Enantiospecific synthesis fromd-fructose of (2S,5R)- and (2R,5R)-2-methyl-1,6-dioxaspiro[4.5]decane [the odor bouquet minor components ofParavespula vulgaris (L.)]

The synthesis of (2S,5R)-(1) and (2R,5R)-2-methyl-1,6-dioxaspiro [4.5]decane (2) from (2RS,5R,8R,9R,10S)-8,9,10-trihydroxy-2-methyl-1, 6-dioxaspiro[4.5]decane (8), obtained in five steps fromd-fructose using Wittig's methodology, reduction, and spiroketalation, has been accomplished by a Corey dideoxygenation at C-8,9, followed by a Barton deoxygenation at C-10, of the appropriately protected derivatives.Enantiospecific synthesis of spirocetals. Part V. For Part IV, see Izquierdo et al. (1992).     

Synthesis and surface properties of chemodegradable anionic surfactants: Sodium carboxylates of cis-1,3-dioxane derivatives

The 2-n-alkyl-5-carboxy-5-methyl-1,3-dioxanes were obtained in good yield from the reaction of aliphatic aldehydes with 2,2-bis(hydroxymethyl)propionic acid in dichloromethane solution, catalyzed by p-toluenesulfonic acid monohydrate. Nuclear magnetic resonance analysis indicated that they were pure cis-isomers with the axial configuration of the carboxylic group at the C-5 carbon atom of the 1,3-dioxane ring. The acids were converted, with retention of the configuration, to their sodium salts by reaction with sodium methoxide or sodium hydroxide in methanol. The physicochemical properties of the acids and sodium salts, as well as their surface properties at the aqueous solution-air interface, were determined. Critical micelle concentration, surface excess concentration, surface area demand per molecule of sodium salts at the monomolecular surface layer, and standard free energy of micellization were determined based on surface tension measurements. Part XXXIV in the series: Chemical Structure and Surface Activity. Part XXXIII: A. Piasecki, A. Sokolowski, B. Burczyk, R. Gancarz, and U. Kotlewska, Synthesis, Surface Properties and Hydrolysis of Chemodegradable Anionic Surfactants: Diastereomerically Pure cis- and trans-2,5-Disubstituted-1,3-dioxanes. J. Colloid Interface Sci. 192:74–82 (1997).     

Up in smoke: II. Germination ofNicotiana attenuata in response to smoke-derived cues and nutrients in burned and unburned soils

Nicotiana attenuata is a native tobacco that is commonly found usually one growing season after fires in the blackbrush, sagebrush and pinyon-juniper forests of the Great Basin desert of North America. This plant also occurs in isolated dry washes and roadsides for many consecutive seasons. Postfire annuals are thought to synchronize their germination from the seed bank with the postfire environment in response to increases in (1) fire-related cues or (2) nutrient supply rates resulting from the mineralization of nutrients by fire, or (3) the removal of allelochemicals produced by the dominant vegetation occupying the sites before the burn or the microbial community associated with the dominant vegetation. We examine the effect of these three changes on the germination ofN. attenuata seed from artificial seed banks made with burned and unburned soil taken in 1993 from under four dominant shrub species (Coleogyne ramosissima, Yucca baccata, Lycium andersonnii, Purshia tridentata) of an area that burned in 1992 and from two dry washes in whichN. attenuata populations have persisted since at least 1988. We utilize our recent discovery that aqueous extracts of wood smoke contain potent germination cue(s) for this species and the established observation that nitrate stimulates germination in manyNicotiana species. In two experiments, we added smoke-derived germination cues and nutrients separately and in combinations to the artificial seed banks, measured germination rates, and inferred the effect of burning by the response of the seed banks to these additions. Germination rates of seed in burned soil were consistently higher than those in unburned soil collected from under all species tested; concentrations of nitrate, P, Mn, and Ca were also higher in burned than unburned soils. Because the addition of more cue and nitrate to burned soil increased germination rates, these soil components may not be at concentrations sufficient to saturate the germination response one year after a fire. The germination of seeds in soil collected from beneath unburnedYucca bacatta plants increased to the same level as that found under burned plants of the same species with the addition of cue and nitrate. Similarly, unburned bitterbrush soil attained the same germination potential with the addition of cue and a complete nutrient solution as burned bitterbrush soil. We conclude that the effect of fire on the germination potential of bitterbrush and yucca soil is due to increases in germination cue and nutrients. However, since the addition of cue and nutrients to the unburned soils under blackbrush and wolfberry shrubs did not elevate the germination potential of these soils to that found in burned soils, we conclude that these species alter the soil so as to inhibit germination and burning reverses this alteration. The presence of persistentN. attenuata populations in washes could not be attributed to particular chemical characteristics of these soils. Additions of cue dramatically increased germination potential of these soils, whereas the addition of nitrate did not. The concentrations of most mineral nutrients resembled those found in the unburned sites with the exception of Mn, Cu, and Zn, which were higher. However, these cations do not influence germination rates. Treatment of soil taken from these washes with cue resulted in significant increases in germination ofN. attenuata seeds in the natural seed bank compared to water-treated controls, demonstrating that the plants growing in washes also produce dormant seeds that require the smoke cue for germination.     

