Phenolic Compounds in a Sahelian Sorghum (Sorghum bicolor) Genotype (CE145–66) and Associated Soils

CE_145–66 is an improved early-maturing grain sorghum genotype, increasingly grown by farmers in the Sahelian part of Senegal. This genotype is known to have negative effects on the following groundnut crop, because of the release of allelopathic phenolic compounds into the soil. We have assessed the synthesis of phenolics in sorghum vegetative parts and the variations in synthesis between years and sites. Total phenols and phenolic acids in the aerial parts and roots of flowering sorghum plants from 52 farmers' fields at two sites (Sagnanème and Médina) in Senegal in 1996 and 1997 were measured. Thirty-eight soil samples, collected after the sorghum harvest, from sorghum rows and interrows also were analyzed for their phenolic content. Total phenols reached 1.1–1.5% of root dry weight and 1.1–2.2% of aerial parts dry weight, with little variation between sites, and large variability between years, presumably due to climatic conditions. Eight phenolic acids and three associated aldehydes were identified by HPLC, with p-hydroxybenzoic, p-coumaric, and ferulic acids the most abundant. Their totals reached 2.9–3.2 mg/g in 1996 and 2.6–2.8 mg/g in 1997 for the aerial part; and 3.3–3.6 mg/g in 1996 and 2.8–3.3 mg/g in 1997 for roots. In soils under sorghum rows, the mean water-soluble total phenols increased from 4.6 in 1997 to 6.7 g/g in 1998 in Sagnanème, and from 3.8 in 1997 to 5 g/g in 1998 in Médina. The concentrations of total phenols and phenolic acids were higher in rows than in interrows. All the phenolic monomers identified in vegetative parts were recovered in associated soil samples, with vanillic and p-hydroxybenzoic acids the most abundant. Finally, variability in plant phenolic content seemed more due to climatic than to cropping or soil factors, as differences between years appear more important than differences between or within sites.     

A chemical basis for differential allelopathic potential of sorghum hybrids on wheat

The basis for differential allelopathic potentials among sorghum (Sorghum bicolor L. Moench) hybrids was investigated by conducting quantitative and qualitative studies of their phenolic contents. Total phenolic content in sorghum plant parts varied within hybrids, among hybrids, and between growing seasons. Inhibition of wheat (Triticum aestivum L.) radicle growth was positively associated (r=0.66) with concentrations of total phenolics contained in plant parts. Extracts from culms contributed the higherst proportion of toxicity from sorghum plants, inhibiting radicle growth up to 74.7%. Concentrations of five phenolic acids,p-hydroxybenzoic (POH), vanillic (VAN), syringic (SYR),p-coumaric (PCO), and ferulic (FER), differed in all plant parts of the three sorghum hybrids. Concentrations of POH, VAN, and SYR were consistently higher than PCO and FER. PCO and FER wer absent from some plant parts, with FER being the most frequently missing. Inhibition of wheat radicle growth was found to be positively associated with the concentration of each phenolic acid. Vanillic acid was most highly associated (r=0.44) with inhition. Thus, above-ground sorghum tissues contained phenolic acids that contributed to allelopathic potential. Additionally, sorghum roots exuded POH, VAN, and SYR that may enhance the overall allelopathic potential of sorghum during growth and after harvest when residues remain on the soil surface or are incorporated prior to planting a subsquent crop.Journal article No. 12245 of the Missouri Agricultural Experiment Station. Product names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the products, and the use of names by USDA implies no approval of the products to the exclusion of others that may be suitable.     

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.     

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.     

Comparative study of allelopathy as exhibited byProsopis Juliflora swartz andProsopis cineraria (L) druce

The allelopathic effects ofProsopis juliflora were studied both in the laboratory and in nature and compared with that ofProsopis cineraria to understand the chemical nature of allelochemics. Both species occupy the same habitats butP. cineraria does not appear to have any toxic effect on other plants under its canopy.P. juliflora is highly allelopathic and does not allow the growth of any other species. Leaf extracts and leaf leachates ofP. juliflora were inhibitory. Decaying leaves were also inhibitory at early stages of decomposition. Live roots were not found to be inhibitory in cogermination and interplanting of seeds. Chemical investigation of the extracts showed the allelopathic compounds to be phenolic in nature in both the species. Slow decomposition and heavy accumulation of leaf litter belowP. juliflora may possibly result in accumulation of toxic substances in soil layers, inhibiting growth of other species.     

