Phytotoxic and antimicrobial activity of volatile constituents ofArtemisia princeps var.orientalis

The volatile constituents ofArtemisia princeps var.orientalis (wormwood) were investigated for phytotoxic and antimicrobial activities. The germination and radicle elongation of receptor plants were inhibited by volatile substances emitted from wormwood leaf and effects were concentration-dependent. Essential oil of the plant extracted by Karlsruker's apparatus suppressed seed germination and seedling elongation of the receptor plants at a threshold concentration of 4.8l/100 ml.Escherichia coli was not susceptible to the wormwood essential oil, but the growth ofBacillus subtilis, Aspergillus nidulans, Fusarium solani, andPleurotus ostreatus was inhibited severely.     

Assessment of allelopathic potential inArtemisia princeps var.Orientalis residues

Field and laboratory studies were conducted to examine the differential phytotoxicity of residues ofArtemisia princeps var.orientalis (wormwood) using various plants as test species. Seedling elongation and dry weights of receptor plants were inversely proportional to the concentration and incubation time of dry leaves ofA. princeps var.orientalis in vermiculite. In seedling growth tests with abandoned field soils (control) and soil underneath wormwood plants (test), the elongation, dry weight, and caloric content of seedlings grown in the soil from under wormwood plants were severely inhibited, thereby suggesting that certain growth inhibitors were released from wormwood and the inhibitor remained in the soil.     

Influence ofArtemisia princeps var.orientalis components on callus induction and growth

An in vitro study was performed to determine the potential application of tissue culture in determining allelopathic potentialof Artemisia princeps var.Orientalis (wormwood). Aqueous extracts and volatile substances ofA. princeps var.Orientalis were tested to determine their effects on callus induction and growth of several tested species. Extracts of 5%A. princeps var.Orientalis caused some reduction in concentration, induction, and growth of callus, although they looked normal, whereas the expiants of most receptor plants did not develop callus at higher concentration. Lettuce andEclipta prostrata were the most sensitive species, andA. princeps var.Orientalis was affected by its own extracts. The growth of calluses in MS 121 medium treated with essential oil ofA. princeps var.Orientalis was inhibited, and the degree of inhibition was proportional to the concentration of the essential oil.     

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.     

Effects of phenolic acids on germination and early growth of herbaceous woodland plants

Four herbaceous plant species from woodland (clearings),Deschampsia flexuosa, Scrophularia nodosa, Senecio sylvaticus, andChamaenerion angustifolium, were tested for their sensitivity to phenolic acids. Seven commonly occurring phenolic compounds were used in a germination experiment in concentrations ranging from 0.01 to 10 mM, i.e., salicylic,p-hydroxybenzoic, syringic, caffeic, vanillic,p-coumaric, and ferulic acids. Germination was delayed rather than inhibited. Radicle elongation was strongly affected; at lower concentrations stimulatory effects were observed, whereas at high concentrations radicle elongation was severely reduced. Salicylic acid was the most effective phenolic compound, whereas caffeic acid caused no effects. Early growth was studied in more detail in a second experiment withDeschampsia flexuosa andSenecio sylvaticus and the phenolic acids, ferulic and p-coumaric acid. Primary root length, number and length of secondary roots, and dry weight were stimulated at 0.01 mM but were inhibited at 10 mM of both compounds. The results are discussed in view of the allelopathic relations between trees and herbaceous understory vegetation.     

HPLC identification of allelopathic compounds fromLantana camara

Aqueous extracts ofLantana camara L. leaves inhibited ryegrass (Lolium multiflorum Lam.) germination and seedling growth. Phytotoxic compounds were fractionated from crude aqueous extracts and fractions were evaluated for their phytotoxicity. Inhibition was most pronounced with the alkaline and acid hydrolysates. Plant inhibition by the crude extract reflected a complex interaction of numerous individual components of diverse chemical compositions and potencies. Presumptive identification of the individual components was accomplished with high-performance liquid chromatography (HPLC). Thirteen phenolic compounds were identified, and most of these compounds were phytotoxic to ryegrass seedlings. Radicle elongation was more sensitive to the toxins than shoot elongation.Florida Agricultural Experiment Station Journal Series No. 7906.     

