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Description of the subject. Essential oils are volatile fractions produced through the special metabolism of plants. One significant biological application of essential oils in recent years has been their use as bioherbicides, due to their important biocide effect. Objectives. This study aims to identify the chemical constituents of Aloysia gratissima (Gillies & Hook.) Tronc. essential oil and assess its biocide potential. Method. This study describes, for the first time, the chemical constitution of the essential oils from seeds of A. gratissima. To verify possible biocide effect, lettuce seeds were submitted to different concentrations of essential oils from A. gratissima leaves (AG-LE-EO), flowers (AG-FL-EO), and seeds (AG-SE-EO) to evaluate germination percentage, mean germination time, synchrony of germination, and seedling length. . Results. At concentrations of 800 µl l-1, synchrony was positively influenced by (AG-LE-EO), increasing from 0.38 to 0.54. On the other hand, AG-SE-EO at concentrations of 800 µl·l-1 reduced lettuce germination from 90.8% to 64.4% and seedling length from 1.35 to 0.8 cm, besides increasing mean germination time, indicating biocide effects. Conclusions. These results are expected to lead to further trials to understand how the constituents of A. gratissima essential oils inhibit germination, making them a potential bioherbicide.

The invasive shrub Lonicera maackii (Amur honeysuckle) is a problematic species in the eastern United States and there is growing evidence that materials from this species have toxic effects on some organisms. We used a sequence of microcosm bioassays to assess the influence of L.maackii leaf leachate on the macroinvertebrate Hyalella azteca, which is a standard aquatic organism for toxicity assessment. In a laboratory setting, H. azteca were exposed to a leaf leachate dilution series (6.25%, 12.5%, 25%, 50%, 100%) in 48-h toxicity tests. This was repeated throughout the growing season to assess the potential for changes in leaf toxicity due to phenology. Strong toxic effects were found when H. azteca was exposed to a L. maackii leachate from autumn (P < 0.05) and, in fact, all organisms died when exposed to any level of concentration in most trials. Mean percent survival also decreased significantly in all dilutions in the spring (P < 0.05 for all treatments); however, little toxicity was detected in growing season trials. These results suggest (a) strong toxic effects of L. maackii foliage on a model aquatic organism that (b) varies throughout the year, potentially in relationship to biochemical changes associated with phenology.

The present study concerns the relationships among leaf litter decomposition, substrate quality, ammonia-oxidizing bacteria (AOB) community composition and nitrogen (N) availability. Decomposition of organic matter affects the biogeochemical cycling of carbon (C) and N. Since the composition of the soil microbial community can alter the physiological capacity of the community, it is timely to study the litter quality effect on N dynamic in ecosystems. The aim of this study was to determine the influence of leaf litter decomposition on N mineralization. The specific objectives of this study were to evaluate the influence of the litter biochemistry of five plants species (Faidherbia albida A.Chev., Azadirachta indica A.Juss., Casuarina equisetifolia L., Andropogon gayanus Kunth and Eragrostis tremula Hochst. ex Steud.) on N mineralization in a tropical ferrous soil (Lixisol), nitrification, and genetic diversity of ammonia-oxidizing bacteria. Denaturing gradient gel electrophoresis (DGGE) of amplified fragments of genes coding for 16S rRNA was used to study the development of bacterial communities during decomposition of leaf litter in soils. Community structure of AOB was determined at two time periods: day 0 and day 140. Ten strains were tested and each of these strains produced a single band. Thus, DGGE DNA band patterns were used to estimate bacterial diversity. Plant secondary compounds such as polyphenols are purported to influence nutrient cycling by affecting organic matter degradation, mineralization rates, N availability and humus formation. In a laboratory study, we investigated the influence of six phenolic acids (ferulic, gallic, vanillic, syringic, p-coumaric and p-HBA acids) commonly found in the plant residues on N mineralization and NH4+ and NO3- production in soils. The results showed that litter type did affect soil nitrification. Faidherbia albida litter was associated with increased inorganic N in soil after 140 days of incubation while A. gayanus and C. equisetifolia litter immobilized N. Azadirachta indica and E. tremula amendments had no significant effects in N mineralization. The results show that the addition of six phenolic acids significantly reduced NH4+ and NO3- compared to the control soil but had no significant effect on N mineralization. For the community of ammonium-oxidizing bacteria, a litter quality effect was noted, but the incubation time effect was more pronounced, except for C. equisetifolia litter. Results confirmed that the N mineralization changed with litter type under controlled conditions and the genetic structure of AOB is highly dependent on litter quality.