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.     

Weed control using allelopathic crop plants

The concept that some crop plants may be allelopathic to common weeds of agricultural lands is receiving greater attention as an alternative weed control strategy. Several crops showing promise are: grain and forage species such as barley (Hordeum), oat (A vena), fescue (Festuca), and sorghum (Sorghum), and the agronomic species of corn (Zea) and sunflower (Helianthus). Among the problems that hinder the conclusive demonstration of allelopathic effects of crop plants are the loss of that capacity through selection and the variability among cultivars. Recent studies to evaluate the allelopathic potential of crop plants have shown that several sunflower varieties inhibit the germination and growth of associated weeds and to a greater extent than found in several biotypes of native sunflower. Aqueous extracts of dried sunflower and rape tissue inhibited or stimulated germination and growth of weeds, and the response depended upon the source of extract, the extract concentration, and the weed species tested. The validity of bioassay results was tested in a 5-year field study with sunflower and oat grown in rotation. Weed density increased in all plots but the extent of increase was significantly less in plots of sunflower than in control plots. The use of crop plants with increased allelochemical production could limit the need for conventional herbicides to early season application with late season control provided by the crop.     

Allelopathic Effects of Humus Phenolics on Growth and Respiration of Mycorrhizal Fungi

Allelopathy due to humus phenolics is a cause of natural regeneration deficiency in subalpine Norway spruce (Picea abies) forests. If inhibition of spruce germination and seedling growth due to allelochemicals is generally accepted, in contrast there is a lack of knowledge about phenolic effects on mycorrhizal fungi. Thus, this work tested effects of a humic solution and its naturally occurring phenolics on the growth and respiration of two mycorrhizal fungi: Hymenoscyphus ericae (symbiont of Vaccinium myrtillus, the main allelochemical-producing plant) and Hebeloma crustuliniforme (symbiont of P. abies, the target plant). Growth and respiration of H. crustuliniforme were inhibited by growth medium with the original humic solution (–6% and –30%), respectively, whereas the same humic solution did not affect growth but decreased respiration of H. ericae (–55%). When naturally occurring phenolics (same chemicals and concentrations in the original humic solution) were added to the growth medium, growth of H. crustuliniforme was not affected, whereas that of H. ericae significantly increased (+10%). We conclude that H. ericae is better adapted to the allelopathic constraints of this forest soil than H. crustuliniforme and that the dominance of V. myrtillus among understory species could be explained in this way.     

Allelopathic potential of well water fromPluchea lanceolata-infested cultivated fields

Pluchea lanceolata, an allelopathic perennial weed, has an extensive deep root and rhizome system. The objective of the present study was to determine the allelopathic potential of well water collected from weed-infested cultivated fields. Results indicate that well water recovered fromP. lanceolata-infested cultivated fields inhibited the shoot growth of pea, chick pea, mustard, and wheat under greenhouse conditions. Two phenolic compounds, phenol and phloroglucinol, were isolated and identified from collected well water using UV spectroscopy. The allelopathic potential of the aqueous extract of the two compounds was determined by growth experiments with 10⁴ M solutions of the compounds. As a consequence of repeated irrigation with well water fromP. lanceolata-infested fields, higher levels of phenolics can accumulate in the soil, which may contribue to increased interference to crop plants. The present study is of significance since it cautions on the use of well water for irrigating cultivated fields infested with the perennial allelopathic weed (P. lanceolata) with dense subterranean systems and emphasizes the importance of controlling such weeds in cultivated fields.     