Effects of bilberry (Vaccinium myrtillus L.) litter on seed germination and early seedling growth of four boreal tree species

Laboratory and greenhouse bioassays were used to test for inhibitory effects of senescent and decomposed leaves and aqueous extract from bilberry (Vaccinium myrtillus L.) against seed germination and seedling growth of aspen (Populus tremula L.), birch (Betula pendula Roth.), Scots pine (Pinus sylvestris L.), and Norway spruce [Picea abies (L.) Karst.]. Aqueous extracts from bilberry leaves were inhibitory to aspen seed germination and seedling growth and also induced root damage and growth abnormalities. Addition of activated carbon removed the inhibitory effects of extracts. Senescent leaves reduced pine and spruce seed germination, but rinsing of seeds reversed this inhibition. Senescent leaves were more inhibitory than decomposed leaf litter, suggesting that the inhibitory compounds in bilberry leaves are relatively soluble and released at early stages during decomposition. Spruce was generally less negatively affected by litter and aqueous extracts than the other tested species. This study indicates that chemical effects of bilberry litter have the potential to inhibit tree seedling recruitment, but these effects were not consistently strong. Phytotoxicity is unlikely to be of critical importance in determining success for spruce seedling establishment.     

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.     

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."     

Allelopathic Potential of Aquatic Plants Associated with Wild Rice (Zizania palustris): I. Bioassay with Plant and Lake Sediment Samples

The allelopathic potential of eight aquatic plants associated with wild rice was investigated using lettuce and wild rice seedling bioassays. Rhizome aqueous extracts of Scirpus acutus, Potamogeton natans, Nymphaea odorata, Nuphar variegatum; shoot extract of Eleocharis smallii; whole plant extract of Myriophyllum verticillatum; and leaf extract of P. natans significantly reduced the root length of lettuce and wild rice seedlings. The lettuce seedling bioassay was more sensitive than the wild rice bioassay. Shoot growth was less affected than the root growth. Water extract of sediments associated with the aquatic plants had little growth inhibitory effect on wild rice. Our study did not yield any conclusive evidence that the wild rice-associated aquatic plants have allelopathic effects on wild rice. We emphasize the use of target species as a bioassay material in allelopathic studies. Further investigation on allelopathic effects of lake sediments associated with the neighboring plants of wild rice is necessary to evaluate their ecological significance.     

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.     

Allelopathy of oats. I. Assessment of allelopathic potential of extract of oat shoots and identification of an allelochemical

The allelopathic potential of oat (Avena sativa L.) extracts was investigated under laboratory conditions. The ethyl ether-, acetone-, and water-soluble fractions obtained from the extract of oat shoots inhibited the germination and growth of roots and hypocotyls of lettuce (Lactuca sativa L.). The inhibitory activity of the water-soluble fraction was maximum, followed by that of ethyl ether-soluble and acetone-soluble fraction. An active principle of the water-soluble fraction was isolated and its structure was determined by spectral data asl-tryptophan.l-Tryptophan inhibited the growth of hypocotyls and roots of lettuce seedlings at concentrations greater than 0.03 and 0.1 mM, respectively. These results suggested thatl-tryptophan may be an allelochemical which affects the growth or germination of different plant species.     

Allelopathy as a factor in the success ofHelianthus mollis Lam.

A method to rapidly screen species suspected of producing allelochemics, using results from simple bioassay tests, is presented. By measuring the osmotic potential ofH. mollis extracts and using mannitol solutions of comparable osmotic potential, the influence of osmotic potential in the bioassay was eliminated. Nested analysis of variance was used to examine the separate influences of (1) extract concentration, (2) source of plants used in extract preparation (edge or center of clones) (3) osmotic potential of the extract, and (4) the differential development of radicles and shoots of species used in the bioassay tests. Bioassay tests for allelopathy showed that extracts made of wholeH. mollis plants significantly inhibited both radicle and shoot development of radish and wheat, but only the radicle of little bluestem. There was a significant increase in the inhibition of radish shoots and wheat radicles at high concentration of the extract, but the radicle of little bluestem was inhibited more at the lower concentration. Extracts prepared from plants collected from the clone center inhibited radish radicle development significantly more than extracts made of plants growing at the clone edge.     

Interactions oftrans-cinnamic acid,its related phenolic allelochemicals,and abscisic acid in seedling growth and seed germination of lettuce

Phenolic compounds have been identified as the most common allelochemicals produced by higher plants. Inhibitions of cinnamic acid, its related phenolic derivatives, and abscisic acid (ABA) on seedling growth and seed germination of lettuce were studied.trans-Cinnamic acid, ando-,m-, andp-coumaric acids inhibited the growth of etiolated seedlings of lettuce at concentrations higher than 10^–4 M and seed germination above 10^–3 M. Coumarin inhibited seedling growth and seed germination at 10^–5 M or above. Chlorogenic acid inhibited seedling growth above 10^–4 M, but did not inhibit seed germination at 10^–5–5×10^–3 M. Low concentrations (below 10^–3 M) of caffeic and ferulic acids promoted the elongation of hypocotyls, but higher concentrations (over 10^–3 M) inhibited seedling growth and seed germination. These phenolic compounds and abscisic acid had additive inhibitory effects both on seedling growth and seed germination. The inhibition on lettuce was reversed by caffeic and ferulic acids at concentrations lower than 10^–3 M except for the inhibition of germination by coumarin. These results suggest that in naturetrans-cinnamic acid,o-, m-, p-coumaric acids, coumarin, and chlorogenic acid inhibit plant growth regardless of their concentration. However, caffeic and ferulic acids can either promote or inhibit plant growth according to their concentration.     

