Spatial and Temporal Dynamics of Root Exudation: How Important is Heterogeneity in Allelopathic Interactions?

Understanding allelopathy has been hindered by the lack of methods available to monitor the dynamics of allelochemicals in the soil. Previous work has demonstrated the feasibility of using polydimethylsiloxane (PDMS) microtubing (silicone tubing microextraction, or STME) to construct sampling devices to monitor the release of lipophilic allelochemicals from plant roots. The objective of this study was to use such sampling devices to intensively monitor thiophene fluxes beneath marigolds over several weeks to gain insight into the magnitude of temporal and spatial heterogeneity in these fluxes. Marigolds were grown in rhizoboxes (20.5 x 20.5 x 3.0 cm) with 16 individual STME samplers per box. Thiophene sampling and HPLC analysis began 45 days after planting. At the end of the study, roots around each sampler were analyzed by HPLC. Results confirmed the tremendous spatial and temporal heterogeneity in thiophene production seen in our previous studies. STME probes show that thiophene concentrations generally increase over time; however, these effects were sampling-port specific. When sampling ports were monitored at 12 h intervals, fluxes at each port ranged from 0 to 2,510 ng day^?1. Fluxes measured over daylight hr averaged 29 % higher than those measured overnight. Fluxes were less than 1 % on average of the total thiophene content of surrounding roots. While the importance of such heterogeneity, or “patchiness”, in the root zone has been recognized for soil nutrients, the potential importance in allelopathic interactions has seldom been considered. The reasons for this variability are unclear, but are being investigated. Our results demonstrate that STME can be used as a tool to provide a more finely-resolved picture of allelochemical dynamics in the root zone than has previously been available.     

Biomimetic measurement of allelochemical dynamics in the rhizosphere

Polydimethylsiloxane (PDMS) materials were used to quantify levels of the photosynthesis inhibitor sorgoleone in the undisturbed rhizosphere of sorghum plants. The materials used included stir bars coated with PDMS (stir bar sorptive extraction), technical grade optical fiber coated with a thin film of PDMS (matrix-solid phase microextraction), and PDMS tubing. PDMS tubing retained the most sorgoleone. As analyzed by high performance liquid chromatography, amounts of sorgoleone retained on the PDMS materials increased with time. Other materials tested (polyurethane foam plugs, C18 and Tenax disks, and resin capsules) proved less suitable, as they were subject to sometimes extensive penetration by fine root hairs. These results demonstrate the potential for PDMS-based materials to monitor the release of allelochemicals in the undisturbed rhizosphere of allelopathic plants. Unlike extraction procedures that recover all available compounds present in the soil, PDMS functions in a manner more analogous to plant roots in sorbing compounds from soil solution or root exudates. Information on chemical dynamics in the rhizosphere is crucial for evaluating specific hypotheses of allelopathic effects, understanding allelopathic mechanisms, and assessing the importance of allelopathic processes in plant communities.     

Root Distribution and Potential Interactions Between Allelopathic Rice,Sprangletop (<Emphasis Type="Italic">Leptochloa</Emphasis> spp.), and Barnyardgrass (<Emphasis Type="Italic">Echinochloa crus-galli</Emphasis>) based on <Superscript>13</Superscript>C Isotope Discrimination Analysis

Weed-suppressive rice cultivars hold promise for improved and more economical weed management in rice. Interactions between roots of rice and weeds are thought to be modulated by the weed-suppressive activity of some rice cultivars, but these phenomena are difficult to measure and not well understood. Thus, above-ground productivity, weed suppression, and root distribution of 11 rice cultivars and two weed species were evaluated in a drill-seeded, flood-irrigated system at Stuttgart, Arkansas, USA in a two-year study. The allelopathic cultivars, PI 312777 and Taichung Native 1 (TN-1), three other weed-suppressive cultivars, three indica-derived breeding selections, and three non-suppressive commercial cultivars were evaluated in field plots infested with barnyardgrass (Echinochloa crus-galli (L.) Beauv.) or bearded sprangletop (sprangletop, Leptochloa fusca (L.) Kunth var. fascicularis (Lam.) N. Snow). The allelopathic cultivars produced more tillers and suppressed both weed species to a greater extent than did the breeding selections or the non-suppressive cultivars. ¹³C isotope discrimination analysis of mixed root samples to a depth of 15 cm revealed that the allelopathic cultivars typically produced a greater fraction of their total root mass in the surface 0–5 cm of soil depth compared to the breeding selections or the non-suppressive cultivars, which tended to distribute their roots more evenly throughout the soil profile. These trends in root mass distribution were apparent at both early (pre-flood) and late-season stages in weed-free and weed-infested plots. Cultivar productivity and root distribution generally responded similarly to competition with the two weed species, but barnyardgrass reduced rice yield and root mass more than did sprangletop. These findings demonstrate for the first time that roots of the allelopathic cultivars PI 312777 and TN-1 explore the upper soil profile more thoroughly than do non-suppressive cultivars under weed-infested and weed-free conditions in flood-irrigated U.S. rice production systems. They raise the interesting prospect that root proliferation near the soil surface might enhance the weed-suppressive activity of allelochemical exudates released from roots. Plant architectural design for weed suppressive activity should take these traits into consideration along with other proven agronomic traits such as high tillering and yield.     

