Assessment of the Allelopathic Potential of <Emphasis Type="Italic">Juniperus ashei</Emphasis> on Germination and Growth of <Emphasis Type="Italic">Bouteloua curtipendula</Emphasis>

Potential allelopathic compounds of Juniperus ashei Buchh. (Ashe juniper) and their effect on a native grass were determined in laboratory and field studies. Solid-phase microextraction and gas chromatography/mass spectrometry were used to determine if monoterpenes found in the essential oils of J. ashei are released in leaf and litter leachate, as well as volatilized from leaf tissue. Camphor, bornyl acetate, and limonene were found in leaf and fresh litter leachates; however, degraded litter did not contain any of these compounds. Camphor was the most common potentially allelopathic compound found in J. ashei leaf and litter leachate and in volatiles from leaf tissue. The effects of leaf and litter tissue on the germination of Bouteloua curtipendula (Michx.) Torr. (side-oats grama) was tested by using the “sandwich agar method”. The highest germination of B. curtipendula (29.6%) occurred in the control, which was significantly higher than fresh litter (13.2%) and degraded litter (16.2%). The lowest germination (6.2%) occurred with J. ashei leaves. In the field experiment, aboveground dry mass of B. curtipendula was evaluated in relation to position within the canopy and intercanopy of J. ashei adult trees when light and water were held constant across locations. Aboveground dry mass of B. curtipendula was significantly greater in the intercanopies of J. ashei (163.7 g m²) compared to the dry mass in the understory (44.8 g m²) and dripline (44.5 g m²), suggesting some negative influence by J. ashei. Chemical analyses indicate that monoterpenes are released through leaching and volatilization from J. ashei, and germination and field studies suggest that these compounds inhibit B. curtipendula.     

Chemical inhibition of fire-prone grasses by fire-sensitive shrub,Conradina canescens

In an investigation of potential chemical activity of fire-sensitive shrubs in Florida's sand pine scrub community, bioassays of foliar washes ofConradina canescens showed significant inhibitory activity on three native grasses that are known to fuel frequent surface fires; inhibition was concentrated seasonally in spring and summer. Application of runoff fromConradina leaves to one of the grasses caused a 50% reduction in growth over a 20-week period. Isolation of the biologically active fractions from the fresh leaves ofC. canescens yielded numerous monoterpenes, a number of which were identified from a GC-MS reference library and/or MS comparison to authentic compounds: 11 from the diethyl ether extract, 11 from steam distillation, and four from the foliar leaf wash. Numerous other monoterpenes present in the extractions were unknown. The terpenoid fraction completely inhibited seed germination of one of the native grasses and of lettuce. Saturated aqueous solutions of nine of the monoterpenes inhibited germination and radicle growth of two native grasses. SEM views of the leaf surfaces ofConradina reveal secretory trichomes that appear to be the source of the monoterpenes as well as the triterpene, ursolic acid. The biological activity ofC. canescens as a fire-sensitive component of the scrub community is reviewed in light of the chemical evidence.     

Effects of Four Monoterpenes on Germination, Primary Root Growth, and Mitochondrial Respiration of Maize

The effects of four monoterpenes—camphor, eucaliptol, limonene, and -pinene—on the oxidative metabolism of mitochondria isolated from maize root (Zea mays), on maize seed germination, and on primary root growth were investigated. The effects of individual monoterpenes on respiration were variable. -Pinene concentrations of 0.05–1.0 mM stimulated respiration with a mixture of substrates composed of NADH, L-malate, succinate, and L-glutamate, and in the absence of exogenously added ADP (basal respiration). However, at concentrations higher than 1.0 mM, -pinene inhibited respiration both in the absence (basal respiration) and presence of ADP (coupled respiration). Limonene at 0.1 mM or above stimulated basal respiration and inhibited in parallel the coupled respiration. Similar effects were promoted by eucaliptol, but at a higher concentration range (1.0 mM or above). Camphor was less active. At 10 mM concentration, it caused stimulation of basal respiration but did not affect coupled respiration. In the concentration range 0.1–10.0 mM, limonene was inactive on seed germination and primary root growth. Camphor and eucaliptol did not inhibit germination but reduced fresh and/or dry weight of roots at 5.0 mM and above. -Pinene inhibited both seed germination and fresh weight of primary roots at 10.0 mM concentration. The results indicate that intact seeds and primary roots are less sensitive than isolated mitochondria. The relatively more lipophilic monoterpenes -pinene and limonene had less activity than the more water-soluble oxygenated monoterpenes camphor and eucaliptol in inhibiting seed germination and/or primary root growth, despite the fact that they had a higher activity on the oxidative metabolism of isolated mitochondria. The findings suggest that the solubility of monoterpenes may be the major factor implicated in these differences.     

