greater early biomass and leaf area. However, the influence of integrating such vigour into breeding lines for improving competitive ability remains to be explored.In replicated controlled environment experiments, the effect of breeding early shoot vigour on root development and below-ground competitiveness was carefully examined. Physical and chemical characteristics of wheat vigour lines were assessed and compared with commercial cultivars in hydroponics and field soil experiments. Measurements included early root growth, rhizosheath size and growth responses in the presence of annual ryegrass, a major weed in wheat production.The vigorous lines exhibited larger leaf widths, increased cell file number, increased total root length and larger rhizosheaths compared with commercial parents. Numerous secondary metabolites with known allelopathic effects on weeds were detected in the roots and the rhizosphere, and significant allelochemical level differences observed between distilled water and soil water extract-treated plants. Although the vigour lines were significantly more competitive than the commercial cultivars against ryegrass, they produced similar levels of phytotoxic secondary metabolites.Competition below-ground was strongly suppressive of ryegrass for the more vigorous genotypes suggesting that breeding with shoot vigour had pleiotropic effects on key root traits for below-ground wheat competitiveness.Results: Weeds are a major biotic stressor impacting crop production. Improving the competitiveness of wheat (Triticum aestivum L.) could provide a useful tool in integrated weed management. While wheat typically exhibits conservative early growth, early vigour has been increased through long-term recurrent selection for greater early biomass and leaf area. However, the influence of integrating such vigour into breeding lines for improving competitive ability remains to be explored.In replicated controlled environment experiments, the effect of breeding early shoot vigour on root development and below-ground competitiveness was carefully examined. Physical and chemical characteristics of wheat vigour lines were assessed and compared with commercial cultivars in hydroponics and field soil experiments. Measurements included early root growth, rhizosheath size and growth responses in the presence of annual ryegrass, a major weed in wheat production.The vigorous lines exhibited larger leaf widths, increased cell file number, increased total root length and larger rhizosheaths compared with commercial parents. Numerous secondary metabolites with known allelopathic effects on weeds were detected in the roots and the rhizosphere, and significant allelochemical level differences observed between distilled water and soil water extract-treated plants. Although the vigour lines were significantly more competitive than the commercial cultivars against ryegrass, they produced similar levels of phytotoxic secondary metabolites.Competition below-ground was strongly suppressive of ryegrass for the more vigorous genotypes suggesting that breeding with shoot vigour had pleiotropic effects on key root traits for below-ground wheat competitiveness.Conclusion: Weeds are a major biotic stressor impacting crop production. Improving the competitiveness of wheat (Triticum aestivum L.) could provide a useful tool in integrated weed management. While wheat typically exhibits conservative early growth, early vigour has been increased through long-term recurrent selection for greater early biomass and leaf area. However, the influence of integrating such vigour into breeding lines for improving competitive ability remains to be explored.In replicated controlled environment experiments, the effect of breeding early shoot vigour on root development and below-ground competitiveness was carefully examined. Physical and chemical characteristics of wheat vigour lines were assessed and compared with commercial cultivars in hydroponics and field soil experiments. Measurements included early root growth, rhizosheath size and growth responses in the presence of annual ryegrass, a major weed in wheat production.The vigorous lines exhibited larger leaf widths, increased cell file number, increased total root length and larger rhizosheaths compared with commercial parents. Numerous secondary metabolites with known allelopathic effects on weeds were detected in the roots and the rhizosphere, and significant allelochemical level differences observed between distilled water and soil water extract-treated plants. Although the vigour lines were significantly more competitive than the commercial cultivars against ryegrass, they produced similar levels of phytotoxic secondary metabolites.Competition below-ground was strongly suppressive of ryegrass for the more vigorous genotypes suggesting that breeding with shoot vigour had pleiotropic effects on key root traits for below-ground wheat competitiveness. [ABSTRACT FROM AUTHOR]

Aim: The main objective of this study was to evaluate the influence of leachates from three typical boreal forest ground layers on young tree growth and to explore the linkages between the chemical composition of the leachates, tree growth, the allocation between belowground and aboveground parts, and ectomycorrhizal colonization. Methods: An original 6-month greenhouse experiment was set up to investigate (i) the effects of lichen (Cladonia spp.) and feather moss (Pleurozium schreberii [Brid.] Mitt.) leachates on jack pine (Pinus banksiana Lamb.) growth and (ii) the effects of feather moss and Sphagnum spp. leachates on black spruce (Picea mariana [Mill.] B.S.P.) growth. Results: Belowground growth and root allocation was reduced by lichen leachates in 2-year-old pine seedlings, while the impact was significant on both below- and aboveground growth in 6-month-old pine seedlings. A substance having the same migration time as usnic acid was detected in the lichen leachates by high-performance liquid chromatography. Sphagnum spp. and feather moss leachates were not found to have any effect on black spruce seedling growth, despite a higher supply of dissolved inorganic N in the feather moss leachates compared to the leachates of Sphagnum spp. and the control. Conclusions: These results demonstrate that ground layer composition plays a crucial role in shaping the plant community in boreal ecosystems by influencing the chemical composition of the soil solution. They suggest that chemical interference may be another mechanism by which lichens promote the self-perpetuation of open woodlands in the closed-crown boreal forest. [ABSTRACT FROM AUTHOR]

Aims: The incidence of herbicide-resistant blackgrass is escalating in wheat fields; the development of alternatives to traditional herbicides is crucial. Allelopathic wheat can suppress blackgrass. Herein, we investigated the influence of allelopathic wheat on herbicide-resistant blackgrass. Methods: Mesosulfuron-methyl-resistant and -susceptible blackgrass were used. We examined i) root interactions between allelopathic wheat and both blackgrass biotypes, ii) allelochemical 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA) production by allelopathic wheat in a root segregation experiment, and iii) responses of allelopathic wheat to root exudates. Signal chemicals in the root exudates of both blackgrass biotypes were characterised. Results: Allelopathic wheat inhibited the growth of roots more than shoots for resistant and susceptible blackgrass. Allelopathic wheat inhibited roots of resistant blackgrass more than those of susceptible blackgrass. Allelopathic wheat significantly shaped blackgrass root behaviour. Resistant blackgrass caused a lower relative increase in DIMBOA production by allelopathic wheat. Root segregation with 30 μm nylon led to greater growth inhibition of blackgrass and a relative increase in allelochemical DIMBOA. Root exudates from blackgrass induced DIMBOA production, but more so for susceptible blackgrass. The concentration of (−)-loliolide, a potential signal chemical, was much lower in the root exudates from resistant than susceptible blackgrass. Conclusions: Allelopathic wheat can interfere with the growth of mesosulfuron-methyl-resistant blackgrass through allelochemical-mediated root interactions. Such allelopathic interference can provide insight into weed management. [ABSTRACT FROM AUTHOR]

Aims: Soil temperature and moisture impact plants not only during growth and survival but also during seed germination and interaction of seeds with the chemical environment. The quantitative impacts of either temperature and moisture or plant specialized metabolites (PSM) on germination are widely studied. However, the combined effect of PSM and moisture or temperature on germination remains poorly understood. Methods: We addressed this issue by studying the effect of PSM extracted from four Mediterranean woody plants on germination speed and final percentages of a subordinate herbaceous plant, Linum perenne. Results: By using hydro- and thermal time threshold models, we show how PSM interact with temperature and moisture levels to limit germination at dry and upper thermal limits, with the magnitude of effects depending on the source plant. PSM effects on germination, also observed on natural soils, persisted after their removal from the seed environment. Conclusions: We conclude that the impact of climate change on reproduction of herbaceous plants can be modulated by effects of PSM from woody plants, which might exacerbate the negative impacts of global changes on biodiversity. [ABSTRACT FROM AUTHOR]