Effect of Phenolic Acids in Soil under and Between Rows of a Prior Sorghum (Sorghum bicolor) Crop on Germination, Emergence, and Seedling Growth of Peanut (Arachis hypogea)

Sorghum bicolor is an allelopathic crop that reduces the yield of succeeding crops. We have assessed its effect on the germination, emergence, and seedling growth of Arachis hypogea sown in soil that had had a prior sorghum cropping. A. hypogea was sown on rows and interrows of a previous sorghum crop in 1997 and 1998 in Senegal. Seedling establishment (germination rate and seedling weight) was better between rows than on rows of the previous crop. The highest concentrations of phenolic compounds occurred in the rows in 1998, while contents of row and interrow soils were similar in 1997. Vanillic acid was the main component of the six chemicals found in 1997 soils, whereas the 1998 soil samples contained mainly p-hydroxybenzoic acid, p-hydroxybenzaldehyde, vanillic, and p-coumaric acids (10 phenolics identified). The germination of peanut seeds in water (control), soil water extracts, and mixtures of pure phenolics (equivalent to those in 1997 and 1998 soil samples) was tested. All extracts inhibited germination compared to controls, but there was no significant difference among treatments, i.e., the inhibition was the same for seeds in soil solutions and those in the respective phenolic mixtures. Similarly, there were no significant differences among the germination rates in soil water extracts of rows and interrows or in the pure phenolic mixtures of rows and interrows. We propose a geometrical sowing pattern for peanuts between the rows of the previous sorghum crop to escape the latter's "allelopathic heritage."     

Significance of medium chainn-alkanes as accompanying compounds in hemipteran defensive secretions: An investigation based on the defensive secretion ofCoridius janus

A mixture oftrans-2-hexenal;n-tridecane (60:40, w/w), the natural combination present in the defensive secretion ofCoridius janus (Hemiptera; Pentatomidae) was shown by comparison with similar aldehyde mixtures having longer and shorter chainn-alkanes, to be the optimal combination as a fumigant and a repellent against three test insect species,Anoplolepis longipes, Sitotroga cerealella, andCulex quinquefasciatus. Toxic values obtained for the three insects, respectively: 1/LC₅₀, 1.72, 4.54, and 6.66 ppm while repellencies were 63%, 50%, and 69%. This study revealed that amongt-2-hexenal:n-alkane combinations those with medium carbon chains,viz C-11 and C-12, also possessed high toxicities and repellencies comparable to that of the natural combination while those with shorter and longern-alkanes possessed lower activity. Toxicities and repellencies ofn-alkane series were only moderate to low showing highest toxic values forn-tridecane at 1/LC₅₀, 0.39, 2.32, and 2.32 ppm and repellencies at 31%, 30%, and 32% for the three test insects, respectively. This series, nevertheless, showed similar variation, medium length chains, C-11, C-12, and C-13 showing comparatively higher activity than other alkanes of shorter and longer chains. This study also revealed that the fumigant property of both alkane and aldehyde are of equal importance while repellency is heavily dependent on the aldehyde.     