Allelopathic effects of water extracts ofArtemisia princeps var.orientalis on selected plant species

The allelopathic effects of wormwood plants (Artemisia princeps var.orientalis) and their possible phytotoxicity on receptor species were investigated. The aqueous extracts of mature leaf, stem, and root of wormwood plants caused significant inhibition in germination and decreased seedling elongation of receptor plants, whereas germination of some species was not inhibited by extracts of stems and roots. Dry weight growth was slightly increased at lower concentrations of the extract, whereas it was proportionally inhibited at higher concentrations. The calorie value of the organic matter in receptor plants measured by bomb calorimeter was reduced proportionally to the extract concentration. However, results with extracts of juvenile leaf did not correlate with inhibition or promotion of elongation and dry weight.     

Changes in the Composition of Phospholipid Fatty Acids and Sterols of Maize Root in Response to Monoterpenes

Terpenes are thought to be important in plant–plant interactions because of their phytotoxic action on seed germination and growth. Herein, the effects of five volatile monoterpenes on root sterols and phospholipid fatty acid (PLFA) composition have been studied during maize seedling germination. The investigated monoterpenes (camphor, 1–8 cineole, geraniol, menthol, and thymol) were applied at their respective IC₈₀ (concentration causing 80% inhibition). They quantitatively affected free sterols and PLFA composition, thus producing an increase in the percentage of unsaturated PLFAs, stigmasterol of the free sterol fraction, and saturated steryl ester fatty acids. Alcoholic and nonalcoholic monoterpenes appeared to have different modes of action. The former affected unsaturated fatty acid and stigmasterol to a greater extent, and accordingly they could interfere in seedling growth by changes in the membrane lipids.     

The role of monoterpenes in resistance of Douglas fir to western spruce budworm defoliation

We conducted defoliation experiments with 7- to 8-year-old clones of Douglas fir [Pseudotsuga menziesii (Mirb.) Franco var. glauca] to assess the role of monoterpenes as a resistance mechanism to western spruce budworm (Choristoneura occidentalis Freeman) defoliation. The grafted clones were derived from mature trees that showed resistance or susceptibility to budworm defoliation in the forest. All clones were exposed to either budworm defoliation or nondefoliation treatments in 1998 and 1999 under greenhouse conditions. We found that the total concentration of monoterpenes in current-year foliage varied greatly between two consecutive years in clones in the greenhouse and in their corresponding mature trees in the forest. Fractional composition of different monoterpenes was similar between different years and between clones and mature trees, indicating genetic control of this trait. Two different defoliation experiments were conducted to assess the importance of budburst phenology as a factor determining host plant resistance. In the 1998 experiment, budworm feeding was matched to the budburst of each individual plant. Monoterpene concentration was high in 1998, and budworm potential fitness was greater on clones from the resistant mature trees that had lower concentrations of total monoterpenes. In the 1999 experiment, budworm feeding was matched to budburst of the whole population of plants in order to mimic conditions similar to insects feeding on trees in the field. The concentration of monoterpenes was low in 1999, and budworm fitness was not related to monoterpenes. Total monoterpene concentration was negatively related to foliar nitrogen concentration, suggesting that C/N balance may affect monoterpene synthesis in needles. However, tree growth was not related to total monoterpene concentration. We concluded that expression of differences in budworm resistance among Douglas fir genotypes might be caused by interactions among multiple resistance mechanisms such as needle monoterpenes and tree budburst phenology.     

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.     

Biological activity and tentative identification of flavonoid components in velvetleaf (Abutilon theophrasti Medik.) seed coats

The biological activity of aqueous extracts of velvetleaf (Abutilon theophrasti Medik.) seed coats and their flavonoid components against three plant species and five soil fungi was investigated. Aqueous extracts slightly inhibited germination and significantly inhibited radicle growth of all plant species tested. Fungal growth was inhibited or not affected, depending on species, by aqueous extracts. The aqueous extracts were extracted into methanol and separated using paper chromatography for identification of compounds responsible for inhibition of seedling and fungal growth. Six flavonoid compounds were isolated and tentatively identified as delphinidin, cyanidin, quercetin, myricetin, (+)-catechin, and (–)-epicatechin. Bioassays revealed that the flavonoid compounds significantly inhibited germination and radicle growth of all test species at a concentration of 1.0 mM. Individual flavonoids had variable effects on fungi but appeared to inhibit growth and sporulation of potential seed-decomposing fungi rather than beneficial fungi. These results indicate that the flavonoid complement of velvetleaf seed coats may function in a dual defensive role against competing seedlings and seed-attacking fungi.Journal article No. 9878 of the Missouri Agricultural Experiment Station.     