Aims: Separating the effect of resource competition from allelopathy in plants is challenging and it has never been attempted in closely related co-occurring bryophytes. In peatlands, peat mosses (Sphagnum spp.) show niche differentiation along water table level (WTL) gradient. Our aim was to evaluate whether the hummock species, S. magellanicum would be a winner at low WTL due to its allelopathic advantage and the hollow species, S. angustifolium would win by virtue of its superior competitive ability but not of allelopathy at high WTL due to dilution of its allelochemicals. Methods: We used a nested, field experimental design, with two WTL treatments—low WTL (hummock habitat) and high WTL (hollow habitat)—and three different inter-specific interactions: 1) monoculture; 2) mixed culture without activated charcoal; and 3) mixed culture with activated charcoal added to the neighbor. We measured growth and biochemical traits of the two species and compared the index of relative neighbor effect on each other. Results: We discovered a trade-off between biomass production (competitive outcome) and phenolic content (allelopathy) in these species. At low WTL, allelopathy of the hummock species is the main mechanism to suppress the hollow species, whereas at high WTL, competition is the main driver to suppress the hummock species. Conclusions: Competitive advantage in Sphagnum is mediated by both resource competition and allelopathy of the co-occurring species through niche differentiation along a WTL gradient. Unlike vascular plants, Sphagnum mosses can serve as excellent model organisms in studying allelopathic interaction since they bypass the complexity of plant-soil interactions. [ABSTRACT FROM AUTHOR]

Background: Allelopathy is an ecological phenomenon consisting of both positive and negative effects between organisms determined by the release of secondary metabolites into the environment. Root exudation represents the most important pathway of releasing allelochemicals. Once released into the soil, allelochemicals interact with the organic and inorganic soil phases, as well as with soil microorganisms. The set of these interactions fix allelochemicals bioavailability and phytotoxic level. Scope: Here we critically review the interactions between plant allelochemicals and physical, chemical and biological soil characteristics by reporting the literature available and pointing out both positive and negative relationships affecting allelochemicals phytotoxicity and nutrient availability. In addition, we have reported a qualitative balance of allelochemicals in the soil. Thirdly, we reviewed the exudation process of allelochemicals and the transport mechanisms across plasma membranes. Conclusions: A two-way relationship exists between soil characteristics and allelochemicals. The level of phytotoxicity is not affected only by a single soil characteristic, but they are closely linked to each other and exert a multiple-effect on retention, transport and transformation processes of allelochemicals in soil. Further efforts are needed to better understand the interactions involved in soil allelopathy and to create new opportunities for a sustainable control of agroecosystems. [ABSTRACT FROM AUTHOR]

Background and aims: The yield and quality of medicinal tubers of Rehmannia glutinosa are seriously restricted by replanting problems. However, the origin and identification of the potential allelochemicals associated with this issue have not been well described. Therefore, in a series of related studies, we aimed to identify the key phytotoxic metabolites released from R. glutinosa. Methods: R. glutinosa residues were harvested and extracted for metabolic profiling and bioassay-directed identification. Metabolites in the leaf, tuber, and soil surrounding R. glutinosa roots were profiled using high pressure liquid chromatography coupled to a quadrupole/time of flight mass spectormeter. A Petri dish-filter paper and modified Parker bioassay were used to evaluate the phytotoxicity of plant residues and candidate allelochemicals. Results: Seventeen metabolites specific to R. glutinosa were identified/annotated using non-targeted metabolic profiling combined with known standards. Assay results suggested an iridoid glycoside and several phenylethanoid glycosides as key allelochemicals originating from R. glutinosa. Conclusion: Identification of bioactive metabolites in planta and in the rhizosphere has increased our understanding of the phytochemistry of R. glutinosa. Further evaluation of the soil persistence and ecological functions of these bioactive compounds will be important for determination of the specific mechanisms associated with R. glutinosa replanting issues. [ABSTRACT FROM AUTHOR]

Background and aims: Arbuscular mycorrhizal fungi (AMF) play important roles in plant community structure and ecosystem functioning. The allelopathic disruption of arbuscular mycorrhizal mutualism is a potentially powerful mechanism by which non-native species can negatively impact native plant diversity. However, there is limited understanding of this mechanism on woody species in forest ecosystems. In this study, we carried out a set of experiments to explore the allelopathy of Eucalyptus urophylla on mycorrhizal mutualists. Methods: First, we examined allelopathic effects on the mycorrhizal growth responses of woody species by collecting leachates from the understory of an E. urophylla plantation. Second, we examined if AMF could counteract the allelopathy of E. urophylla by treating the target species with and without aqueous extract of E. urophylla and AMF inoculum. We conducted a third in vitro experiment to characterize the effects of identified putative allelochemicals (IPAs) of E. urophylla on AMF spore germination, namely Glomus mosseae, Claroideoglomus etunicatum and mixed spores extracted from field soil. Results: There was a positive correlation between the stimulatory effects of natural leachates of E. urophylla and the mycorrhizal growth responses of target woody species. AMF could counteract the negative impact of E. urophylla allelopathy at a relatively low concentration. The IPAs of E. urophylla had variable effects on germination of AMF spores, from stimulatory to inhibitory, depending on chemical types and AMF species. Moreover, the AMF G. mosseae was the least sensitive to allelopathic inhibition of E. urophylla IPAs. Conclusions: This study does not support the hypothesis that allelopathy degrades mycorrhizal symbioses. Our study first provides preliminary evidence for a positive correlation between allelopathy and mycorrhizal growth responses and suggests that higher mycorrhizal growth responses could better protect woody species from allelopathic inhibition in E. urophylla plantations. [ABSTRACT FROM AUTHOR]

Aims: To test the effects of characteristic ecological gradients in peatlands including oxygen-deficiency and allelopathy on Sphagnum spore persistence.Methods: We determined the initial viability of Sphagnum spores and then stored the spores for 60 days, either dry, in ultrapure water, peatland surface water or Sphagnum water leachate (the latter two solutions rich in allelochemicals from Sphagnum). We varied oxygen concentration by injecting air at three different concentrations during the storage experiment. After retrieval from experimental storage, spore germinability index was assessed.Results: Germinability index after air injection was lower than under oxygen-deficiency. Germinability index in the peatland surface water and Sphagnum leachate water was higher than in ultrapure water, under oxygen-deficiency. Overall, dissolved oxygen and high pH negatively affected spore germinability and, consequently, viability.Conclusions: These results indicate that once dispersed onto bryophyte substrates or waterlogged hollows, Sphagnum spores can maintain viability better than when exposed to dry conditions or in water without allelochemicals. Extreme longevity of Sphagnum spores may be attributed to the oxygen-deficient, waterlogged and acidic Sphagnum-dominated peatland habitat with allelopathic substrates. [ABSTRACT FROM AUTHOR]