Methyl jasmonate as an allelopathic agent: sagebrush inhibits germination of a neighboring tobacco,Nicotiana attenuata

Artemisia tridentata ssp. tridentata is the dominant and defining shrub in the Great Basin Desert, with well-documented allelopathic tendencies that have generally been ascribed to its most abundantly released secondary metabolites. However, as a minor component, sagebrush releases a highly biologically active substance, methyl jasmonate (MeJA), which is known to function as both a germination inhibitor and promoter in laboratory studies. Nicotiana attenuata is a tobacco species native to the Great Basin Desert and grows in newly burned juniper–sagebrush habitats for 2–3 yr following a fire. With a combination of field and laboratory studies, we examined the role of MeJA release from sagebrush by both air and water transport in inhibiting N. attenuata seed germination. We demonstrated that sagebrush interacts allelopathically with the seed bank of N. attenuata through its release of MeJA. In the field, seeds buried 0–40 cm from sagebrush plants for 4 months in net bags had significantly reduced germination compared to seeds buried similarly but protected in plastic bags. Moreover, germination on soils collected from underneath sagebrush plants was reduced by 60% compared to seeds placed on soils collected between sagebrush plants or outside of the sagebrush population. Exposure to A. tridentata seeds and seedlings did not affect N. attenuata germination, suggesting that established sagebrush plants only influence the tobacco's seed bank. In the laboratory, exposure of seeds to sagebrush emissions resulted in germination delays of up to 6 d. Exposure to volatile and aqueous MeJA also inhibited germination of N. attenuata seeds at quantities that are released naturally by sagebrush: 3.5 g/hr and 1.12 g/seed cup (56 ng/seed), respectively. A. tridentata seeds were significantly more resistant to MeJA, being inhibited at 336 g MeJA (16.8 g/seed), 300 times greater than the level of aqueous MeJA required to inhibit N. attenuata seeds. MeJA inhibited N. attenuata germination regardless of the seed's dormancy state and the specific epimer (trans- or cis-) of MeJA. Germination on sagebrush chaff that had been heated to reduce MeJA content was negatively correlated with the amount of MeJA remaining in the chaff. Germination of a nondormant, conspecific tobacco, N. trigonophylla, which grows in the same area but is not associated with fire, is less sensitive than N. attenuata to the extracts of sagebrush litter, but similarly sensitive to MeJA. Additionally, four of five other tobacco species that are not known to be associated with sagebrush are less sensitive to MeJA, suggesting an evolved sensitivity to MeJA. To determine the proportion of germination inhibition of a sagebrush extract that could be attributed to MeJA, we serially diluted sagebrush extracts with water and restored the quantity of MeJA of the original extract by adding appropriate quantities of synthetic MeJA; 16–60% of the inhibitory activity of the original extract could be attributed to the MeJA. We conclude that MeJA release from sagebrush plays an allelopathic role for N. attenuata seed banks, but other unidentified compounds are also involved.     

Effects of Root Isoquinoline Alkaloids from <Emphasis Type="Italic">Hydrastis canadensis</Emphasis> on <Emphasis Type="Italic">Fusarium oxysporum</Emphasis> Isolated from <Emphasis Type="Italic">Hydrastis</Emphasis> Root Tissue

Goldenseal (Hydrastis canadensis L.) is a popular medicinal plant distributed widely in North America. The rhizome, rootlets, and root hairs produce medicinally active alkaloids. Berberine, one of the Hydrastis alkaloids, has shown antifungal activity. The influence of a combination of the major Hydrastis alkaloids on the plant rhizosphere fungal ecology has not been investigated. A bioassay was developed to study the effect of goldenseal isoquinoline alkaloids on three Fusarium isolates, including the two species isolated from Hydrastis rhizosphere. The findings suggest that the Hydrastis root extract influences macroconidia germination, but that only the combined alkaloids—berberine, canadine, and hydrastine—appear to synergistically stimulate production of the mycotoxin zearalenone in the Fusarium oxysporum isolate. The Hydrastis root rhizosphere effect provided a selective advantage to the Fusarium isolates closely associated with the root tissue in comparison with the Fusarium isolate that had never been exposed to Hydrastis.     