Crude extracts of asteraceous weeds

Petrol and ethanolic extracts of six asteraceous weeds were added to artificial diets to screen for growth inhibition and mortality of the variegated cutworm,Peridroma saucia (Hbn). Petrol and ethanolic extracts ofArtemisia tridentata andChamomilla suaveolens and ethanolic extracts ofChrysothamnus nauseosus andCentaurea diffusa severely inhibited larval growth at five times the natural concentrations. The twoC. suaveolens extracts and the ethanol extract ofA. tridentata were active at the natural concentration (100%) and were further examined at 20, 40, 60, and 80% of this level. Inhibition of larval growth was directly related to concentration for each of the three extracts tested. EC₅₀s (effective concentration to inhibit growth by 50% relative to controls) for the three extracts were 36–42% of the naturally occurring level in the plants. Nutritional indices were calculated for secondinstarP. saucia feeding on the activeA. tridentata EtOH extract and the petrol extract fromC. suaveolens. Addition of the activeA. tridentata EtOH or theC. suaveolens petrol extract to the diet resulted in significant reduction in the relative growth rate of larvae, although theA. tridentata extract was much more inhibitory. Dietary utilization was significantly lower for larvae fed theA. tridentata EtOH extract.     

BIOFUMIGANT COMPOUNDS RELEASED BY FIELD PENNYCRESS (<Emphasis Type="Italic">Thlaspi arvense</Emphasis>) SEEDMEAL

Defatted field pennycress (Thlaspi arvense L.) seedmeal was found to completely inhibit seedling germination/emergence when added to a sandy loam soil containing wheat (Triticum aestivum L.) and arugula [Eruca vesicaria (L.) Cav. subsp. sativa (Mill.) Thell.] seeds at levels of 1.0% w/w or higher. Covering the pots with Petri dishes containing the soil-seedmeal mixture decreased germination of both species at the lowest application rate (0.5% w/w), suggesting that the some of the phytotoxins were volatile. CH₂Cl₂, MeOH, and water extracts of the wetted seedmeal were bioassayed against wheat and sicklepod (Senna obtusifolia (L.) H. S. Irwin & Barneby) radicle elongation. Only the CH₂Cl₂ extract was strongly inhibitory to both species. Fractionation of the CH₂Cl₂ extract yielded two major phytotoxins, identified by gas chromatography–mass spectrometry and NMR as 2-propen-1-yl (allyl) isothiocyanate (AITC) and allyl thiocyanate (ATC), which constituted 80.9 and 18.8%, respectively, of the active fraction. When seeds of wheat, arugula and sicklepod were exposed to volatilized AITC and ATC, germination of all three species was completely inhibited by both compounds at concentrations of 5 ppm or less. In field studies, where seedmeal was applied at 0.50, 1.25, and 2.50 kg/m² and tarped with black plastic mulch, all of the treatments significantly reduced dry weight of bioassay plants compared to the tarped control, with the highest seedmeal rate decreasing dry matter to less than 10% of the control 30 d after seedmeal application. Field pennycress seedmeal appears to offer excellent potential as a biofumigant for high-value horticultural crops for both conventional and organic growers.Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by USDA implies no approval of the product to the exclusion of others that may also be suitable.     

Mechanisms of Hydroquinone-Induced Growth Reduction in Leafy Spurge

Field observations indicate leafy spurge (Euphorbia esula) is inhibited by the presence of Antennaria microphylla. Hydroquinone (HQ), one of several compounds isolated from A. microphylla has been shown to inhibit leafy spurge seed germination, root elongation, and callus culture growth. The present study was designed to analyze the effects of HQ on water relations and photosynthesis of leafy spurge. Plants grown in 0.25 mM HQ had consistently higher leaf diffusive resistance and lower transpiration rates than control plants (P < 0.05). Chlorophyll fluorescence was significantly lower than controls (P < 0.05) towards the end of the treatment period. At the end of the treatment, tissue from 0.25 mM HQ plants had higher levels of ¹³C, indicating there had been a sustained interference with stomatal function. These data suggest that a disruption of the plant water balance is one mechanism of leafy spurge inhibition by A. microphylla.     

Seed germination and growth inhibitory cadinenes fromEupatorium adenophorum spreng

Chloroform extract of the aerial parts ofEupatorium adenophorum Spreng was fractionated and examined for growth inhibition. Bioassay-directed fractions of the plant materials afforded three known cadinenes and -sitosterol. The effects of different fractions as well as isolated cadinenes were determined usingAllium cepa, Raphanus sativus, andCucumis sativus seeds. Three-day exposure to these cadinenes significantly inhibited germination and seedling growth of all three assay seeds. The degree of inhibition was dependent upon seed species and the concentrations of the compounds tested. Cadinene(1) was found to be more inhibitory to the seeds tested and the activity of the cadinene(3) was less than that of(1) and(4).     