Background & Aims: Oak seedling establishment is difficult and may be partly explained by litter-mediated interactions with neighbors. Litter effects can be physical or chemical and result in positive or negative feedback effects for seedlings. Mediterranean species leaves contain high levels of secondary metabolites which suggest that negative litter effects could be important.Methods: Seedlings of Quercus ilex and Quercus pubescens were grown for two years in pots with natural soil and litter inputs from 6 Mediterranean woody species, artificial litter (only physical effect) or bare soil.Results: Litter types had highly different mass loss (41-80%), which correlated with soil organic C, total N and microbial activity. Litter of Q. pubescens increased soil humidity and oak seedlings aerial biomass. Litters of Cotinus coggygria and Rosmarinus officinalis, containing high quantities of phenolics and terpenes respectively, decomposed fast and led to specific soil microbial catabolic profiles but did not influence oak seedling growth, chemistry or mycorrhization rates.Conclusions: Physical litter effects through improved soil humidity seem to be predominant for oak seedling development. Despite high litter phenolics content, we detected no chemical effects on oak seedlings. Litter traits conferring a higher ability to retain soil moisture in dry periods deserve further attention as they may be critical to explain plant-soil feedbacks in Mediterranean ecosystems. [ABSTRACT FROM AUTHOR]

Background and aim: Fires affect what happens to litter in ecosystems. Biological and chemical effects of burnt litter on plants are not as of yet fully understood. We aimed to assess the effects of heat-treated leaf litter on germination and seedling root growth of Eucalyptus globulus.Methods: Litter from E. globulus, Acacia dealbata, Pinus pinaster, and Quercus suber was collected in Portugal, on Humic Cambisol, and heated between 25 °C and 600 °C. Those materials were then characterized by: 13C CPMAS NMR spectroscopy, proximate lignin and cellulose, and elemental analyses. Afterwards, they were used as substrate in bioassays with Eucalyptus seeds.Results: Heating changed litter composition (P < 0.05), consistently across species: alkyl C, O-alkyl C, and methoxyl + N-alkyl C decreased more than 50%; and aromatic C increased more than 5-fold. Unheated and lightly heated litters inhibited germination and growth, with maximum inhibition by Quercus and Eucalyptus litters, down to 17% of the control (P < 0.05). Severely charred materials had neutral or stimulatory effects, up to 191% of the control (P < 0.05). These responses were associated with concentrations of dominant C types in litters (P < 0.05).Conclusion: Litter charring, as it occurs during wildfires, is potentially critical for the success of E. globulus’s regeneration from seeds. [ABSTRACT FROM AUTHOR]

Background and aims: Invasive weeds may exert negative impact on other plant species and soil processes. The observed negative impact of an invasive weed species may be driven by allelopathy or nutrient availability. Methodology: Sorghum halepense is one of the worst invasive weeds in crop fields. We quantified the species richness in the S. halepense-invaded communities and communities not yet invaded by the weed. Sorghum soil and no-sorghum soil were analysed for total phenolics, microbial activity, available nitrogen (N) and organic carbon. Manipulative experiments were carried out to understand the interference potential of S. halepense. Soil was amended with root or shoot leachate of S. halepense, and its impact on plant growth and soil properties was studied. Results: Out of four S. halepense-sites, lower plant species richness was observed in one site compared to uninvaded sites. S. halepense-invaded soil had higher levels of total phenolics and lower levels of available N. Higher inhibition in the root growth of Brassica juncea or Bidens pilosa was observed in root leachate-amended soil than shoot leachate-amended soil. Shoot leachate-amended soil had higher levels of total phenolics and available N than root leachate-amended soils. Significant reduction in the available N was observed in soil amended with root leachate. Significant decline in the total phenolics over a period of time was observed in soil amended with root leachate or shoot leachate of S. halepense. Higher CO release was observed 24 h after amending soil with root leachate or shoot leachate of S. halepense. Conclusions: Sorghum halepense interference potential in its soil is likely due to lower levels of available N. Greater reduction in root dry weight of assay species in root leachate amended soil compared to shoot leachate amended soil was likely due to lower levels of available N in root leachate-amended soil. Relative interference potential of both root and shoot leachates or extracts should be evaluated in allelopathy bioassays and further experiments should be designed to distinguish the role of allelochemicals and nutrient availability in plant growth inhibition. [ABSTRACT FROM AUTHOR]

Background and aims: Belowground competition and allelopathic interference of neighbouring plants play important roles in shaping plant performance. We assessed the effect of belowground interactions involving root competition, root exudates and ectomycorrhizal (ECM) association between Kalmia and black spruce on above- and belowground biomass increment and foliar nutrient content of black spruce seedlings. We predicted that root competition and root exudates of Kalmia would exert a negative impact on black spruce performance, while ECM in black spruce would exert positive impacts on its performance. Methods: In a greenhouse experiment, using the neutral growth medium of peat-vermiculite, we planted Kalmia and black spruce seedling in pairs. We controlled root competition by placing fine wire mesh between the seedlings and applied benomyl fungicide to control ectomycorrhizae. We used activated charcoal to neutralize the potential allelopathic effects associated with root exudates. After 8 months, we evaluated the performance of black spruce seedlings in terms of biomass, abundance of ectomycorrhizal short roots and foliar nutrient concentrations. To determine treatment effects on soil processes, we also analysed soil pH, total phenols, C:N ratio and bioavailable nitrogen (NO− and NH+) at the end of the experiment. Results: We found a significant effect of root competition on root-shoot ratio and biomass increment in black spruce seedlings, which achieved the highest root-shoot ratio and biomass increment when major root competition was controlled. Kalmia produced twice the root biomass than black spruce. Surprisingly, charcoal and benomyl treatments-which remove the effects of Kalmia allelopathy on black spruce and suppress ECM growth, respectively-had little or no significant effect on overall biomass of black spruce. In contrast, Kalmia biomass increment and root-shoot ratio were maximized in benomyl treatment. Soil pH and C:N ratio were significantly higher in free competition than root exclusion and benomyl treatments and with activated charcoal. Conclusions: Kalmia root competition mediated by high root biomass is the predominant driver shaping neighbourhood interactions between Kalmia and black spruce, which in turn shape the performance of black spruce seedlings. Root exudates of Kalmia have a very weak effect on black spruce seedling growth. [ABSTRACT FROM AUTHOR]

Background and aims: Bothriochloa spp. are non-native warm-season grasses invading native grasslands of the southern and central Great Plains, altering ecological services these grasslands supply. Our study investigated potential allelopathic effects of the invasive grass species B. ischaemum on native grass germination, growth, and survival. Methods: Leachate or litter from Andropogon gerardii (native) or B. ischaemum were applied to two native grass species ( A. gerardii; Schizachyrium scoparium). Leachate and litter were also added to B. ischaemum and a water control was included. Germination, above- and belowground biomass, and survival were determined. Results: Application of B. ischaemum leachate or litter significantly reduced the germination, growth, and survival of both A. gerardii and S. scoparium but had no conspecific effects, while A. gerardii treatments had no effect on any species. Conclusions: Bothriochloa spp. may gain a competitive advantage through allelopathic biochemicals. It is unclear if these allelopathic effects directly hinder competitors or indirectly hinder them through alterations in soil microbial communities, however, reductions in germination of native seeds strongly support direct allelopathic effects. Greater phenolic content in native grass leachates suggest allelopathic biochemical production may not be unique to non-native species and may be a mechanism for maintenance of plant species biodiversity in native systems. [ABSTRACT FROM AUTHOR]