Description of the subject. The present study focuses on the description of the allelopathic interactions between wild and crop species that may occur in a given ecosystem. Objectives. The objective is the evaluation of the allo- and autoinhibition activity of root exudates of barley (Hordeum vulgare L. subsp. vulgare) and great brome (Bromus diandrus Roth.) seedlings by water-soluble allelochemicals. Method. The allelopathic activities of five Tunisian barley genotypes (modern varieties and landraces), one Saudi Arabian barley landrace and great brome were assessed using a modified laboratory bioassay named “seedling-after-seedling agar method”. Results. The barley or the great brome reduced, to a greater extent, the root growth compared to the shoot growth of receiver species. The response of the root system architecture of the great brome towards barley root exudates was studied in detail. All the measured root traits were highly sensitive to the presence of barley. In our conditions, the allelopathic activity of barley root exudates had no apparent relationship with the size of the root and a prominent action of genetic determinants in the allelopathic potential between genotypes is proposed. The alloinhibitory activity of barley or great brome root exudates deferred between the receiver species but was always higher than the autoinhibition potential. The autoinhibition in barley proved to depend on whether the genotypes used as donor and receiver are identical or different, suggesting a specific interaction of allelochemicals with the receiver plant. These molecules seem to be the main actors in the allelopathic barley potential as external factors such variations of pH have no evident relevance in the inhibition process. Conclusions. Barley and great brome exude molecules in their surroundings. This affects the growth of the receiver plants, suggesting that these compounds might contribute to the plant community dynamics. Évaluation du potentiel allélopathique des composés hydrosolubles de l’orge (Hordeum vulgare L. subsp. vulgare) et du grand brome (Bromus diandrus Roth.) moyennant un bio-essai modifié Description du sujet. La présente étude se focalise sur la description des interactions allélopathiques entre des espèces sauvages et cultivées qui peuvent survenir dans un écosystème donné. Objectifs. L’objectif est l'évaluation de l’activité d’auto- et d’alloinhibition des exsudats racinaires de l’orge (Hordeum vulgare L. subsp. vulgare) et du grand brome (Bromus diandrus Roth.) via les allélochimiques hydrosolubles. Méthode. Les activités allélopathiques de cinq génotypes tunisiens d’orge (variétés modernes et orges locales), d’une orge locale d’Arabie saoudite et du grand brome ont été évaluées moyennant un bio-essai modifié nommé « seedling-after-seedling agar method ». Résultats. L’orge et le grand brome ont réduit dans une grande mesure la croissance des racines des espèces receveuses comparativement à celle des pousses. La réponse de l’architecture du système racinaire du grand brome à l’égard des exsudats racinaires de l’orge a été étudiée en détail. Il s’est avéré que tous les traits des racines analysés ont été très sensibles en présence de l’orge. Dans nos conditions, l’activité allélopathique des exsudats racinaires de l’orge n’avait aucune relation apparente avec la taille des racines et une action prépondérante des déterminants génétiques dans le potentiel allélopathique entre les génotypes est ainsi proposée. L’activité d’alloinhibition des exsudats racinaires de l’orge ou du grand brome était différente entre les espèces receveuses mais toujours plus élevée par rapport au potentiel d’autoinhibition. L’autoinhibition chez l'orge a été dépendante des génotypes utilisés comme donneurs et receveurs qui étaient identiques ou différents, suggérant ainsi une interaction spécifique des allélochimiques avec la plante receveuse. Ces molécules semblent être les principaux acteurs du potentiel allélopathique chez l’orge, étant donné que les facteurs externes comme les variations du pH n’ont aucune pertinence évidente dans le processus d’inhibition. Conclusions. L’orge et le grand brome relâchent dans leur environnement des molécules qui affectent la croissance des plantes receveuses, suggérant ainsi que ces composés pourraient contribuer à la dynamique des communautés végétales.

