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.     

Inhibition of seedling growth of crop species by recirculating root exudates ofBidens pilosa L.

The allelopathic effects of root exudates ofBidens pilosa L. on seedling growth ofLactuca sativa L.,Phaseolus vulgaris L.,Zea mays L., andSorghum bicolor (L.) Moench were studied using a root exudate recirculating system that allows continuous exposure of crop plants to allelopathic chemicals. This system maintains an undisturbed rhizosphere and eliminates competition and physical contact between the donor and acceptor plants. Comparison of responses to hydrophobic and hydrophilic root exudates is made possible by removal of hydrophobic compounds using XAD-4. Treatments consisted ofB. pilosa, B. pilosa with an Amberlite XAD-4 resin column attached to the donor pot to remove hydrophobic allelochemicals, and a donor pot without weeds.B. pilosa significantly inhibited seedling growth of all crop species tested. The crop species varied in response to the root exudates, withL. sativa being most sensitive. Larger and olderB. pilosa plants caused greater inhibition of seedling growth ofL. sativa andP. vulgaris than did smaller (younger)B. pilosa plants.B. pilosa with XAD-4 caused significantly less inhibition to all crop species, exceptZ. mays, thanB. pilosa without XAD-4, indicating that the hydrophobic exudates played an important role in the allelopathic growth inhibition. Variability in species response toB. pilosa with and without XAD-4 was probably due to differences in sensitivity to hydophobic and hydrophilic allelochemicals.Supported by a grant under USDA Agreement No. 83-CRSR-2-2293. Journal Series No. 2887 of the Hawaii Institute of Tropical Agriculture and Human Resources.     

Autointoxication mechanism ofOryza sativa I. Phytotoxic effects of decomposing rice residues in soil

The aqueous extracts of decomposing rice residues in soil exhibited inhibition on the radicle growth of lettuce and rice seeds and the growth of rice seedlings. The phytotoxicity was found in extracts obtained from the early stage of decomposition (first month), and gradually declined thereafter. The inhibition was also found in extracts obtained from rice fields, and was persistent for 4 months. The root initiation of hypocotyl cuttings of mungbeans was suppressed by extracts of decaying rice residues and extracts obtained from paddy soil. Five phytotoxins,p-hydroxybenzoic,p-coumaric, vanillic, ferulic, ando-hydroxyphenylacetic acids, and several unknowns were found in the decomposing rice residues under waterlogged conditions. At 25 ppm,o-hydroxyphenylacetic acid revealed significant inhibition on the radicle growth of rice and lettuce seeds and suppressed root initiation of mungbean seedlings. It was concluded that the growth of rice seedlings was retarded by decaying rice residues in soil; thus, this appeared to be an autointoxication phenomenon.Paper No. 176 of the Scientific Journal Series, Institute of Botany, Academia Sinica. This study was financially supported by the National Science Council, the Republic of China.     

Possible allelopathic constituents ofCoffea arabica

The aqueous extracts of leaves, stems, and roots ofCoffea arabica significantly inhibited the seed germination and radicle growth of rye grass, lettuce, and fescue. When the extracts were diluted to 1% solution, significant suppression of lettuce growth was still found and was particularly pronounced in the extract of young seedlings. The paper chromato-gram of the ether fraction of an aqueous extract of coffee leaves was bioassayed with lettuce seeds and revealed a remarkable inhibition throughout the chromatogram except for the segment ofR _f 0.00–0.12. Paper without spotting extract was used as a standard. The phytotoxins present in coffee tissue were identified by paper and thin-layer chro-matography and mass spectrometry. The compounds include caffeine, theobromine, theophylline, paraxanthine, scopoletin, and chlorogenic, ferulic,p-coumaric,p-hydroxybenzoic, caffeic, and vanillic acids. All compounds except caffeic acid exhibited significant phytotoxicity to lettuce growth at a concentration of 100 ppm.Paper no. 222 of the Scientific Journal Series, Institute of Botany, Academia Sinica, Taiwan, and Journal Article 3582 of the Agricultural Experiment Station, Oklahoma State University, Stillwater, Oklahoma. The abstract was published in the Proceedings of 63rd Annual Meeting of American Societies for Experimental Biology, held on April 1–10, 1979, Dallas, Texas.     