Aims: The allelopathic activity of canola ( Brassica napus) germplasm was investigated using the important Australian weed, annual ryegrass ( Lolium rigidum) as the target species. Methods: Three different canola plant densities (10, 20, and 30 seedlings/beaker) of each of 70 world-wide genotypes were tested in vitro in close proximity to annual ryegrass seedlings. Results: The allelopathic activity of canola, as measured by reduction in annual ryegrass root and shoot growth, increased with canola crop seedling densities. Density did not consistently influence shoot length of annual ryegrass. Greater shoot length suppression was observed in genotype cv. Rivette and BLN3343CO0402. The Australian genotype cv. Av-opal and the breeding line Pak85388-502 suppressed root length of ryegrass more than other genotypes, even at low densities. At the lowest density, the least allelopathic genotypes were cv. Barossa and cv. Cescaljarni-repka, although they became more allelopathic at higher density. An overall inhibition index was calculated to rank each of the canola genotypes. There were significant differences between canola genotypes in their ability to inhibit root and shoot growth in ryegrass. Conclusion: Considerable genetic variation exists among canola genotypes for their allelopathic effects on annual ryegrass. Further investigation is required to determine the allelopathic mechanisms, particularly to identify the responsible allelochemical(s) and the gene(s) controlling the trait. This research suggests that highly allelopathic canola genotypes can be potential for controlling weeds such as annual ryegrass in integrated weed management programs. [ABSTRACT FROM AUTHOR]

Background and Aims: Soil micro-circumstance and biological stress resistance were studied to validate our hypothesis that the allelopathic potential that was enhanced by breeding resulted partially from rhizophere microbes associated with the different varieties. Methods: The rhizosphere soils from four wheat genotypes with different allelopathic potential were collected so as to compare their soil micro-environments and bio-pressure tolerances. Results: The levels of these three categories such as bacteria, fungi, and actinomycetes ranged among 1.54-26.59 × 10, 0.43-4.12 × 10, and 1.36-18.25 × 10 CFU/g soil, respectively. Wheat 22 Xiaoyan with greater allelopathic potential had higher levels of microorganisms than the other three genotypes having weak allelopathy. The soil microbial carbon and nitrogen analyses suggested that wheat could create an active microhabitat with high activities of key soil enzymes such as urease, catalase, sucrase, and dehydrogenase. Using the approximate concentrations detected in wheat rhizosphere soils, the leachates of all four wheat materials significantly inhibited the growth of the weed Descurainia sophia and take-all pathogen Gaeumannomyces graminis var . Tritici. Conclusions: Wheat exudates provided carbon and nitrogen resources for the relevant microorganism. Meanwhile, the rhizosphere soil microbes contributed to allelopathic potential of wheat by positive feedback. [ABSTRACT FROM AUTHOR]

Aims: We compared the degree of arbuscular mycorrhizal fungi (AMF) colonization on an invasive, Triadica sebifera, and two native, Baccharis halimifolia and Morella cerifera, woody species that occur in coastal Mississippi, USA. Specifically, we investigated how the degree of colonization affects growth of these species and assessed potential allelopathic effects of T. sebifera on the two native species. Methods: A field study and a greenhouse experiment were used to determine the degree of AMF colonization on focal woody species. Seedling growth of these species was compared between active (fungicide untreated) and AMF-suppressed (fungicide treated) soils in the greenhouse experiment. In a second greenhouse experiment, we tested the potential allelopathic effects of T. sebifera on the native species by growing the natives in soils from T. sebifera invaded or uninvaded regions, with and without activated carbon (AC). Results: The invasive species, Triadica sebifera, exhibited a higher degree of AMF colonization compared to the native species and significantly higher total biomass in active soils compared to AMF-suppressed soils. Seedling biomass and AMF colonization of native B. halimifolia and M. cerifera did not differ between T. sebifera invaded and uninvaded soils, irrespective of the application of AC. Conclusions: Results suggest that invasive T. sebifera benefits from the presence of AMF, which might facilitate establishment of this invader. Results also suggest that allelopathy is not a likely mechanism of T. sebifera invasion in coastal transition ecosystems. A higher degree of AMF colonization, relative to native co-occurring species, may partly explain the successful invasion of T. sebifera into coastal plant communities of the southeastern USA. [ABSTRACT FROM AUTHOR]

Background and aims: Successful invasion by exotic plant species can modify the abundance and composition of soil microbial communities. Eupatorium adenophora and Chromolaena odorata are exotics that have become highly invasive plants in China. Several studies have investigated mechanisms of their successful invasions including phenotypic plasticity, genetic differentiation, and allelopathy, but little is known about their effects on soil microorganisms. Moreover, whether soil microbial community changes could cause feedback effects on these plant species is also not known. We seek a belowground microbiological mechanism supporting successful invasions by these exotic plants. Methods: In this study, two invasive ( E. adenophora and C. odorata) and two native plant species ( Eupatorium japonicum and Eupatorium heterophyllum) were used to compare the soil feedback (on plant growth) before and after soil sterilization and from plant-root exudates. Bacterial and fungal biomass and community composition were also examined. Results: We found that soil sterilization significantly increased biomass of native species and did not affect the invasive species' biomass. After root exudates from these plants had acted on the soil biota for 10 months, soil sterilization significantly decreased the growth of E. adenophora and C. odorata and continued to significantly increase the biomass of two native species. Denaturing gradient gel electrophoresis revealed that these four plant species modified fungal rather than bacterial communities in soil. Conclusions: Higher abundance of Paraglomus sp. in soil with C. odorata is likely to provide C. odorata roots with more soil nutrients. Considered together, these results strongly suggest that invasive E. adenophora and C. odorata created a belowground feedback that may be a mechanism contributing to their success as invasive species. [ABSTRACT FROM AUTHOR]

Aims: Weed control in rice is challenging, particularly in light of increased resistance to herbicides in weed populations including Echinochloa crus- galli (L.) Beauv. Indica rice cultivars can produce high yields and suppress barnyardgrass, but have not been commercially acceptable in the U.S. due to inferior agronomic traits and grain quality. Our objectives were to combine high yield and weed-suppressive characteristics from indica cultivars with commercially acceptable grain quality and plant types from long-grain cultivars grown in the southern U.S. Methods: Crosses between indica and commercial tropical japonica (cv. Katy, and cv. Drew) rice were evaluated for weed suppression and agronomic traits in a breeding program. Results: In some tests, the selection STG06L-35-061 was nearly as weed suppressive as PI 312777, the suppressive parent, and more suppressive than its tropical japonica parents. Its main crop yield is commercially acceptable, and intermediate between PI 312777 and Katy. Its milling quality and cooking quality are similar to long-grain commercial cultivars, and it has resistance to rice blast disease. Marker analyses identified introgressions from the indica parents on chromosomes 1 and 3 of STG06L-35-061 that require further analysis as possible sources of weed suppressive traits. Conclusions: STG06L-35-061 might be suitable for organic rice or reduced input conventional systems. [ABSTRACT FROM AUTHOR]

Aims: The possible involvement of the chemical-mediated interaction in allelopathy between rice and barnyard grass was investigated. Methods: Effcts of rice seedlings and rice root exudate on the alleloapthic activity of barnyard grass were determined and a key compound invovled in the allelopathic interaction between rice and barnyard grass was isolated. Results: Allelopathic activity of barnyard grass was increased by the presence of rice seedlings. Rice root exudates also elevated the allelopahtic activity of barnyard grass. A key compound, which increased the allelopathic activity of barnyard grass, in the rice root exudates was isolated and determined as momilactone B. Momilactone B increased the allelopathic activity of barnyard grass at concentrations greater than 3 μM, and increasing the momilactone B concentration increased the activity. Conclusions: Momilactone B is known to act as a potent rice allelochemical and to possess strong growth inhibitory activity against barnyard grass. The present research suggests that barnyard grass may response to the presence of neighboring rice by sensing momilactone B in rice root exudates and increase allelopathic activity. Thus, momilactone B may not only act as a rice allelochemical but also play an important role in rice-induced allelopathy of barnyard grass. The induced-allelopathy may provide a competitive advantage for barnyard grass through the growth inhibition of competing plant species including rice. Barnyard grass allelopathy may be one of the inducible defense mechanisms by chemical-mediated plant interaction between rice and barnyard grass. Rice allelopathy was also reported to be increased by the presence of barnyard grass through increasing production and secretion of momilactone B into surrounding environments. During the evolutional process, rice and barnyard grass may have developed the chemical cross talk to activate the defense mechanisms against some biotic stress conditions by detection of certain key compounds. [ABSTRACT FROM AUTHOR]