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.     

Antimicrobial Activity of Exocrine Glandular Secretion of Chrysomela Larvae

The exocrine glandular secretions of leaf beetle larvae of the taxon Chrysomela are well-known defensive devices used against some generalist predators. Salicylaldehyde is the major repellent component of secretions emitted by larvae of Chrysomela vigintipunctata and C. lapponica, which feed on salicin-rich Salicaceae. In this study, we examined whether salicylaldehyde is also active against the entomopathogenic fungus Metarhizium anisopliae. The germination and growth of this fungus was strongly inhibited when salicylaldehyde was applied directly onto the blastospores. The salicylaldehyde concentration of the larval secretions of the tested willow feeding Chrysomela larvae was much higher than the one necessary to display this antifungal activity. Additionally, salicylaldehyde was shown to reduce germination and growth of M. anisopliae via the gas phase over a distance of 45 mm. Further studies on the antimicrobial activity of the salicylaldehyde-containing secretions of Chrysomela larvae revealed that they act nonspecifically against prokaryotic (Escherichia coli) and eukaryotic cells (Saccharomyces cerevisiae and Trichoplusia ni). All antimicrobial and cytotoxic effects detected proved to be due to salicylaldehyde. The larval secretions of the birch-feeding allospecies of C. lapponica, that do not contain salicylaldehyde, but mainly carboxylic acids and their esters, showed no detectable effects on bacteria or fungi and no cytotoxic effects against insect cells. The results are discussed with respect to their ecological relevance.     

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.     

Essential Oil of <Emphasis Type="Italic">Artemisia scoparia</Emphasis> Inhibits Plant Growth by Generating Reactive Oxygen Species and Causing Oxidative Damage

We investigated the chemical composition and phytotoxicity of the essential oil extracted from leaves of Artemisia scoparia Waldst. et Kit. (red stem wormwood, Asteraceae). GC/GC-MS analyses revealed 33 chemical constituents representing 99.83% of the oil. The oil, in general, was rich in monoterpenes that constitute 71.6%, with β-myrcene (29.27%) as the major constituent followed by (+)-limonene (13.3%), (Z)-β-ocimene (13.37%), and γ-terpinene (9.51%). The oil and β-myrcene were evaluated in a dose–response bioassay under laboratory conditions for phytotoxicity against three weeds—Avena fatua, Cyperus rotundus, and Phalaris minor. A significant reduction in germination, seedling growth, and dry matter accumulation was observed in the test weeds. At the lowest treatment of 0.07 mg/ml Artemisia oil, germination was reduced by 39%, 19%, and 10.6% in C. rotundus, P. minor, and A. fatua, respectively. However, the inhibitory effect of β-myrcene was less. In general, a dose-dependent effect was observed and the growth declined with increasing concentration. Among the three weeds, the inhibitory effect was greatest on C. rotundus, so it was selected for further studies. We explored the explanation for observed growth inhibition in terms of reactive oxygen species (ROS: lipid peroxidation, membrane integrity, and amounts of conjugated dienes and hydrogen peroxide)-induced oxidative stress. Exposure of C. rotundus to Artemisia oil or β-myrcene enhanced solute leakage, indicating membrane disintegration. There were increased levels of malondialdehyde and hydrogen peroxide, indicating lipid peroxidation and induction of oxidative stress. We conclude that Artemisia oil inhibits plant root growth through generation of ROS-induced oxidative damage.     

Simultaneous effects of ferulic andp-coumaric acids on cucumber leaf expansion in split-root experiments

Experiments were conducted to determine how plant responses to mixtures of allelochemicals may change as the proportion of roots in contact with allelochemicals is modified. Thirteen-day-old cucumber seedlings were treated with ferulic and/orp-coumaric acid in a split-root nutrient culture system. Leaf areas were determined just prior to treatment and at harvest, 24 hr after treatment. Ferulic acid was more inhibitory to cucumber leaf expansion thanp-coumaric acid. The effects of ferulic andp-coumaric acids on leaf expansion were additive. For individual acids, mixtures of ferulic andp-coumaric acids in the same container and combinations of ferulic andp-coumaric acids in separate containers, the inhibition of leaf expansion was directly related to the product of the concentration of the acid(s) and the proportion of roots treated with the acid(s). Pretreatment with 0.2 or 0.4 mM ferulic acid and subsequent treatment with 0.5 mM ferulic acid failed to show evidence of acclimation by cucumber seedlings.The use of trade names in this publication does not imply endorsement by the North Carolina Agricultural Research Service of products named, nor criticism of similar ones not mentioned.     