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.     

Allelopathy of oats. II. Allelochemical effect ofl-Tryptophan and its concentration in oat root exudates

l-Tryptophan caused growth inhibition of roots and hypocotyls (or coleoptiles) of cockscomb (Amaranthus caudatus L.), lettuce (Lactuca sativa L.), cress (Lepidium sativum L.), timothy (Phleum pratense L.), rice (Oryza sativa L.), wheat (Triticum aestivum L.), and oat (Avena sativa L.), increasing the dose ofl-tryptophan increased the inhibition. The concentrations for 50% inhibition of the root growth were 0.14, 0.15, 0.21, 0.79, 0.95, 1.7, and 2.4 mM for cockscomb, cress, lettuce, timothy, rice, wheat, and oat, respectively; the concentrations for 40% inhibition of the hypocotyl (or coleoptile) growth were 0.28, 0.33, 0.43, 2.7, 4.5, 7.2, and 15 mM for cockscomb, cress, lettuce, timothy, rice, wheat and oat, respectively. The levels ofl-tryptophan in oat seedlings and in its root exudates were 29.3 mg/kg fresh wt and 0.25 mM under light conditions, and 21.1 mg/kg fresh wt and 0.18 mM under dark conditions, respectively. The presence ofl-tryptophan in the root exudates coupled with its effect on growth suggested thatl-tryptophan may play an important role in the growth inhibition of other plants in nature.     

Allelopathy ofSasa cernua

Sasa (Sasa cernua Makino) is a very serious weed pest. Its allelopathy was studied using lettuce, wheat, timothy, and green amaranth as testing species. Cultured in the rhizosphere soil ofSasa cernua, the seedlings were inhibited by 42–80% compared with the controls cultured in normal soil and vermiculite. The phenolic fraction extracted with 1 M NaOH from the rhizosphere soil ofS. cernua caused significant inhibitions on the seed germination and seedling growth of lettuce, timothy, green amaranth, and barnyard grass.p-Coumaric, ferulic, vanillic, andp-hydroxybenzoic acids andp-hydroxybenzaldehyde were identified as the main allelochemicals in sasa soil by HPLC and [¹H]NMR. Their contents in the rhizosphere soil were 5640, 1060, 860, 810 and 630 μg/100 g soil. The neutral fraction inhibited the seed germination and seedling growth of lettuce in the TLC direct bioassay. Volatile compounds released from sasa leaves also inhibited the growth of lettuce, wheat, timothy, and green amaranth grown under light, and the growth of etiolated seedlings of barley and wheat. These results confirm thatS. cernua produces typical allelopathy through its rhizosphere soil and air space.     

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.     

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.     

Hydroxamic Acid Content and Toxicity of Rye at Selected Growth Stages

Rye (Secale cereale L.) is an important cover crop that provides many benefits to cropping systems including weed and pest suppression resulting from allelopathic substances. Hydroxamic acids have been identified as allelopathic compounds in rye. This research was conducted to improve the methodology for quantifying hydroxamic acids and to determine the relationship between hydroxamic acid content and phytotoxicity of extracts of rye root and shoot tissue harvested at selected growth stages. Detection limits for an LC/MS-MS method for analysis of hydroxamic acids from crude aqueous extracts were better than have been reported previously. (2R)-2-β-d-Glucopyranosyloxy-4-hydroxy-(2H)-1,4-benzoxazin-3(4H)-one (DIBOA-G), 2,4-dihydroxy-(2H)-1,4-benzoxazin-3(4H)-one (DIBOA), benzoxazolin-2(3H)-one (BOA), and the methoxy-substituted form of these compounds, (2R)-2-β-d-glucopyranosyloxy-4-hydroxy-7-methoxy-(2H)-1,4-benzoxazin-3(4H)-one (DIMBOA glucose), 2,4-hydroxy-7-methoxy-(2H)-1,4-benzoxazin-3(4H)-one (DIMBOA), and 6-methoxy-benzoxazolin-2(3H)-one (MBOA), were all detected in rye tissue. DIBOA and BOA were prevalent in shoot tissue, whereas the methoxy-substituted compounds, DIMBOA glucose and MBOA, were prevalent in root tissue. Total hydroxamic acid concentration in rye tissue generally declined with age. Aqueous crude extracts of rye shoot tissue were more toxic than extracts of root tissue to lettuce (Lactuca sativa L.) and tomato (Lycopersicon esculentum Mill.) root length. Extracts of rye seedlings (Feekes growth stage 2) were most phytotoxic, but there was no pattern to the phytotoxicity of extracts of rye sampled at growth stages 4 to 10.5.4, and no correlation of hydroxamic acid content and phytotoxicity (I₅₀ values). Analysis of dose–response model slope coefficients indicated a lack of parallelism among models for rye extracts from different growth stages, suggesting that phytotoxicity may be attributed to compounds with different modes of action at different stages. Hydroxamic acids may account for the phytoxicity of extracts derived from rye at early growth stages, but other compounds are probably responsible in later growth stages.     