Difference in hydroxamic acid content in roots and root exudates of wheat (Triticum aestivum L.) and rye (Secale cereale L.): Possible role in allelopathy

Hydroxamic acids (Hx) produced by some cereal crops have been associated with allelopathy. However, the release of Hx to the soil by the producing plant-an essential condition for a compound to be involved in allelopathy-has not been shown. GC and HPLC analysis of roots and root exudates of wheat (Triticum aestivum L.) and rye (Secale cereale L.) cultivars, with high Hx levels in their leaves, demonstrated the presence of these compounds in the roots of all cultivars analyzed and in root exudates of rye. Moreover, bioassays employing root exudates collected from wheat and rye seedlings demonstrated that only rye exudates inhibited root growth of wild oats,Avena fatua L., a weed whose root growth is inhibited by Hx. These results suggest that rye could potentially interfere with the growth ofAvena fatua in nature and that this interference could be due to the release of Hx to the soil by way of roots.     

Allelopathic effect of alfalfa (Medicago sativa) on bladygrass (Imperata cylindrica)

Greenhouse and laboratory experiments were conducted at the Agricultural and Water Resources Research Center Station, Baghdad, in 1985 and 1986 to investigate the possible allelopathic potential of alfalfa (Medicago saliva L.) and its decomposed residues on bladygrass (Imperata cylin-drica L. Beauv.), a noxious weed in Iraq, and to isolate, characterize, and quantify possible allelopathic agents in alfalfa residues and root exudates. Results indicated that decomposed alfalfa roots and their associated soil produced a 51–56% reduction in bladygrass seed germination. Root and shoot length of bladygrass seedlings were reduced by an average of 88%. Decayed and undecayed mixtures of alfalfa roots and soil at 0.0151 (w/w) inhibited bladygrass seedlings reproduced from rhizomes by 30 and 42%. It was found that root exudates of alfalfa seedlings caused significant reduction in shoot and root dry weights of bladygrass seedlings when alfalfa and bladygrass were grown together in nutrient culture. Caffeic, chlorogenic, isochloro-genic,p-coumaric,p-OH-benzoic, and ferulic acids were detected in alfalfa root exudates and residues. The highest amount (126 fig phenolic acids/g soil) of these compounds was found in alfalfa root residues after six months of decomposition in soil.     

Root-secreted Allelochemical in the Noxious Weed <Emphasis Type="Italic">Phragmites Australis</Emphasis> Deploys a Reactive Oxygen Species Response and Microtubule Assembly Disruption to Execute Rhizotoxicity

Phragmites australis is considered the most invasive plant in marsh and wetland communities in the eastern United States. Although allelopathy has been considered as a possible displacing mechanism in P. australis, there has been minimal success in characterizing the responsible allelochemical. We tested the occurrence of root-derived allelopathy in the invasiveness of P. australis. To this end, root exudates of two P. australis genotypes, BB (native) and P38 (an exotic) were tested for phytotoxicity on different plant species. The treatment of the susceptible plants with P. australis root exudates resulted in acute rhizotoxicity. It is interesting to note that the root exudates of P38 were more effective in causing root death in susceptible plants compared to the native BB exudates. The active ingredient in the P. australis exudates was identified as 3,4,5-trihydroxybenzoic acid (gallic acid). We tested the phytotoxic efficacy of gallic acid on various plant systems, including the model plant Arabidopsis thaliana. Most tested plants succumbed to the gallic acid treatment with the exception of P. australis itself. Mechanistically, gallic acid treatment generated elevated levels of reactive oxygen species (ROS) in the treated plant roots. Furthermore, the triggered ROS mediated the disruption of the root architecture of the susceptible plants by damaging the microtubule assembly. The study also highlights the persistence of the exuded gallic acid in P. australis’s rhizosphere and its inhibitory effects against A. thaliana in the soil. In addition, gallic acid demonstrated an inhibitory effect on Spartina alterniflora, one of the salt marsh species it successfully invades. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Allelochemicals fromPolygonum sachalinense Fr. Schm. (Polygonaceae)