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.     

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.     

Effects of salicylic acid on plant-water relationships

Soybean seedlings [Glycine max (L.) Merr.] were used as the test species to study the allelopathic influence of salicylic acid (SA) on short- and long-term plant water status. Plants were grown in greenhouse conditions in nutrient culture medium amended with SA. Treatments were initiated 10 days after germination and continued for either 14 or 28 days. The threshold for inhibition of seedling growth over a 28-day treatment was 0.15 mM SA. Seedlings grown with 0.3 mM SA consistently had higher leaf diffusive resistance and lower transpiration and water potentials than control plants. The stable carbon isotope ratio (¹³C:¹²C) in tissue from both the 0.15 and 0.30 mM SA-treated plants was significantly higher than control seedlings, indicating SA caused a chronic water stress during the 28-day treatment. These data show that an interference with plant-water relationships is one mechanism whereby this allelochemical inhibits plant growth.This study was supported by National Science Foundation grant STI-8902066.     

Role of allelopathy as a possible factor associated with the rising dominance ofBunias orientalis L. (Brassicaceae) in some native plant assemblages

Leaf extracts ofBunias orientalis were shown to inhibit seed germination of a variety of cultivar plant species and of species cooccurring withB. orientalis in the field. Root exudate solutions and leaf litter leachates ofB. orientalis were tested for their allelopathic activity using seedling growth assays. Additionally, in comparative seedling growth assays soil cores removed from denseB. orientalis stands were tested bimonthly for elevated allelopathic effects. The impact of root exudates on seedling growth was generally weak and varied between species. Similar results were obtained for the effect ofB. orientalis leaf litter leachates on seedlings grown in sand culture relative to the effect of leaf litter leachates of a plant species mixture. When soil as a growth substrate was used, no consistent differences in seedling growth were obtained between the two litter leachate treatments. In the soil core experiment seedlings grown in soil cores collected from a denseB. orientalis stand unexpectedly showed better performance than seedlings grown in soil cores collected from a nearby mixed plant stand withoutB. orientalis, at least in early spring and late autumn. Predominating nutrient effects are, therefore, assumed to conceal a potentially increased allelopathic effect of soil beneath denseB. orientalis stands. It is concluded that other factors than allelopathy must be investigated to explain the rapid establishment of dense stands of this alien plant species.     

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.     

Effects of ferulic andp-coumaric acids in nutrient culture of cucumber leaf expansion as influenced by pH

Cucumber seedlings were grown in 5 mM MES [2-(N-morpholino)ethanesulfonic acid] -buffered nutrient solutions adjusted to a pH of 5.5, 6.25, or 7.0. Nutrient solutions were changed on alternate days. Seedlings were treated for a two-day period with various concentrations (0–1 mM) of ferulic acid,p-coumaric acid, or mixtures of these phenolic acids when 16 days old. Leaf growth, dry weight, and water utilization of the seedlings; pH of the solutions; and disappearance of the phenolic acids from nutrient solutions were monitored. Leaf area expansion of cucumber seedlings was inhibited by both ferulic andp-coumaric acid, and the magnitude of these inhibitions was influenced by concentration and pH. Inhibition of leaf area expansion was greater at pH 5.5 and nominal at pH 7.O. Ferulic acid was more inhibitory thanp-coumaric acid. The effect of pH on growth was best described by data for mean relative rates of leaf expansion. For example, the mean relative rates of leaf expansion by both acids at 0.5 mM for the 16- to 18-day growth period (treatment period) were reduced by 45, 31, and 8% for the pH 5.5, 6.25, and 7.0 treatments, respectively. The dry weight of seedlings at harvest (day 22) was significantly reduced for seedlings grown in the pH 5.5 and 6.25 treatments, but not for the pH 7.0 treatment. There was, however, one exception; the dry weight of seedlings treated withp-coumaric acid solutions adjusted to a pH of 5.5 was not significantly reduced. Water utilization by the seedlings was reduced by both ferulic andp-coumaric acid. Again, the impact of ferulic acid was greater thanp-coumaric acid. The effect of ferulic acid on water utlization decreased with increasing pH of the nutrient solution. The pH effects were not so consistent forp-coumaric acid. The effects of equimolar mixtures of the two phenolic acids were additive for all variables measured. There was a linear correlation between mean relative rates of leaf expansion and water utilization.Paper No. 9693 of the Journal Series of the North Carolina Agricultural Research Sevice, Raleigh, North Carolina. Mention of a trademark or a propriety product does not constitute a guarantee or warranty of the product by the Agricultural Research Service and does not imply its approval to the exclusion of other products that may be suitable.     

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.