Background and aims: Nitrate leaching from intensively cropped soils represents a huge environmental problem. In order to diversify the range of nitrogen management strategies, this investigation is focused on the effects of ribwort plantain, Plantago lanceolata L., and its allelochemicals on soil N mineralization. Methods: High-performance liquid chromatography was used in this study for phytochemical analysis of the major allelochemicals aucubin, catalpol, and verbascoside. Soil incubation experiments demonstrated a significant suppression of soil N mineralization caused by the incorporation of the iridoid glycoside (IG) aucubin, leaf material of two varieties ( P. lanceolata cv. Libor and cv. Arterner), and an aqueous extract of P. lanceolata leaves. Results: Throughout the growing season, the two varieties conspicuously differed in aucubin and verbascoside contents as well as in leaf dry weight. In soil incubation experiments, incorporated leaf material of both varieties affected long-term low soil nitrate concentrations. Experimental aucubin application resulted in an inhibitory effect on soil N mineralization. This was not true for the IG catalpol. Furthermore, we observed a negative relationship between IG concentrations and inorganic soil nitrogen concentrations when the soil was incubated with aqueous P. lanceolata leaf extract of different concentrations. Conclusion: This study enforced the hypothesis that allelochemicals of P. lanceolata have an active role in a suppression effect on soil N mineralization. Further research may be necessary to investigate the specific effects of P. lanceolata allelochemicals on the nitrogen cycle. [ABSTRACT FROM AUTHOR]

Background and aims: Festuca rubra plants are asymptomatically infected by the systemic fungal endophyte Epichloë festucae and high infection rates have been found in natural grasslands. Our objective was to determine the effect of endophyte infection on the competitive ability of Festuca rubra in binary mixtures against other grassland species. Methods: In two glasshouse experiments, endophyte-infected (E+) and non-infected (E−) plants of two half-sib lines (PEN, RAB) of Festuca rubra were grown in monocultures and in binary mixtures against five grassland species: Trifolium repens, Trifolium pratense, Trifolium subterraneum, Lotus corniculatus and Plantago lanceolata. Results: Biomass production of F.rubra was affected by endophyte infection and plant line but not by competition with T.pratense. In the PEN line E+ plants produced less root biomass than E−; and in the RAB line E+ plants produced less shoot biomass than E−. In spite of these differences shoot and root biomass production of Trifolium pratense were more inhibited when growing with endophyte-infected plants of Festuca rubra than with E− plants, regardless of the plant line. In both F.rubra lines, the endophyte increased the phosphorus concentration in shoots. Regardless of F.rubra plant line, root biomass of the five target species was more inhibited when growing with endophyte-infected than with endophyte-free plants. Conclusions: Irrespective of F.rubra line, endophyte-infected plants had a better competitive ability than non-infected plants, as indicated by the lower relative yield of companion plants when growing in mixture with E+ plants. We suggest an allelopathic effect of E+ red fescue on other grassland species. [ABSTRACT FROM AUTHOR]

Background and aims: Increasing evidence suggests that several plants, particularly non-mycorrhizal species, are negatively affected by the presence of arbuscular mycorrhizal fungi (AMF). Mechanisms explaining suppressive effects of AMF are, however, still poorly understood. Here we test whether growth suppression of the non-host weed Stellaria media in the presence of AMF can be explained by mycorrhizal alellopathy. Methods: We grew S. media in microcosms where an active AM mycelium was supported by neighboring wheat ( Triticum aestivum) plants. To test for allelopathy, we added activated carbon (AC) to the soil substrate. In addition, we performed two complementary experiments where extracts from roots extensively colonized by AMF (AM exudates) were directly applied to S. media seeds and seedlings. Results: Stellaria media plants grown in microcosms with AM mycelium showed an 8-fold biomass reduction compared to microcosms where AMF were absent. The addition of AC, which is thought to reduce allelopathic effects by binding organic compounds, did not greatly mitigate the negative effect of AM mycelium on S. media growth. Moreover, AM exudates did not significantly reduce S. media germination and growth. Conclusions: Results from this study confirm that non-hosts like S. media can be highly suppressed in the presence of AMF. However, we found no evidence that mycorrhizal allelopathy was a major mechanism responsible for growth suppression of S. media in the presence of AMF. Other mechanisms might therefore be more significant in explaining suppressive effects of AMF on non-host plant species. [ABSTRACT FROM AUTHOR]

In the northern China steppe, overgrazing has decreased the abundance of many species that were originally dominant, but increased the abundance of Artemisia frigida. We aimed to determine whether the adaptive and competitive abilities of A. frigida are associated with allelopathy. Soil nutrient characteristics could not explain the poor growth of the originally dominant species. Volatile compounds released from A. frigida leaves and aqueous extracts (0.025, 0.05, 0.075, 0.10, and 0.15 g ml) from A. frigida leaves and roots and from soil under A. frigida inhibited seed germination and seedling growth of three dominant species ( Leymus chinensis, Stipa krylovii, and Cleistogenes squarrosa). Allelopathic activity varied according to extract concentration, test species, and extract source. Germination was most strongly inhibited in S. krylovii, followed by L. chinensis and then C. squarrosa. Seedling growth was most strongly inhibited in L. chinensis, followed by S. krylovii and then C. squarrosa. Gas chromatography-mass spectrometry analyses of the leaf volatiles identified 27 compounds, primarily monoterpene or sesquiterpene compounds and their oxygen-containing derivatives, such as eucalyptol, beta.-myrcene, 1,6-octadien-3-ol,3,7-dimethyl, 3-carene, bicyclo[2.2.1]heptan-2-one,1,7,7-trimethyl-,(1R), cis-sabinenehydrate, camphene, and alpha-Pinene. These findings suggest that allelochemicals from A. frigida can modify the surrounding micro-habitat. The responses of target plants to allelopathy of A. frigida may be one reason for changes in plant community succession in the northern China steppe. [ABSTRACT FROM AUTHOR]

lower herb richness and diversity in the presence of Cistus ladanifer is attributed to the allelochemicals exudate from its leaves and stem. The objective of the present study was to determine allelopathic activities of C. ladanifer exudates and the persistence of potential allelochemicals in soil environment. The results showed that the aglycone flavonoids degrade very slowly in the soil, remaining a very long time with no further external input. Although the amounts of aglycone flavonoids in soils are low (of the order of μg/g soil), their high persistence may facilitate inhibition of the germination and growth of other species or an indirect effect by altering the soil's characteristics, providing a possible explanation for the phytotoxic activity attributed to C. ladanifer. [ABSTRACT FROM AUTHOR]