Reversible sorption of phenolic acids by soils may provide some protection to phenolic acids from microbial degradation. In the absence of microbes, reversible sorption 35 days after addition of 0.5-3 micromole/g of ferulic acid or p-coumaric acid was 8-14% in Cecil A(p) horizon and 31-38% in Cecil B(t) horizon soil materials. The reversibly sorbed/solution ratios (r/s) for ferulic acid or p-coumaric acid ranged from 0.12 to 0.25 in A(p) and 0.65 to 0.85 in B(t) horizon soil materials. When microbes were introduced, the r/s ratio for both the A(p) and B(t) horizon soil materials increased over time up to 5 and 2, respectively, thereby indicating a more rapid utilization of solution phenolic acids over reversibly sorbed phenolic acids. The increase in r/s ratio and the overall microbial utilization of ferulic acid and/or p-coumaric acid were much more rapid in A(p) than in B(t) horizon soil materials. Reversible sorption, however, provided protection of phenolic acids from microbial utilization for only very short periods of time. Differential soil fixation, microbial production of benzoic acids (e.g., vanillic acid and p-hydroxybenzoic acid) from cinnamic acids (e.g., ferulic acid and p-coumaric acid, respectively), and the subsequent differential utilization of cinnamic and benzoic acids by soil microbes indicated that these processes can substantially influence the magnitude and duration of the phytoxicity of individual phenolic acids

Results of a literature survey indicate that weed population density and biomass production may be markedly reduced using crop rotation (temporal diversification) and intercropping (spatial diversification) strategies. Crop rotation resulted in emerged weed densities in test crops that were lower in 21 cases, higher in 1 case, and equivalent in 5 cases in comparison to monoculture systems. In 12 cases where weed seed density was reported, seed density in crop rotation was lower in 9 cases and equivalent in 3 cases when compared to monocultures of the component crops. In intercropping systems where a main crop was intersown with a @'smother@' crop species, weed biomass in the intercrop was lower in 47 cases and higher in 4 cases than in the main crop grown alone (as a sole crop); a variable response was observed in 3 cases. When intercrops were composed of two or more main crops, weed biomass in the intercrop was lower than in all of the component sole crops in 12 cases, intermediate between component sole crops in 10 cases, and higher than all sole crops in 2 cases. It is unclear why crop rotation studies have focused on weed density, whereas intercropping studies have focused on weed biomass. The success of rotation systems for weed suppression appears to be based on the use of crop sequences that create varying patterns of resource competition, allelopathic interference, soil disturbance, and mechanical damage to provide an unstable and frequently inhospitable environment that prevents the proliferation of a particular weed species. The relative importance and most effective combinations of these weed control tactics have not been adequately assessed. In addition, the weed-suppressive effects of other related factors, such as manipulation of soil fertility dynamics in rotation sequences, need to be examined. Intercrops may demonstrate weed control advantages over sole crops in two ways. First, greater crop yield and less weed growth may be achieved if intercrops are more effective than sole crops in usurping resources from weeds or suppressing weed growth through allelopathy. Alternatively, intercrops may provide yield advantages without suppressing weed growth below levels observed in component sole crops if intercrops use resources that are not exploitable by weeds or convert resources to harvestable material more efficiently than sole crops. Because of the difficulty of monitoring the use of multiple resources by intercrop/weed mixtures throughout the growing season, identification of specific mechanisms of weed suppression and yield enhancement in intercrop systems has so far proven elusive. Significant advances in the design and improvement of weed-suppressive crop rotation and intercropping systems are most likely to occur if three important areas of research are addressed. First, there must be continued attention to the study of weed population dynamics and crop-weed interference in crop rotation and intercropping systems. More information is needed concerning the effects of diversification of cropping systems on weed seed longevity, weed seedling emergence, weed seed production and dormancy, agents of weed mortality, differential resource consumption by crops and weeds, and allelopathic interactions. Second, there needs to be systematic manipulation of specific components of rotation and intercropping systems to isolate and improve those elements (e.g., interrow cultivation, choice of crop genotype) or combinations of elements that may be especially important for weed control. Finally, the weed-related impacts of combining crop rotation and intercropping strategies should be assessed through careful study of extant, complex farming systems and the design and testing of new integrated approaches. Many aspects of crop rotation and intercropping are compatible with current farming practices and could become more accessible to farmers if government policies are restructured to reflect the true environmental costs of agricultural production.