The root exudates fromPolygonum sachalinense in a recirculating system significantly inhibited lettuce seedling growth. The rhizomes and roots ofP. sachalinense were extracted with 80% acetone. Bioassay of the neutral-acidic fraction on the TLC agar plate showed the inhibitory activity corresponded to the two yellow pigment bands. Two orange needles were isolated and identified as anthraquinone compounds: emodin and physcion. Both compounds exhibited inhibitory activities against the seedling growth of several testing plant species. Glucosides were isolated fromP. sachalinense and were identified as emodin-1-O-β-D-glucoside and physcion-1-O-β-D-glucoside, respectively. On plant growth bioassay, these glucosides showed no phytotoxic activity against lettuce seedlings. The concentrations of emodin, physcion, and their glucosides from rhizome with roots, aerial parts, fallen leaves, and soil were determined. The rhizome with roots and fallen leaves contained emodin and physcion at relatively high concentrations. Emodin also occurs in the soil of this plant community with effective concentrations in the fall. The results indicate that these anthraquinones are responsible for the observed interference and are potent allelopathic substances.     

植物根系及其分泌物对微生物生长及活性的影响

通过无土培养获得植物（黑麦草、高羊茅及紫花苜蓿）的根系及其分泌物,以摇瓶培养的方式考察了其对微生物生长及活性的影响。结果表明,黑麦草、高羊茅及紫花苜蓿的根系及其分泌物能支持微生物的生长,并促进原油的降解;黑麦草与高羊茅处理间的差异不显著,但这两种草本植物与紫花苜蓿处理闻有显著差异;根系对微生物生长和原油降解的刺激效应持续时间比根系分泌物的刺激效应持续时间要长。     

Enhanced oxidative desulfurization of model fuels using a film-shear reactor

A film-shear reactor was used to enhance the oxidative desulfurization of thiophenes in fuels. With selected conditions, one pass of a model fuel through the film-shear reactor provided up to 55% removal of benzothiophene in only seconds at temperatures as low as 10 °C. Recirculation experiments showed that, if the flow rate and all other experimental parameters were held constant, the extent of thiophene removal increased as the residence time increased. Experiments using various concentrations of hydrogen peroxide and different fuel:oxidant ratios showed that, above a minimum amount, an increase in oxidant concentration did not lead to increased thiophene removal.     

Differences Among Rice Cultivars in Root Exudation, Methane Oxidation, and Populations of Methanogenic and Methanotrophic Bacteria in Relation to Methane Emission

Greenhouse experiments were conducted under subtropical conditions to understand the mechanism of rice cultivar differences in methane (CH₄) emission. Three rice cultivars were studied. Differences in CH₄ emission rates among the three rice cultivars became evident in the middle and late growth stages. Rice root exudates per plant measured as total released C were significantly different among rice cultivars. The effect of root exudates on CH₄ production in soil slurry differed accordingly. The amount of root exudates was not significantly different among rice cultivars when computed on a dry matter basis, indicating that it is positively correlated to root dry matter production. The root CH₄-oxidizing activity differed among rice cultivars. IR65598 had a higher oxidative activity than IR72 and Chiyonishiki. Root air space was not significantly different among rice cultivars at the late growth stage, indicating that it is probably not a factor contributing to cultivar differences in CH₄ emission. The population level of methanogenic bacteria differed significantly in soil grown to different rice cultivars, but not in roots, at booting stage and ripening stage. Methanotrophic bacteria population differed significantly in roots among rice cultivars at ripening. Rice cultivars with few unproductive tillers, small root system, high root oxidative activity, and high harvest index are ideal for mitigating CH₄ emission in rice fields.     