Effects of Temperature and Photoperiod on Phytotoxic Root Exudates of Cucumber (Cucumis sativus) in Hydroponic Culture

In order to elucidate the effects of temperature and photoperiod on the quality and quantity of plant root exudates, a Japanese cucumber (Cucumis sativus, cv. Shougoin-Aonaga-Fushinari) was grown hydroponically in growth chambers under controlled temperature and photoperiod conditions with or without the addition of activated charcoal (AC) to the nutrient solutions. Fresh AC was used to trap the organic compounds exuded from cucumber roots every two weeks. Cucumber plants without AC were severely retarded in root growth and in the accumulation of dry matter, especially at high temperature and long photoperiod, compared to those with AC. The growth inhibitors, adsorbed on the AC or accumulated in the nutrient solution without AC, were extracted by organic solvents and analyzed by GC-MS. Benzoic acid and its derivatives, cinnamic acid derivatives, and fatty acids were identified. The rate of root exudation in vegetative and reproductive stages for some of these organic acids increased with the elevation of temperature and the elongation of photoperiod, and the mean rate was two or more times higher than the minimum exudation at low temperature with short photoperiod. Some of the identified compounds significantly inhibited the germination and/or root growth of lettuce and cucumber.     

Phytotoxic Effects of Leukamenin E (an ent-kaurene diterpenoid) on Root Growth and Root Hair Development in Lactuca sativa L. seedlings

Leukamenin E, an ent-kaurene diterpenoid isolated from Isodon racemosa (Hemsl) Hara, showed phytotoxic effects on root growth and root hair development of lettuce seedlings (Lactuca sativa L.). Lower concentrations (10 μM) of leukamenin E did not affect root growth, but at concentrations higher than 50 μM, the rate was inhibited. The influence of leukamenin E on root growth rate was closely correlated with alterations in the mitotic index. A low incidence of aberrant mitosis image was observed when lettuce roots were treated with higher concentrations (100 and 200 μM) of leukamenin E. This suggests that inhibition of root growth may be due to inhibition of cell division. All tested concentrations of the diterpenoid (10 μM or more) inhibited root hair development in a dose-dependent manner. At a concentration of 80 μM, leukamenin E completely blocked root hair initiation. Application of Ag⁺—an ethylene action inhibitor—to lettuce seedlings inhibited root hair elongation similar to the diterpenoid. Enhanced root hair length was stimulated by exogenous ethephon—an ethylene-releasing agent—and could be reversed by addition of leukamenin E. This suggests that leukamenin E may act as a potential ethylene action antagonist in the inhibition of lettuce root hair development. We conclude that leukamenin E may curb root hair development by interfering with ethylene action at concentrations above 10 μM and inhibits root growth via inhibition of cell division at concentrations above 50 μM. Lan Ding and Linlin Qi contributed equally to this work.     

Allelopathy in agroecosystems: Wheat phytotoxicity and its possible roles in crop rotation

The germination rates of cotton and wheat seeds were significantly affected by various extracts of wheat mulch and soils collected from the wheat field. This toxicity was even more pronounced against seedling growth. Five allelochemics: ferulic,p-coumaric,p-OH benzoic, syringic, and vanillic acids, were identified from the wheat mulch and its associated soil. Quantitatively, ferulic acid was found at higher concentrations thanp-coumaric acid in the soil. Various concentrations of ferulic andp-coumaric acids were toxic to the growth of radish in a bioassay. The functional aspects of allelochemic transfer from decaying residue to soil and the subsequent microbial degradation within agroecosystems are discussed, particularly as they relate to wheat crop rotation, with wheat and cotton, in Pakistan.     