Allelopathic pollen of Phleum pratense (Poaceae) has been shown to reduce pollen germination and seed set of heterospecific Poaceae. Danthonia compressa has a mixed breeding system, i.e.,both cleistogamous and chasmogamous florets. Previous studies revealed that extracts from 1-5 pollen grains of P. pratense reduce pollen germination and seed set although direct field evidence was lacking. Therefore, we tested whether the presence or absence of pollen from P. pratense affected seed set in D. compressa. In 1991, inflorescences of P. pratense were clipped to reduce pollen dispersal in an abandoned hayfield. In an adjacent abandoned hayfield, P. pratense flowered unimpeded. Comparing the clipped versus unclipped fields, the mean number of pollen grains of P. pratense/stigma of D. compressa decreased from 7.6 to 0.19 while mean seed set/culm increased from 50% to 76.9%. All of the increased seed set was related to increased seed set in chasmogamous florets. In 1992, when P. pratense was allowed to flower in both fields, mean seed set/culm was less than 50%. In a third population of D. compressa, at a site where P. pratense was not present, mean seed set/culm was over 76% in both years. Cleistogamy in D. compressa maintained a minimum seed set of 41.9% in all fields. Hence, there is an effective barrier against allelopathic pollen of P. pratense. The implications of loss of chasmogamously produced seed are unclear. There were no differences in mean seed mass or size between chasmogamous and cleistogamous florets. Hence fitness may be unaffected. Although loss of chasmogamous seeds may result in increased sibling competition in cleistogamously produced seedlings, this hypothesis has yet to be tested. The relative numbers of chasmogamous and cleistogamous florets did not change in 1991-1992, although there were more chasmogamous florets produced by D. compressa at the site where P. pratense did not exist. Tests of whether pollen allelopathy acts as a selection pressure on the relative numbers of chasmogamous and cleistogamous florets in D. compressa will require long-term field studies.

Following several decades of popularity after the Second World War, the use of synthetic herbicides is now experiencing a backlash within the agriculture industry.The increase in organic farming and concerns about potential negative effects on human health and the environment is creating a demand for pesticide-free food and alternative weed management techniques. International research has now explored the potential, limitations and impacts of non-chemical alternatives and the effect of different strategies on the entire agro- or natural ecosystem. Through the re-evaluation of techniques previously considered uneconomical or impractical, this text provides a comprehensive examination of non-chemical weed management.

Phenolics are one of the many secondary metabolites implicated in allelopathy. To establish that allelopathy functions in a natural ecosystem, the allelopathic bioassay must be ecologically realistic so that responses of appropriate bioassay species are determined at relevant concentrations. It is important to isolate, identify, and characterize phenolic compounds from the soil. However, since it is essentially impossible to simulate exact field conditions, experiments must be designed with conditions resembling those found in natural systems. It is argued that allelopathic potential of phenolics can be appreciated only when we have a good understanding of 1) species responses to phenolic allelochemicals, 2) methods for extraction and isolation of phenolic allelochemicals, and 3) how abiotic and biotic factors affect phenolic toxicity.