Allelopathic effects ofPolygonum aviculare L. II. Isolation,characterization, and biological activities of phytotoxins

In earlier work, we found thatPolygonum aviculare had pronounced allelopathic effects against several test species. Four inhibitors were isolated from livingPolygonum plants, three of which were glucosides. Four different inhibitors were isolated fromPolygonum residues and soil underPolygonum stands, and none of these occurred in soil fromCynodon dactylon (L.) Pers. stands. Three of these were glycosides containing both fructose and cellobiose as the sugars. Color reactions of all the inhibitors indicated that they are phenolic in nature. All the inhibitors reduced seed germination and/or seedling growth ofChenopodium album L. Moreover some of them inhibited growth of different strains ofRhizobium andAzotobacter.     

Allelopathic effects of phenolic mixtures on respiration of two spruce mycorrhizal fungi

Investigations were conducted to determine whether the allelopathic effect (inhibition of oxygen consumption) of mull- and mor-type humic solutions on ectomycorrhizal fungi was due to the solutions' phenolic contents. In the first experiment, two concentrations (10^–3 M and 10^–7 M) of binary equimolar phenolic mixtures were tested on the oxygen consumption ofLaccaria laccata andCenococcum graniforme. The high concentrations of most of the mixtures induced an increase of fungal respiration, whereas the 10^–7 M treatments all reduced the rate of respiration. In the second experiment, the effects on respiration of the phenolic mixture reproducing mull- and mortype humic solutions were compared with the effects of natural humic solutions. The resulting data suggest that allelopathic effect of humic solutions can at least in part be attributed to their phenolic contents.     

Toxicity of Phenolic Compounds Extracted from Bark Residues of Different Ages

In Quebec, Canada, industrial bark wastelands cover several hundred hectares of land. Bark residue that has piled up for decades tends to remain free of vegetation for years. To assess the revegetation potential of such sites, we sought to determine those factors responsible for poor plant growth. Phenolic compounds from fresh to 20-year-old bark residues were extracted with four solvents and quantified by high-performance liquid chromatography (HPLC). We simulated solutions (mixtures of standard phenolic compounds) to evaluate the potential toxic effects of phenolic compounds on the rhizobial growth, germination index, plant growth, nodule number, and nitrogen fixation activity of two legume species under laboratory conditions. The concentration of individual phenolic compounds varied from none detected to 350 μg/g bark residue. The extracted phenolic compounds differed among solvents and bark residues. The highest concentration of total phenolic compounds was from fresh bark; most of these were soluble in water or 0.1 M NaOH. For older bark residues, the total phenolic content depended on solvent strength, generally in the order of 2.0 M NaOH > 0.1 M NaOH = hot water > cold water. The biological activity of the simulated bark extracts was not established with the rhizobial growth inhibition test but with the germination index and rhizobium–legume symbiosis tests. With these, the toxicity of the simulated phenolic extracts decreased from fresh to the older bark residues. Plant dry weight, nodule number, and nitrogen fixation activity of vetch (Vicia sativa L.) were less negatively affected by high concentrations of phenolics than birdsfoot trefoil (Lotus corniculatus L.), although birdsfoot trefoil grew at lower concentrations. The rhizobium–legume symbiosis has potential for revegetating bark wastelands with less than 1 year old and older bark residues.     

Growth Inhibitory Effects of Nutgrass (Cyperus rotundus) on Rice (Oryza sativa) Seedlings

Growth inhibitory effects of aqueous extracts and leachates of leaves and tubers of Cyperus rotundus L. were investigated by using rice (Oryza sativa L.) seedling as a bioassay material. Both the extracts and leachates of Cyperus were inhibitory to the growth of rice seedlings. Growth inhibition was more pronounced in the presence of aqueous extracts than the leachates. The extract and leachate of leaves had higher total phenolic contents than those of the tubers. Soil amendment with fresh leaves of Cyperus reduced plant height, leaf area, and root and shoot weight of rice seedlings. Total phenolic content was higher in soil amended with fresh Cyperus leaves than the unamended control soil. Nineteen compounds were tentatively identified from the aqueous extracts of leaves and tubers by ethyl acetate extraction followed by gas chromatography–mass spectroscopy (GC-MS). Dicarboxylic, phenolic, and fatty acids were the major compounds. Our results suggest that Cyperus may affect the growth and establishment of rice seedlings after sowing or transplanting, especially when Cyperus plants are mixed in soil during land preparation by ploughing in rain-fed rice culture.     