Root exudates have a key role in communication between plants and microbes in the rhizosphere. Fusarium wilt of watermelon, caused by Fusarium oxysporum f. sp. niveum (Fusarium oxysporum), drastically reduces watermelon yields in continuous cultivation systems, but it can be significantly alleviated using watermelon/aerobic rice intercropping system as shown by the research carried out in this laboratory. It is important to evaluate the interaction between root exudates from the two crops and the pathogen and thus to clarify the mechanism of disease suppressiveness in the intercropping system. The effects of phenolic acids, sugars and free amino acids in root exudates from watermelon (REW) and rice (RER) on the growth of Fusarium oxysporum were studied. The results obtained are listed as follows: (1) REW significantly increased spore germination and sporulation, whereas RER had inhibitory effects on those two parameters. (2) HPLC analysis showed that salicylic acid, p-hydroxybenzoic acid and phthalic acid were identified in exudates from both plants, but p-coumaric acid was only detected in rice and ferulic acid only in watermelon. Moreover, of the total rice exudates a high proportion (37.9 %) of p-coumaric acid was detected and the total amount of phenolic acids was 1.4-fold as high as that in watermelon. (3) Considerable differences in the components and contents of both sugars and amino acids were found between REW and RER exudates. (4) Exogenously applied alanine (Ala) increased spore germination and sporulation. In contrast, addition of exogenous p-coumaric acid reduced spore germination and sporulation, relative to controls. It was concluded that the rice root exudates had anti-fungal properties while that from watermelon promoted pathogen growth. This discovery provided a scientific basis for practicing watermelon/aerobic rice intercropping to control Fusarium wilt in watermelon. [ABSTRACT FROM AUTHOR]

Allelopathic interactions implicate the inhibition of cell division by allelochemicals. To examine the effects of autotoxic agents on cell cycle and plant growth, germinated cucumber seeds ( Cucumis sativus L.) were incubated in solutions containing the aqueous root extracts of cucumber at 1:100, 1:50, 1:25 and 1:10 ( w: v), or the hydrophobic root exudates of cucumber at 25, 50 and 100 mg·L−1. Aqueous root extracts and hydrophobic root exudates inhibited radicle elongation by 36.47–60.18% and 38.24–62.50%, respectively. The mitosis-specific genes were down-regulated in roots exposed to aqueous root extracts and hydrophobic root exudates. Meanwhile, exposure to either aqueous root extracts or hydrophobic root exudates decreased the proportion of 2C (C-value) and increased the proportion of 8C, leading to an increased mean C-value. We conclude that autotoxic agent-induced inhibition of radicle growth was partly attributed to the down-regulation of cell cycle-related genes and endoreduplication was enhanced under our experimental condition. [ABSTRACT FROM AUTHOR]

Changes to ecosystem abiotic parameters are regarded as possible mechanisms facilitating plant invasion and community composition shifts. This study compared the hydrophobic chemical signatures of soil from exotic bitou bush ( Chrysanthemoides monilifera spp. rotundata) invaded, indigenous acacia ( Acacia longifolia var. sophorae) dominated and bare sand (unvegetated) habitats using a novel, rapid, capturing technique which utilised Amberlite® XAD4 resin filled bags that were placed in situ. The hydrophobic chemical signature of the bitou bush soil extract was significantly different to the acacia soil and bare sand extracts. High concentrations of 18 sesquiterpenes dominated the hydrophobic signature of the bitou bush extract. Low concentrations of all three extracts did not significantly affect the seedling growth of three indigenous test species under laboratory conditions, however, at higher concentrations, the extracts from soil inhabited by plants, whether exotic or indigenous, similarly inhibited the seedling growth of two species, while seedling growth of the third species was inhibited by extracts from all three soil types. These results do not support the hypothesis that exotic invasive species are more likely to exhibit allelopathic effects than indigenous plant species. [ABSTRACT FROM AUTHOR]

Root exudates are implicated in the chemical defense of plants, but testing such hypotheses has been hindered by the difficulties of quantifying allelochemical concentrations in soil. Here we describe a new, simple method to quantify the dynamics of non-polar root exudates in soil. Novel soil probes were constructed using stainless steel wire inserted into polydimethylsiloxane (PDMS) tubing. Probes were inserted into soil for 24 h, removed and extracted, and analyzed by HPLC. Lipophilic thiophenes produced by roots of Tagetes and Rudbeckia species were chosen as candidate compounds to test the method. Probes recovered microgram quantities of the highly phytotoxic thiophenes 5-(3-buten-1-ynyl)-2,2′-bithienyl (BBT) and α-terthienyl per probe per day from the root zone of Tagetes patula, and distribution of thiophenes beneath plants was spatially and temporally heterogeneous. Flux-proportional sampling of soil provides a means to test hypotheses about the role of root exudates in plant–plant and other interactions. [ABSTRACT FROM AUTHOR]

Although residue management seems a key factor in residue-mediated weed suppression, very few studies have systematically compared the influence of different residue management strategies on the establishment of crop and weed species. We evaluated the effect of several methods of pre-treatment and placement of winter rye ( Secale cereale L.) and winter oilseed rape ( Brassica napus L.) residue on seedling emergence under field conditions. For both species two cultivars, differing in allelochemical content, were used. Residues incorporated in the upper soil layer exerted a large inhibitory effect on the establishment of the relatively early emerging lettuce ( Lactuca sativa L.) and spinach ( Spinacia oleracea L.) seedlings, whereas the inhibitory effect on the slightly later emerging Stellaria media L. seedlings was variable, and often a stimulatory effect on the very late emerging Chenopodium album L. seedlings was observed. Differences between cover crop cultivars were minor. For winter oilseed rape residue, pre-treatment strongly affected the time-course of residue-mediated effects. Finely ground residues were only inhibitory to seedling establishment during the first two to three weeks, whereas cut residues became inhibitory after this period. For winter rye, residue placement was most important. Residue incorporation gave variable results, whereas placement of winter rye residue on top of the soil inhibited the emergence of all receptor species. In conclusion, the optimal residue management strategy for weed suppression depends both on the cover crop species used and the target weed species. [ABSTRACT FROM AUTHOR]

Chemical interference is increasingly suggested as a mechanism facilitating exotic plant invasion and plant community composition. In order to explore this further, we employed a comprehensive extract-bioassay technique that facilitated detection and demarcation of phytotoxicity, direct allelopathy and indirect allelopathy of bitou bush ( Chrysanthemoides monilifera spp. rotundata) compared to an indigenous dominant of the invaded system, acacia ( Acacia longifolia var. sophorae). Extracts of the leaves and roots of both species exhibited phytotoxic effects against five indigenous plant species. Evidence for allelopathy between co-evolved indigenous plants was detected between acacia and Isolepis nodosa. Allelopathy between bitou bush and four indigenous plant species was also detected. Therefore we propose that both the acacia and bitou bush have the potential to chemically inhibit the establishment of indigenous plants. Eventual dominance of bitou bush is predicted, however, based on more ubiquitous effects on seedling growth. [ABSTRACT FROM AUTHOR]

Nitrification is a key biological process for the control of soil NO3 − availability and N losses from terrestrial ecosystems. The study investigates the causes for the absence of net nitrification activity in the soil of a Mediterranean monospecific woodland of Arbutus unedo, focusing in particular on the possible role of chemicals produced by this plant. The mineral N pool, net rates of mineralization and nitrification were measured in the soil top 10 cm over 18 months. Raw extracts of leaves and roots of Arbutus unedo and soil underneath Arbutus plant canopy were purified using chromatographic techniques and the structure of chemicals was defined using spectroscopic and spectrometric methods. Leaf extracts (raw, aqueous and organic fractions) were tested for their toxicity on net nitrification, using a test soil. Field and laboratory incubations showed soil NO3 − concentration below the detection limit over the whole study period, despite the significant NH4 + availability. Toxicity tests indicated that more than 400 μg of extract g−1 dry soil were needed to have more than 50% reduction of net NO3 − production. Gallocatechin and catechin were among the most abundant chemicals in the extracts of leaves, roots and soil. Their soil concentration was significantly higher than the annual calculated input via leaf litter, and it was in the range of toxic concentrations, as deduced from the dose-response curve of the toxicity test. Data support the hypothesis that plant produced chemicals might be involved in the limited net nitrate production in this Mediterranean woodland. [ABSTRACT FROM AUTHOR]