Low-temperature selective catalytic reduction of NO with NH3 based on MnOx-CeOx/ACFN

MnO _x -CeO _x /ACFN were prepared by the impregnation method and used as catalyst for selective catalytic reduction of NO with NH3 at 80°C-150°C. The catalyst was characterized by N₂-BET, scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR). The fraction of the mesopore and the oxygen functional groups on the surface of activated carbon fiber (ACF) increased after the treatment with nitric acid, which was favorable to improve the catalytic activities of MnO _x -CeO _x /ACFN. The experimental results show that the conversion of NO is nearly 100% in the range 100°C-150°C under the optimal preparation conditions of MnO _x -CeO _x /ACFN. In addition, the effects of a series of performance parameters, including initial NH₃ concentration, NO concentration and O₂ concentration, on the conversion of NO were studied. __________ Translated from Chemical Industry and Engineering Progress, 2007, 27(1): 87–91 [译自: 化工进展]     

Enantiospecific synthesis of (R)-1,7-dioxaspiro [5.5]undecane [major component of olive fruit fly (Dacus Oleae) sex pheromone] from d-Fructose

The synthesis of the title compound13 has been carried out through the preparation of its precursor, (3R,4R,5S,6R)-3,4,5-trihydroxy-1,7-dioxaspiro[5.5]undecane (6), obtained fromd-fructose using Wittig's methodology, reduction, and spiroketalation. Compound6 was transformed into13 by a Barton deoxygenation at C-5 followed by a Corey dideoxygenation at C-3,4 of the appropriately protected derivatives.Enantiospecific synthesis of spiroacetals. Part II. For Part I, see Izquierdo and Plaza (1990).     

Inhibitory Effect of Termite Fecal Pellets on Fungal Spore Germination

The dampwood termite Zootermopsis angusticollis lines nest chambers and galleries with fecal pellets. The antifungal properties of feces were tested by recording germination rates of spores of the fungus Metarhizium anisopliae that had been incubated with various concentrations of fecal material. The presence of fecal pellet material significantly decreased the germination rates of spores relative to those of control spore solutions lacking fecal material. Spore germination rates were inversely proportional to the amount of fecal matter present in the spore–feces suspensions but were independent of incubation time. The fungistatic effect of the fecal material is virtually immediate and does not require prolonged contact with spores to inhibit germination. This mechanism of biochemical protection may reduce risks of fungal infection in termite nests.     

Phenylacetic acid as a phytotoxic compound of corn pollen

Phenylacetic acid (PAA), one of the phytotoxic compounds in corn (Zea mays) pollen, was identified by GC-MS and by direct comparison with a pure commercial sample of PAA. Bioassays were carried out by testing whole pollen, methylene chloride extract of the pollen, and pure PAA on germination and radical growth ofAmaranthus leucocarpus andEchinochloa crusgalli. The effect of corn pollen was compared with that ofZea mexicana (Teosinte), one of the wild relatives of cultivated maize.     

Allelochemical regulation of reproduction and seed germination of two BrazilianBaccharis species by phytotoxic trichothecenes

The potent phytotoxic trichothecene roridins and baccharinoids occur naturally in the Brazilian plants,Baccharis coridifolia andB. megapotamica. Biosynthesis of roridins inB. coridifolia appears to be linked to pollination, and the phytotoxins then accumulate in the seed. The roles of the phytotoxins in pollination, seed maturation, and germination of theBaccharis species were investigated. The high production of roridins occurred only in seeds resulting from intraspecific pollination, and the concentration of the toxins in the seeds generally increased with seed maturity. Removal of seed coats from trichothecene-producing BrazilianBaccharis species (B. coridifolia andB. megapotamica) and non-trichothecene-producing AmericanBaccharis species (B. halimifolia andB. glutinosa) resulted in improved seed germination ofB. halimifolia andB. glutinosa but complete inhibition of seed germination ofB. coridifolia andB. megapotamica. Addition of seed coat extracts of the BrazilianBaccharis species of dilute solutions (10^–6g/ml) of roridins or baccharinoids to the decoated seeds ofB. coridifolia andB. megapotamica resulted in germination, while seeds ofB. halimifolia andB. glutinosa were killed by the phytotoxins. Roridins interacted with gibberellic acid, a germination promoter, but not with abscisic acid, a germination inhibitor. The results from this study suggest that macrocyclic trichothecenes have a regulatory role(s) on reproduction and germination of BrazilianBaccharis species in their natural habitat.     

Perception byTrogoderma species of chirality and methyl branching at a site far removed from a functional group in a pheromone component

Responses to enantiomers of (Z)- and (E)-trogodermal (14-methyl-8-hexadecenal) suggest that fourTrogoderma species utilize the (R)-(–) configuration at C-14. Removal of the C-14 methyl branch decreased the response. These results demonstrate the high specificity associated with the configuration at a chiral center, or the methyl branch, distant in terms of numbers of bonds from a functional group.Coleoptera: Dermestidae.     