Chemical Stimulants of Leaf-Trenching by Cabbage Loopers: Natural Products, Neurotransmitters, Insecticides, and Drugs

Larvae of the cabbage looper, Trichoplusia ni (Lepidoptera: Noctuidae), often transect leaves with a narrow trench before eating the distal section. The trench reduces larval exposure to exudates, such as latex, during feeding. Plant species that do not emit exudate, such as Plantago lanceolata, are not trenched. However, if exudate is applied to a looper's mouth during feeding on P. lanceolata, the larva will often stop and cut a trench. Dissolved chemicals can be similarly applied and tested for effectiveness at triggering trenching. With this assay, I have documented that lactucin from lettuce latex (Lactuca sativa), myristicin from parsley oil (Petroselinum crispum), and lobeline from cardinal flower (Lobelia cardinalis) elicit trenching. These compounds are the first trenching stimulants reported. Several other constituents of lettuce and parsley, including some phenylpropanoids, monoterpenes, and furanocoumarins had little or no activity. Cucurbitacin E glycoside found in cucurbits, another plant family trenched by cabbage loopers, also was inactive. Lactucin, myristicin, and lobeline all affect the nervous system of mammals, with lobeline acting specifically as an antagonist of nicotinic acetylcholine receptors. To determine if cabbage loopers respond selectively to compounds active at acetylcholine synapses, I tested several neurotransmitters, insecticides, and drugs with known neurological activity, many of which triggered trenching. Active compounds included dopamine, serotonin, the insecticide imidacloprid, and various drugs such as ipratropium, apomorphine, buspirone, and metoclopramide. These results document that noxious plant chemicals trigger trenching, that loopers respond to different trenching stimulants in different plants, that diverse neuroactive chemicals elicit the behavior, and that feeding deterrents are not all trenching stimulants. The trenching assay offers a novel approach for identifying defensive plant compounds with potential uses in agriculture or medicine. Cabbage loopers in the lab and field routinely trench and feed on plants in the Asteraceae and Apiaceae. However, first and third instar larvae enclosed on Lobelia cardinalis (Campanulaceae) failed to develop, even though the third instar larvae attempted to trench. Trenching ability does not guarantee effective feeding on plants with canal-borne exudates. Cabbage loopers must not only recognize and respond to trenching stimulants, they must also tolerate exudates during the trenching procedure to disable canalicular defenses.     

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.     

Allelopathy of yellow fieldcress (Rorippa sylvestris): Identification and characterization of phytotoxic constituents

Both the neutral and acidic fractions of the acetone extract of yellow fieldcress (Kireha-inugarashi,Rorippa sylvestris Besser) inhibited lettuce seed germination. Salicylic,p-hydroxybenzoic, vanillic, and syringic acid were identified in the acidic fraction. In the neutral fraction, hirsutin (8-methylsulfinyloctyl isothiocyanate), 4-methoxyindole-3-acetonitrile, and pyrocatechol were identified. Bioassay using a root exudate recirculating system showedR. sylvestris during flowering inhibited the lettuce seedling growth. Hirsutin (13g/plant/day) and pyrocatechol (9.3g/plant/day) were the major compounds released into the rhizosphere. Several combinations of pyrocatechol,p-hydroxybenzoic acid, vanillic acid, and hirsutin reduced lettuce seedling growth. These compounds seemed to be 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.     