Belowground mechanisms involved in plant competition are still poorly understood. Since plant species are differently affected by soil organisms, changes in soil community composition might affect interspecific competition with consequences for plant community structure. We studied whether soil community composition affects competition between the grass Holcus lanatus L. and the legume Lotus corniculatus L. We established three different soil communities by adding no soil organisms (control), microorganisms <30 μm, and a soil suspension including microorganisms >30 μm, AMF and nematodes to gamma-sterilized soil. Nodulation and aboveground biomass of Lotus was decreased in the sterilized control soil and in the presence of Holcus. Contrastingly, the grass grew better in the presence of the legume than in monoculture and was not affected by soil community composition. Legume monocultures tended to produce the greatest aboveground biomass of the plant combinations when soil microorganisms were present, while the root biomass in legume monocultures was the lowest. Then, in a second experiment, we used natural (not sterilized) soil and added activated carbon to test whether the reduced nodulation of Lotus in interspecific competition is caused by allelopathic compounds of Holcus. In the natural soil, nodulation and flowering of Lotus was reduced, but the aboveground biomass was not affected by the competition with Holcus. Contrary to our expectations, activated carbon had a strong negative effect on the nodulation, growth and flowering of Lotus and shifted the interspecific competition in favour of Holcus. Probably, activated carbon impeded the nodulation by disrupting the communication between the legume and N2-fixing bacteria. We suggest that interruption of plant-microbe communications by activated carbon might be widespread and will confound interpretations on the role of allelopathy. Generally, we observed that the symbiosis of the legume with N2-fixing bacteria plays a crucial role in the grass-legume competition. When the symbiosis was deterred, the legume was outcompeted by the grass. [ABSTRACT FROM AUTHOR]

The allelopathic effect of white micromeria ( Micromeria fruticosa) leaves on germinating wheat (Triticum aestivum L.) was examined in sterilized and non-sterilized clay and sandy soils. Seed germination and seedling development were evaluated and the volatile compounds in the soil and in the seeds analyzed. The germination of the seeds and subsequent development of the seedlings were inhibited by addition of the leaves to the soil. The degree of inhibition was found to be concentration dependent, and the extent of inhibition was higher in the sandy soil than the clay. Volatile components from M. fruticosa and their derivatives were found in the soil and in the seeds. Soil sterilization inhibited the degradation of the volatiles but its effect on seedling emergence was not significant. We suggest that the active compounds pulegone and isomenthol are the probable cause of the inhibitory effects. The results demonstrate that leaf residues in soil systems can cause allelopathic effects by releasing volatiles. The compounds are metabolized in the seeds and are able to lead to inhibition of germination. [ABSTRACT FROM AUTHOR]

In order to develop biological control methods that are effective against the root-knot nematode Meloidogyne incognita (Kofoid and White) chitwood, the activity of ethanolic and aqueous extracts of wild plant species distributed on Okinawa Island on the viability and mobility of second stage M. incognita juveniles (J2s) was evaluated. Eleven of the 29 extracts immobilized at least half of the J2 stage nematodes in an in vitro assay. Aqueous extracts of Bidens pilosa L. var. radiata Scherff, Hydrocotyle dichondroides Makino, Oxalis corymbosa DC., Oxalis corniculata L., and Stenactis annus (L.) Cass gave 90% or better immobilization activity. Among these, B. pilosa var. radiata had the highest activity. Significant immobilization, lethality, repellence and egg hatching inhibition were observed with extracts from each B. pilosa plant part, but especially from leaves. The effects of plant extracts on the mobility of M. incognita were higher than on the free-living nematode Panagrolaimus sp., suggesting that M. incognita could be suppressed using B. pilosa extracts without significantly affecting beneficial nematodes. [ABSTRACT FROM AUTHOR]

A 2-year field investigation was carried out during 2003–2004 to determine the effectiveness of intercropping single and double rows of sorghum, soybean and sesame in a cotton crop on the suppression of purple nutsedge ( Cyperus rotundus L.). Results revealed that all three intercrops were effective in inhibiting purple nutsedge density (70–96%) and dry matter production (71–97%) during both years of experimentation. Control in the second year was more effective than in the first year. The seed cotton yield was also depressed by the intercrops but its suppression (8–23%) was far less severe than that of purple nutsedge and its loss was compensated by greater total economic returns. Intercropping of sorghum and sesame produced greater than 20% net benefits (up to 60%) in comparison with the control (cotton alone). Soybean intercropping produced comparable net benefits (95–103%). Sesame two rows intercrop treatment appeared the most profitable with net benefit of 51–59% with good purple nutsedge control (73–92% density suppression, 77–95% dry weight suppression) during both years of experimentation. [ABSTRACT FROM AUTHOR]

Allantoin (5-ureidohydantoin) plays an essential role in the assimilation, metabolism, transport, and storage of nitrogen in numerous higher plants, but its ecological implications are largely unknown. In this study allantoin was found in tissues of 11 rice ( Oryza sativa) varieties tested, and its structure was characterised by X-ray diffraction analysis to confirm the fact that allantoin was actually obtained from the rice plants. Furthermore, the endogenous allantoin was exuded from the rice roots into the rhizosphere soils and had a great diversity of biological effects on associated weeds and microbes by soil interactions once released. However, allantoin levels in tissues or soils could not be distinguished between the allelopathic and non-allelopathic rice varieties. Field experiments showed that levels of allantoin released from rice varieties varied with their growth stages and reached the maximal levels at the stem elongation or panicle initiation to booting stages and then decreased dramatically. Allantoin could significantly stimulate the germination and growth of Echinochloa crus-galli and populations of soil bacteria and actinomycetes at selected test concentrations (30–500 μg/g), but had no effect on soil fungi. The half-life ( t 1/2 ) of allantoin in autoclaved soil (20.2 ± 2.5 h, r 2 = 0.95) was almost three-times longer than in non-autoclaved soil (7.3 ± 1.9 h, r 2 = 0.92), indicating that rapid biodegradation or transformation of allantoin occurs in paddy soil. The results suggest that not only may allantoin play a role in the transport and storage of nitrogen in rice tissues but it may also participate in species interactions between rice and other organisms in paddy soil. [ABSTRACT FROM AUTHOR]

Wheat varietal autotoxicity and varietal allelopathy were assessed based on plant extract and root exudate bioassays under laboratory conditions. Aqueous extract of wheat differed in varietal autotoxicity and varietal allelopathy, inhibiting wheat germination by 2–21%, radicle growth by 15–30%, and coleoptile growth by 5–20%, depending on the combination of the receiver and donor. Extracts of cv Triller or cv Currawong were more allelopathic to other wheat varieties than cv Batavia and cv Federation. Triller extract was more autotoxic than Federation. Assessment of root exudates by the equal-compartment-agar-method further identified the significant differences in varietal autotoxicity and varietal allelopathy of root exudates between wheat varieties, with root exudates of Triller or Batavia showing stronger autotoxic or allelopathic effects than Currawong or Federation. The varietal autotoxicity and allelopathy of root exudates also showed a characteristic radial inhibitory pattern in the agar growth medium. These results suggest that careful selection of suitable wheat varieties is necessary in a continuous cropping system in order to minimize the negative impacts of varietal allelopathy and varietal autotoxicity. Factors affecting autotoxicity in the field and strategies in autotoxicity management are discussed. [ABSTRACT FROM AUTHOR]