Evaluation of Synergism in the Feeding Deterrence of Some Furanocoumarins on Spodoptera littoralis

The phagodepression activity of five furanocoumarins (FC), bergapten, xanthotoxin, psoralen, imperatorin, and angelicin, has been studied against larvae of Egyptian cotton leafworm Spodoptera littoralis (Boisduval) (Lepidoptera: Noctuidae) using a leaf-disk choice bioassay. The dose range used was 0–10 g/cm² for linear furocoumarins and 0–30 g/cm² for angelicin, the angular furanocoumarin. Dose–feeding deterrency activity correlations were governed by various sigmoidal functions, except in the case of imperatorin and bergapten, which had dose–response curves showing irregular traces with two maxima. All five FCs induced various degrees of phagodepression in S. littoralis; at 3 g/cm², the relative feeding deterrence was bergapten = xanthotoxin > psoralen = imperatorin = angelicin. Structure–activity correlations indicated that a methoxy group on C-5 or C-8 enhanced the activity. Comparison of experimental feeding deterrence of binary mixtures of imperatorin with xanthotoxin, bergapten, or psoralen at various relative concentrations with the calculated additive activity of each combination indicated that a proportion of 40% or more of imperatorin may exert a greater antifeedant effect on S. littoralis than the sum of individual compounds. The effect of angelicin also was examined in combination with psoralen, both of which are present in Psoralea plants. Their mixtures yielded a clear synergistic effect in the 20–80% angelicin range (coactivity coefficient = 124–133). By contrast, the effect of angelicin on xanthotoxin was only additive. In the dose–response curves of imperatorin/bergapten combinations, synergism was found at >60% imperatorin relative concentration and 1 g/cm², whereas lower proportions led to antagonism. The threshold between the two opposing effects was found to depend on the total FC concentration employed. Some natural systems contain FCs in the range of synergistic proportions recorded here and, thus, may have been produced by the host to increase its defensive effect at a lower metabolic cost.     

Phenolic Metabolites in Leaves of the Invasive Shrub, Lonicera maackii, and Their Potential Phytotoxic and Anti-Herbivore Effects

Lonicera maackii is an invasive shrub in North America for which allelopathic effects toward other plants or herbivores have been suspected. We characterized the major phenolic metabolites present in methanol extracts of L. maackii leaves. In addition, we examined the effects of methanol–water extracts of L. maackii leaves on seed germination of a target plant species and on feeding preference and growth rate of a generalist insect herbivore. A total of 13 individual major and minor compounds were detected in crude leaf extracts by high-performance liquid chromatography coupled to electronspray ionization-tandem mass spectrometry (ESI-MS/MS). Extracts were dominated by two major flavones, apigenin and luteolin, and their glucoside derivatives, apigenin-7-glucoside and luteolin-7-glucoside. Quantities of these compounds, along with chlorogenic acid, varied between two sampling points. Leaf extracts that contained these compounds were inhibitory to seed germination of Arabidopsis thaliana. In addition, treatment of artificial diet with leaf extracts deterred feeding of the generalist herbivore, Spodoptera exigua, in choice experiments but had no effect on growth rate in short-term no-choice bioassays. Purified apigenin tended to deter feeding by S. exigua and inhibited seed germination of A. thaliana. We conclude that leaves of L. maackii contain phenolic compounds, including apigenin and chlorogenic acid, capable of having biological effects on other plants and insects.     

<Emphasis Type="Italic">Bidens pilosa</Emphasis> L. Exhibits High Sensitivity to Coumarin in Comparison with Three Other Weed Species

Nine natural plant compounds were screened for phytotoxicity to Bidens pilosa L. a troublesome weed in field and plantation crops. The sensitivity of three other weed species to coumarin, the most active identified compound, was also evaluated. Coumarin, at a concentration of 500 μM, had little effect on germination and growth of Senna obtusifolia L., Euphorbia heterophylla L., and Ipomoea grandifolia L. when compared with its effects on B. pilosa L. In a concentration range of 10–100 μM, coumarin caused a dose-dependent inhibition of germination and growth of B. pilosa L. The measurements of some parameters of energy metabolism revealed that coumarin-treated root tissues exhibited characteristics of seedlings in an earlier stage of growth, including higher respiratory activity and higher activities of alcohol dehydrogenase and lipoxygenase. These results suggest that coumarin inhibition of germination and growth of B. pilosa L. was not a consequence of an impairment of energy metabolism. Rather, it seems to act as a cytostatic agent, retarding germination. At concentrations above 50 μM, coumarin increased lipoxygenase activity and the level of conjugated dienes of root extracts, suggesting that it may induce oxidative stress in seedling roots.     