Allelopathic activity in wheat-conventional and wheat-no-till soils: Development of soil extract bioassays

The primary objective of this research was to determine if soil extracts could be used directly in bioassays for the detection of allelopathic activity. Here we describe: (1) a way to estimate levels of allelopathic compounds in soil; (2) how pH, solute potential, and/or ion content of extracts may modify the action of allelopathic compounds on germination and radicle and hypocotyl length of crimson clover (Trifolium incarnatum L.) and ivyleaved morning glory (Ipomoea hederacea L. Jacquin.); and (3) how biological activity of soil extracts may be determined. A water-autoclave extraction procedure was chosen over the immediate-water and 5-hr EDTA extraction procedures, because the autoclave procedure was effective in extracting solution and reversibly bound ferulic acid as well as phenolic acids from wheat debris. The resulting soil extracts were used directly in germination bioassays. A mixture of phenolic acids similar to that obtained from wheat-no-till soils did not affect germination of clover or morning glory and radicle and hypocotyl length of morning glory. The mixture did, however, reduce radicle and hypocotyl length of clover. Individual phenolic acids also did not inhibit germination, but did reduce radicle and hypocotyl length of both species. 6-MBOA (6-methoxy-2,3-benzoxazolinone), a conversion product of 2-o-glucosyl-7-methoxy-1,4-benzoxazin-3-one, a hydroxamic acid in living wheat plants, inhibited germination and radicle and hypocotyl length of clover and morning glory. 6-MBOA, however, was not detected in wheat debris, stubble, or soil extracts. Total phenolic acids (FC) in extracts were determined with Folin and Ciocalteu's phenol reagent. Levels of FC in wheat-conventionaltill soil extracts were not related to germination or radicle and hypocotyl length of either species. Levels of FC in wheat-no-till soil extracts were also not related to germination of clover or morning glory, but were inversely related to radicle and hypocotyl length of clover and morning glory. FC values, solute potential, and acidity of wheat-no-till soil extracts appeared to be independent (additive) in action on clover radicle and hypocotyl length. Radicle and hypocotyl length of clover was inversely related to increasing FC and solute potential and directly related to decreasing acidity. Biological activity of extracts was determined best from slopes of radicle and hypocotyl length obtained from bioassays of extract dilutions. Thus, data derived from the water-autoclave extraction procedure, FC analysis, and slope analysis for extract activity in conjunction with data on extract pH and solute potential can be used to estimate allelopathic activity of wheat-no-till soilsThe use of trade names in this publication does not imply endorsement by the North Carolina Agricultural Research Service of products named, or criticism of similar ones not mentioned.     

Oviposition stimulants inBarbarea vulgaris forPieris rapae andP. napi oleracea: isolation,identification and differential activity

The closely related butterflies,Pieris rapae andP. napi oleracea, readily laid eggs onBarbarea vulgaris in greenhouse cages. When offered a choice between cabbage andB. vulgaris, P. rapae showed no preference, butP. napi oleracea preferredB. vulgaris. Bioassays of extracts ofB. vulgaris foliage revealed the presence of oviposition deterrent(s) in l-butanol extracts as well as stimulants in the postbutanol water extracts. However, the deterrent effect was apparently outweighed by the strong stimulatory effect in the whole plants. The postbutanol water extract was preferred over an equivalent cabbage extract by both species, but more significantly in the case ofP. napi oleracea. The stimulants were isolated by open column chromatography and HPLC, and the activity was associated with three glucosinolates.P. napi oleracea was more sensitive thanP. rapae to the natural concentration of compounds1 and3, whereas both species were strongly stimulated to oviposit by natural concentrations of compound2. Compounds1 and2 were identified as (2R)-glucobarbarin and (2S)-glucobarbarin, respectively, and3 was identified as glucobrassicin, on the basis of their UV, mass, and NMR spectra. When the pure compounds were tested at the same concentrations applied to bean plants, the (2R)-glucobarbarin at 0.2 mg/plant was preferred over a standard cabbage extract by both butterfly species. However, at a dose of 0.02 mg/plant,P. rapae preferred the cabbage extract whereasP. napi oleracea still preferred the (2R)-glucobarbarin. No such difference in response of the two species to the same two concentrations of (2S)-glucobarbarin was obtained. The results indicate a distinct difference in sensitivity of these butterflies to the epimers of glucobarbarin, and the differences in behavioral responses of the two butterfly species depend to a large extent on the concentration of stimulant present.