Resource competition and chemical interference are mechanisms of interaction among plants that may occur simultaneously. However, both mechanisms are rarely considered together when modelling plant growth. We propose a new empirical model that estimates biologically significant parameters on both plant competition and chemical interference. The model is tested with data sets from different density-dependent experiments done with two species (the grass Lolium rigidum Gaud. and the legume Glycine max soya L.) subjected to a noxious chemical environment when growing (allelochemicals and herbicides, respectively). Hypotheses on the effect of allelochemicals and its interaction with density are tested using maximum likelihood ratio tests in order to ask, for these species, whether chemical interference is playing a significant role in the interactions among plants or on the contrary, whether interactions among plants are sufficiently explained by the resource competition. In all cases a significant interaction between chemicals and density is observed. This interaction is inconsistent with the hypothesis of only resource competition having an influence of plant biomass and suggests a significant density-dependent effect of chemicals on plant growth. [ABSTRACT FROM AUTHOR]

Although Acroptilon repens (L.) DC. (Russian knapweed) is known to concentrate zinc (Zn) in upper soil layers, the question of whether the elevated Zn has an allelopathic effect on restoration species has not been addressed. Experiments were conducted to investigate whether soils collected from within infestations of A. repens (high-Zn) inhibit the germination or growth and development of desirable restoration species, compared to soils collected adjacent to an A. repens infestation (low-Zn). Four bioassay species [ Sporobolus airoides (Torrey) Torrey (alkali sacaton), Pseudoroegneria spicata (Pursh) A. Love (bluebunch wheatgrass), Psathyrostachys juncea (Fischer) Nevski (Russian wildrye) and A. repens] were germinated in a growth chamber and grown in a greenhouse in both soils and received treatments for the alleviation of Zn toxicity (P, Fe, Fe-oxide, and soil mixing) to isolate the effects of elevated soil Zn on plant performance. Percent germination, total plant biomass, tiller and stem number, inflorescence number, and tissue metal levels were compared among soil types and treatments for each species. There was no evidence from any of the indicators measured that high-Zn soils reduced plant performance, compared to low-Zn soils. Tissue Zn levels barely approached the lower range of phytotoxic levels established for native grasses. Older plants with longer exposure times may accumulate higher Zn concentrations. S. airoides and A. repens both had higher biomass in the high-Zn soil, most likely due to increased macronutrient (N and P) availability. As the Zn levels in the soils used in this study were much higher than any levels previously reported in soils associated with A. repens, it is unlikely that the elevation of soil Zn by A. repens will hinder germination or growth and development of desirable grasses during establishment. [ABSTRACT FROM AUTHOR]

A few rice ( Oryza sativa L.) varieties or rice straw produce and release allelochemicals into soil in which interfere with the growth of neighboring or successive plants. Allelopathic rice PI312777 and Huagan-1 at their early growth stages released momilactone B, 3-isopropyl-5-acetoxycyclohexene-2-one-1, and 5,7,4′-trihydroxy-3′,5′-dimethoxyflavone into soil at phytotoxic levels, but non-allelopathic rice Huajingxian did not. Both allelopathic and non-allelopathic rice residues released momilactone B and lignin-related phenolic acids ( p-hydroxybenzoic, p-coumaric, ferulic, syringic and vanillic acids) into the soil during residue decomposition to inhibit successive plants. The results indicated that allelochemicals involved in rice allelopathy from living and dead plants are substantially different. Interestingly, the concentrations of the allelochemicals released from the allelopathic rice seedlings in soil increased dramatically when they were surrounded with Echinochloa crus-galli. The concentrations of the allelochemicals were over 3-fold higher in the presence of E. crus-galli than in the absence of E. crus-galli. However, the same case did not occur in non-allelopathic Huajingxian seedlings surrounded with E. crus-galli. In addition to allelochemical exudation being promoted by the presence of E. crus-galli, allelopathic rice seedlings also increased allelochemical exudation in response to exudates of germinated E. crus-galli seeds or lepidimoide, an uronic acid derivative exuded from E. crus-galli seeds. These results imply that allelopathic rice seedlings can sense certain allelochemicals released by E. crus-galli into the soil, and respond by increased production of allelochemicals inhibitory to E. crus-galli. This study suggests that rice residues of both allelopathic and non-allelopathic varieties release similar concentrations and types of allelochemicals to inhibit successive plants. In contrast, living rice plants of certain allelopathic varieties appear to be able to detect the presence of interspecific neighbors and respond by increased allelochemicals. [ABSTRACT FROM AUTHOR]

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 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. [ABSTRACT FROM AUTHOR]

Autotoxic species are those which adversely affect their own seeds’ germination and/or seedling development. Cistus ladanifer L (labdanum or jara) has been shown to have a pattern of allelopathic behaviour against the herbs that share its habitat. The present work studied whether an autotoxic effect also exists. The aqueous solution obtained from washing jara leaves was found by itself to inhibit germination and cotyledon emergence of the species’ seeds. When these same trials were carried out in soils, autotoxicity was observed only from leaves and soils collected in winter. This was so both in soils collected away from the influence of the jaral to which was added the greatest concentration of aqueous extract prepared from the leaves, and in soils collected within the jaral, except that in the latter group of soils germination was inhibited with or without the addition of C. ladanifer extracts. This autotoxic behaviour could be involved in the species’ own population control, and would explain the scant self-regeneration within established C. ladanifer stands . [ABSTRACT FROM AUTHOR]

A field experiment was conducted to investigate the causes of poor canola growth through surface-retained wheat stubble. The experiment was designed to separate the role of biochemical factors such as stubble phytotoxicity, nitrogen (N) immobilisation and disease incidence from the physical effects of the stubble including changes in the seedbed microclimate on crop growth and yield, as these had been previously identified as possible mechanisms for the poor growth. Treatments included: two stubble cultivars, three levels of decomposition, two levels of nitrogen and inert plastic mulch. A novel approach involving leachates collected from intact field cores with a rainfall simulator was also used to assess the phytotoxic impacts of the stubble. Surface-retained wheat stubble (5 t/ha) reduced the canola vegetative biomass by 46% and yield by 26%, consistent with previous field observations. There was no impact of stubble variety or level of stubble decomposition on canola growth and yield, and an inert plastic mulch designed to simulate the physical effects of the wheat stubble had a similar effect on growth and yield as wheat stubble. Leachates collected from intact field cores through surface stubble also had no impact on canola germination or radicle elongation in petri-dishes in the absence of soil, nor on emergence of canola in pots of soil. Together, these observations suggest phytotoxicity was not contributing to the poor growth. N-immobilisation was evident in stubble-retained treatments although tissue N concentrations were adequate in all treatments and the growth limitation could not be overcome with added N. The principal mechanism of growth reduction was associated with the physical impact of the stubble including a reduction in photosynthetically active radiation, the red:far red ratio of incident light under the stubble, and the temperature above the stubble layer. These effects led to elongated hypocotyls, reduced investment in early leaf and root growth, delayed emergence and slower leaf area development as well as an increase in seedling disease complex, effects apparent in both wheat stubble and inert plastic mulch treatments. The results suggest that physical, rather than biochemical factors are the main cause of poor growth of canola in surface-retained wheat stubble, and that much of the impact could be avoided if the stubble were moved away from directly above the emerging seedlings. [ABSTRACT FROM AUTHOR]