Root exudates of a legume tree as a nitrogen source for a tropical fodder grass

Exudation of nitrogenous compounds from the roots of dinitrogen-fixing plants is a potential source of nitrogen for adjacent plants in intercropping systems. We studied (1) the extent of N exudation from the roots of a tropical legume tree Gliricidia sepium (Jacq.) Kunth ex Walp., and (2) the ability of a C4 fodder grass Dichantium aristatum (Poir) C.E. Hubbard and its mycorrhizal symbionts to absorb N from tree exudates in a glasshouse experiment. Root exudates of ¹⁵N-labelled trees were collected in hydroponic culture and applied with irrigation water on grass grown in separate pots. During the 10-week experiment, the trees exuded 34.1 ± 5.0 mg of N, which represented 1.7 ± 0.2% of their total N by the end of the experiment. The total amount exuded would have been enough to supply 16% of grass N content by the end of the experiment. The grass, however, absorbed only 3.8–7.5% of ¹⁵N in exudates and gained 0.8–1.1% of its N from exudates. The low absorption of exudate N by grass was explained by probable soil microbial immobilisation and by the dilution of exuded N in the substantially larger pool of soil mineral N. A close contact between the root systems of N donor and recipient plants directly or via their mycorrhizal symbionts seems to be a precondition of the apparently direct N transfer earlier observed in field studies of the same soil-plant system.     

Subterranean Herbivore-induced Volatiles Released by Citrus Roots upon Feeding by <Emphasis Type="Italic">Diaprepes abbreviatus</Emphasis> Recruit Entomopathogenic Nematodes

Herbivore-induced volatile emissions benefit plant hosts by recruiting natural enemies of herbivorous insects. Such tritrophic interactions have been examined thoroughly in the above-ground terrestrial environment. Recently, similar signals have also been described in the subterranean environment, which may be of equal importance for indirect plant defense. The larvae of the root weevil, Diaprepes abbreviates, are a serious pest of citrus. Infestations can be controlled by the use of entomopathogenic nematodes, yet the interactions between the plant, insect and nematode are poorly understood and remain unpredictable. In bioassays that used a root zone six-arm olfactometer, citrus roots (‘Swingle citrumelo’ rootstock) recruited significantly more entomopathogenic nematodes (Steinernema diaprepesi) when infested with root weevil larvae than non-infested roots. Infested plants were more attractive to nematodes than larvae alone. Roots damaged by weevil larvae attracted more nematodes than mechanically damaged roots and sand controls. By dynamic in situ collection and GC-MS analysis of volatiles from soil, we determined that four major terpene compounds were produced by infested plant roots that were not found in samples from non-infested roots or soil that contained only larvae. Solvent extracts of weevil-infested roots attracted more nematodes than extracts of non-infested roots in a two choice sand-column bioassay. These findings suggest that Swingle citrus roots release induced volatiles as an indirect defense in response to herbivore feeding, and that some of these induced volatiles function as attractants for entomopathogenic nematodes.     

Sorghum Allelopathy—From Ecosystem to Molecule

Sorghum allelopathy has been reported in a series of field experiments following sorghum establishment. In recent years, sorghum phytotoxicity and allelopathic interference also have been well-described in greenhouse and laboratory settings. Observations of allelopathy have occurred in diverse locations and with various sorghum plant parts. Phytotoxicity has been reported when sorghum was incorporated into the soil as a green manure, when residues remained on the soil surface in reduced tillage settings, or when sorghum was cultivated as a crop in managed fields. Allelochemicals present in sorghum tissues have varied with plant part, age, and cultivar evaluated. A diverse group of sorghum allelochemicals, including numerous phenolics, a cyanogenic glycoside (dhurrin), and a hydrophobic p-benzoquinone (sorgoleone) have been isolated and identified in recent years from sorghum shoots, roots, and root exudates, as our capacity to analyze and identify complex secondary products in trace quantities in the plant and in the soil rhizosphere has improved. These allelochemicals, particularly sorgoleone, have been widely investigated in terms of their mode(s) of action, specific activity and selectivity, release into the rhizosphere, and uptake and translocation into sensitive indicator species. Both genetics and environment have been shown to influence sorgoleone production and expression of genes involved in sorgoleone biosynthesis. In the soil rhizosphere, sorgoleone is released continuously by living root hairs where it accumulates in significant concentrations around its roots. Further experimentation designed to study the regulation of sorgoleone production by living sorghum root hairs may result in increased capacity to utilize sorghum cover crops more effectively for suppression of germinating weed seedlings, in a manner similar to that of soil-applied preemergent herbicides like trifluralin.     