Allelopathic research of subtropical vegetation in Taiwan

Leucaena leucocephala plantations in Kaoshu, southern Taiwan, exhibit, after several years of growth, a unique pattern of weed exclusion beneathLeucaena canopy. The pattern has been observed in manyLeucaena plantations in Taiwan and is particularly pronounced in the area where a substantial amount ofLeucaena litter has accumulated on the ground. Field data showed that the phenomenon was primarily not due to physical competition involving light, soil moisture, pH, and nutrients. Instead, aqueous extracts ofLeucaena fresh leaves, litter, soil, and seed exudate showed significantly phytotoxic effects on many test species, including rice, lettuce,Acacia confusa, Alnus formosana, Casuarina glauca, Liquidambar formosana, andMimosa pudica. However, the extracts were not toxic to the growth ofLeucaena seedlings. The decomposing leaves ofLeucaena also suppressed the growth of the aforementioned plants grown in pots but did not inhibit that ofLeucaena plants. By means of paper and thin-layer chromatography, UV-visible spectrophotometry, and high-performance liquid chromatography, 10 phytotoxins were identified. They included mimosine, quercetin, and gallic, protocatechuic,p-hydroxybenzoic,p-hydroxyphenylacetic, vanillic, ferulic, caffeic, andp-coumaric acids. The mature leaves ofLeucaena possess about 5% dry weight of mimosine, the amount varying with varieties. The seed germination and radicle growth of lettuce, rice, and rye grass were significantly inhibited by aqueous mimosine solution at a concentration of 20 ppm, while that of the forest species mentioned was suppressed by the mimosine solution at 50 ppm or above. However, the growth ofMiscanthus floridulus andPinus taiwanensis was not suppressed by the mimosine solution at 200 ppm. The seedlings ofAgeratum conzoides died in mimosine solution at 50 ppm within seven days and wilted at 300 ppm within three days. It was concluded that the exclusion of understory plants was evidently due to the allelopathic effect of compounds produced byLeucaena. The allelopathic pattern was clearly shown in the area with a heavy accumulation ofLeucaena leaf litter, which was a result of drought and heavy wind influence.Paper No. 292 of the Scientific Journal Series of the Institute of Botany, Academia Sinica, Taipei, Taiwan. This study was supported in part by a grant to C.H. Chou. Part of this paper was a MS thesis submitted by Y.L. Kuo to the Department of Forestry, National Taiwan University, and presented at the Seminar on Allelochemicals and Pheromones, sponsored by the CCNAA and AIT on June 21–26, 1982.     

Flavonoids released by carrot (Daucus carota) seedlings stimulate hyphal development of vesicular-arbuscular mycorrhizal fungi in the presence of optimal CO2 enrichment

Carbon dioxide has been previously identified as a critical volatile factor that stimulates hyphal growth ofGigaspora margarita, a vesiculararbuscular mycorrhizal fungus, and we determined the optimal concentration at 2.0%. The beneficial effect of CO₂ on fungal development is also visible in the presence of stimulatory (quercetin, myricetin) or inhibitory (naringenin) flavonoids. Sterile root exudates from carrot seedlings stimulate the hyphal development ofG. margarita in the presence of optimal CO₂ enrichment. Three flavonols (quercetin, kaempferol, rutin or quercetin 3-rutinoside) and two flavones (apigenin, luteolin) were identified in carrot root exudates by means of HPLC retention time. Flavonols like quercetin and kaempferol are known to have stimulatory effects on hyphal growth ofG. margarita.     

Allelopathic Potential of <Emphasis Type="Italic">Robinia pseudo-acacia</Emphasis> L.

Robinia pseudo-acacia L. (black locust) is a nonindigenous species currently invading the central part of Japanese grasslands. Several allelochemicals were identified and characterized from the leaf tissue. The growth of both radicle and hypocotyl in the tested species (barnyard grass, white clover, lettuce, and Chinese cabbage) was reduced when grown in soil mixed with the leaves of R. pseudo-acacia at various concentrations. Aqueous leaf extracts, when bioassayed, exhibited a significant suppression of radicle growth. Chromatographic separation of an ethanolic extract of R. pseudo-acacia leaves resulted in isolation of three compounds, identified as robinetin (1), myricetin (2), and quercetin (3) by nuclear magnetic resonance and mass spectroscopy. All inhibited root and shoot growth of lettuce. Robinetin, found in a large amount, caused 50% suppression of the root and shoot growth of lettuce at 100 ppm. The presence of these bioactive substances in leaf tissue suggests a potential role for flavonoids in R. pseudo-acacia invasion in introduced habitats.