Phytogrowth Properties of Limonoids Isolated from Cedrela ciliolata

An epimeric mixture of the limonoid photogedunin, isolated from the heartwood of C. ciliolata, as well as its R- and S-acetate derivative were evaluated for phytotoxicity in mono- and dicotyledoneous plants. A mixture of epimeric photogedunin 1 and 2, a mixture of epimeric photogedunin acetate, (R)-photogedunin acetate 3, and (S)-photogedunin acetate 4 inhibited seed germination, seedling growth, and root and hypocotyl/coleoptyle growth in all species assayed. The concentration of phytochemicals required for 50% inhibition ranged from 4.5 to 300 M. Inhibitory plant responses appeared to require that the OH-group at C-23 be acetylated in photogedunin, since the nonacetylated compound showed less phytotoxic activity.     

Quantitative analysis of partial acylglycerols and free fatty acids in palm oil by 13C nuclear magnetic resonance spectroscopy

The chemical shifts, spin-lattice relaxation times (T ₁), and one-bond C−H coupling constants of the glycerol carbons of mono-, di-, and triacylglycerols in CDCI₃ solution are presented and discussed. The glycerol carbons have low T ₁ values (<1.0 s) and full nuclear Overhauser effect and also exhibit broader linewidths than the aliphatic carbons, suggesting that the glycerol carbons are at or near the T ₁ minimum for the dipole-dipole relaxation mechanism. Therefore, for quantitative measurement of the composition of partial acylglycerols (relative to the triacylglycerols) in palm oil, the nuclear magnetic resonance (NMR) spectrum of the β-carbons, which lie exclusively in the region δ68.3–72.1 ppm, should preferably be acquired at medium or low magnetic fields and at an elevated temperature in order to ensure that the condition for extremely narrow spectral lines is satisfied. The chemical shifts and spinlattice relaxation times of the aliphatic C-2 and C-3 carbons and of the carbonyl carbons (C-1) of acyl groups present in palm oil are also presented and discussed. The presence of free fatty acid in the palm oil is easily detected and quantified in the spectrum of the aliphatic carbons. The presence of partial acylglycerols in palm oil can also be detected and/or quantified in the NMR spectra of the C-2 and the carbonyl carbons. The quantitative analysis of the glycerol carbons of a known mixture of acylglycerols obtained by using this method is presented.     

Selective Phytotoxic Activity of Withanolides from Iochroma australe to Crop and Weed Species

Phytotoxic effects of Iochroma australe extract and 4,7,20-oxowithanolides were examined in Petri dish bioassays at 10, 100, and 1000 ppm. The extract and the major constituent (17S,20R,22R)-4,7,20-trihydroxy-1oxowitha-2,5,24-trienolide (2) reduced growth of the radicle of the weeds Sorghum halepense (Monoct.) and Chenopodium album (Dicot.). Neither the extract nor withanolides had significant effect on germination or radicle length of the commercial crop species Lactuca sativa. From our experimental data we conclude that the withanolides tested here are natural products with selective herbicidal activity against weed species.     

Release of allelochemical agents from litter,throughfall, and topsoil in plantations ofEucalyptus globulus Labill in Spain

Natural leachates ofEucalyptus globulus (throughfall, stemflow, and soil percolates) were collected daily during rainy spells in the vegetative period (February–July), and their effects on the germination and radicle growth ofLactuca sativa were measured. Concurrently, the effects ofL. sativa of topsoil and leachates from decaying litter were determined. The results suggest that toxic allelochemicals released byEucalyptus globulus may influence the composition and structure of the understory of the plantation and that this effect is attributable mainly to the decomposition products of decaying litter rather than to aerial leachates. The soil may neutralize or dilute allelopathic agents, at least below the top few cms.