Effects of a Coumarin Derivative, 4-Methylumbelliferone,on Seed Germination and Seedling Establishment in <Emphasis Type="Italic">Arabidopsis</Emphasis>

The root system is central for plant adaptation to soil heterogeneity and is organized primarily by root branching. To search for compounds that regulate root branching, a forward chemical genetics screen was employed, and 4-methylumbelliferone (4-MU), a coumarin derivative, was found to be a potent regulator of lateral root formation. Exogenous application of 4-MU to Arabidopsis thaliana seeds affected germination and led to reduced primary root growth, the formation of bulbous root hairs, and irregular detached root caps accompanied by reorganization of the actin cytoskeleton in root tips before seedling establishment. Abundant lateral roots formed after exposure to 125 μM 4-MU for 22 days. Molecular, biochemical, and phytochemical approaches were used to determine the effect of 4-MU on root growth and root branching. Arabidopsis seedlings grown in the presence of 4-MU accumulated this compound only in roots, where it was partially transformed by UDP-glycosyltransferases (UGTs) into 4-methylumbelliferyl-β-D-glucoside (4-MU-Glc). The presence of 4-MU-Glc in seedling roots was consistent with the upregulation of several genes that encode UGTs in the roots. This shows that UGTs play an integral role in the detoxification of 4-MU in plants. The increased expression of two auxin efflux facilitator genes (PIN2 and PIN3) in response to 4-MU and the lack of response of the auxin receptor TIR1 and the key auxin biosynthetic gene YUCCA1 suggest that auxin redistribution, rather than auxin biosynthesis, may directly or indirectly mediate 4-MU-induced root branching.     

Studies on 3‐(2‐Ethylhexyl)thiophene Polymers

The regio‐regularity and properties of the poly[3‐(2‐ethylhexyl)thiophene] were studied, and compared with the poly[3′‐(2‐ethylhexyl)‐2,2′,5′,2′′‐terthiophene]. The thermal stability of the polymers was improved due to the addition of thiophene rings with (2‐ethylhexyl)thiophene. Due to the additional thiophenes at the 2,5 position of ethylhexylthiophene, even though the absorption wavelength was red‐shifted, there is no characteristic change in photoemission for the latter polymer. Addition of thiophene rings to the 3‐(2‐ethylhexyl)thiophene improves the thermal properties without loosing the 3‐(2‐ethylhexyl)thiophene characteristics.     

Hormonomic Changes Driving the Negative Impact of Broomrape on Plant Host Interactions with Arbuscular Mycorrhizal Fungi

Belowground interactions of plants with other organisms in the rhizosphere rely on extensive small-molecule communication. Chemical signals released from host plant roots ensure the development of beneficial arbuscular mycorrhizal (AM) fungi which in turn modulate host plant growth and stress tolerance. However, parasitic plants have adopted the capacity to sense the same signaling molecules and to trigger their own seed germination in the immediate vicinity of host roots. The contribution of AM fungi and parasitic plants to the regulation of phytohormone levels in host plant roots and root exudates remains largely obscure. Here, we studied the hormonome in the model system comprising tobacco as a host plant, Phelipanche spp. as a holoparasitic plant, and the AM fungus Rhizophagus irregularis. Co-cultivation of tobacco with broomrape and AM fungi alone or in combination led to characteristic changes in the levels of endogenous and exuded abscisic acid, indole-3-acetic acid, cytokinins, salicylic acid, and orobanchol-type strigolactones. The hormonal content in exudates of broomrape-infested mycorrhizal roots resembled that in exudates of infested non-mycorrhizal roots and differed from that observed in exudates of non-infested mycorrhizal roots. Moreover, we observed a significant reduction in AM colonization of infested tobacco plants, pointing to a dominant role of the holoparasite within the tripartite system.     

Root Chemistry in Populus tremuloides: Effects of Soil Nutrients,Defoliation, and Genotype

Although genetic, environmental, and G x E effects on aboveground phytochemistry have been well documented in trembling aspen (Populus tremuloides), little work has focused on the same factors affecting tissues underground. Belowground plant defenses are likely important mediators of root-feeding herbivores that can strongly influence plant fitness. We used a common garden of potted aspen trees to explore the individual and interactive effects of soil nutrient availability, foliar damage, genotype, and their interactions, on concentrations of phytochemicals in aspen roots. Our common garden experiment employed 12 aspen genotypes that were planted into either low- or high-nutrient soil environments. Half of the trees were subjected to defoliation for two successive years, while the others were protected from damage. At the end of the growing season after the second defoliation, we harvested the trees to obtain root samples for which we assessed levels of phenolic glycosides, condensed tannins, nitrogen, and starch. Phenolic glycosides were most affected by genotype, while the other root phytochemicals were most responsive to soil nutrient conditions. The effects of defoliation were observed in interaction with soil nutrient environment and/or genotype. Interestingly, the effect of defoliation on phenolic glycosides was mediated by soil nutrients, whereas the effect of defoliation on condensed tannins was observed in concert with effects of both soil nutrients and genotype. Comparison of data from this study with an earlier, related study revealed that concentrations of phenolic glycosides and condensed tannins are lower in roots than leaves, and less responsive to defoliation. That soil nutrient environment affects root phytochemical concentrations is not unexpected given the intimate association of roots and soil, but the complex interactions between soil nutrients, aboveground damage, and genotype, and their effects on root phytochemistry, are intriguing. Variation in root chemistry could have wide-reaching effects on soil microbial communities, nutrient cycling, and herbivores. Additionally, the response of phytochemicals to damage across organs can link different, spatially separated herbivores as they use different parts of the same plant resource.     

Monooxygenase- and Dioxygenase-Catalyzed Oxidative Dearomatization of Thiophenes by Sulfoxidation,cis-Dihydroxylation and Epoxidation

Enzymatic oxidations of thiophenes, including thiophene-containing drugs, are important for biodesulfurization of crude oil and drug metabolism of mono- and poly-cyclic thiophenes. Thiophene oxidative dearomatization pathways involve reactive metabolites, whose detection is important in the pharmaceutical industry, and are catalyzed by monooxygenase (sulfoxidation, epoxidation) and dioxygenase (sulfoxidation, dihydroxylation) enzymes. Sulfoxide and epoxide metabolites of thiophene substrates are often unstable, and, while cis-dihydrodiol metabolites are more stable, significant challenges are presented by both types of metabolite. Prediction of the structure, relative and absolute configuration, and enantiopurity of chiral metabolites obtained from thiophene enzymatic oxidation depends on the substrate, type of oxygenase selected, and molecular docking results. The racemization and dimerization of sulfoxides, cis/trans epimerization of dihydrodiol metabolites, and aromatization of epoxides are all factors associated with the mono- and di-oxygenase-catalyzed metabolism of thiophenes and thiophene-containing drugs and their applications in chemoenzymatic synthesis and medicine.     

Phytotoxicity of <Emphasis Type="Italic">Phytolacca americana</Emphasis> Leaf Extracts on the Growth,and Physiological Response of <Emphasis Type="Italic">Cassia mimosoides</Emphasis>

We examined the allelochemical effects of control soil, native soil (treated soil), and leaf extracts of Phytolacca americana (pokeweed) on the germination rate and seedling growth of Cassia mimosoides var. nomame. We also studied the resulting changes in root-tip ultrastructure and peroxidase isozyme biochemistry. P. americana leaf extract inhibited seed germination, seedling growth, and biomass when compared to control and treated soil. Root and shoot growth in treated soil was stimulated relative to control soil, but root growth was inhibited by 50% in the leaf extract treatment. Biomass of C. mimosoides seedlings grown on leaf extract was reduced sevenfold when compared to the control seedlings. The amounts of total phenolic compounds in the leaf extract, treated soil, and control soil were 0.77, 0.14, and 0.03 mg l⁻¹, respectively. The root tips of C. mimsoides treated with leaf extracts of P. americana showed amyloplasts and large central vacuoles with electron-dense deposits inside them when compared to control root tips. The activity of guaiacol peroxidase (GuPOX) in whole plant, roots, and shoots of C. mimosoides increased as leaf extract increased; maximum activity was observed in extract concentrations of 75% and higher. Root GuPOX activity was three times higher than in shoots. Therefore, we conclude that inhibition of C. mimosoides growth is related to the phenolic compounds in the P. americana leaf extract and the ultrastructure changes in root-tip cells and increased GuPOX activity is a response to these allelochemicals.