Effect of gamma irradiation on allelopathic potential ofSorghum bicolor against weeds and nitrification

The effect of low doses of gamma irradiation on the allelopathic potential ofSorghum bicolor against weeds and nitrification was investigated. The results revealed that all test doses (500, 1000 and 1500 rad) significantly increased the allelopathic activity of root exudates, aqueous extracts, and decaying residues aganst seed germination and seedling growth ofAmaranthus retroflexus. The results also indicated that all test doses stimulated the allelopathic potential of aqueous extracts and decaying residues against nitrification activity. The possible application of this approach in biological control is briefly discussed.     

Allelopathic potential ofErica vagans,calluna vulgaris,andDaboecia cantabrica

In laboratory tests the allelopathic potential ofErica vagans, Calluna vulgaris, andDaboecia cantabrica was determined. Aqueous extracts of flowers ofD. cantabrica and leaves ofC. vulgaris inhibit root and hypocotyl growth of red clover, the former causing 51% inhibition of germination. Intact aerial parts of the Ericaceae here studied drastically reduced the growth of red clover and 100% inhibition of germination was caused by flowers ofD. cantabrica. Inhibition of aqueous extracts remains after Chromatographic separation, and two well-defined inhibition zones may be observed. Hydrosoluble organic compounds (phenol-like compounds) could probably be responsible for the inhibitions detected.     

Phenolic inhibitors inErica australis L. and in associated soil

Fifteen phenolic compounds were identified in methanolic extracts obtained from flowers, leaves, stems, and roots ofErica australis L. and the associated soil. It is possible that the compounds identified may be connected with allelopathic phenomena.     

Environmental and biotic factors affecting the phenolic content of different cultivars ofSorghum bicolor

Levels of phenolic acids in healthy plants ofSorghum bicolor differ considerably with cultivar and always decrease as the plant matures. Laboratory- and field-grown plants show significant differences in phenolics. Environmental factors, particularly light intensity, influence the concentration of phenolics in sorghum. Attack by insects and pathogenic fungi also increase the phenolic content of the plants to varying degrees dependent on the cultivar and the stage of growth of the plant.     

Effects of pine-produced chemicals on selected understory species in aPinus ponderosa community

Pinus ponderosa accounted for more than 98% of all tree and shrub stratum stems in a climax community with low herb coverage and aboveground biomass, 35% and 60 g/m², respectively. Because of our previous report that nitrification and nitrifying bacteria in the same community were allelopathically inhibited, we speculated that the pine-produced allelochemics might also directly influence the development and growth of the herb stratum. In most cases decaying needles, needle leachate, and field soils significantly reduced germination and radicle growth ofAndropogon gerardii andA. scoparius, pine-associated herbaceous species. Additionally, growth ofAndropogon scoparius seedling radicles was reduced 28–56% by pine needle extracts, 33% by pine bark extracts, and 67% by soil hydrolysate extracts.Andropogon seed germination was reduced 20–25% by pine needles and soil. Phytotoxins identified in various plant parts and associated soils were caffeic acid, chlorogenic acid, quercetin, and condensed tannins. Pine needle water and soil hydrolysate extracts were most inhibitory to the radicle growth of the test species. Thus it appears that the limited growth of the herbaceous stratum in the pine community may be accounted for, in part, by allelopathy. Such allelopathic interactions may have an adaptive ecological significance in various forest and other plant communities.     

Allelopathic potential ofIpomoea tricolor (Convolvulaceae) in a greenhouse experiment

The allelopathic potential ofIpomoea tricolor, a plant used in Mexican agriculture to control weeds, and tricolorin A, the major phytogrowth inhibitor present in the so-called resin glycosides of this plant, have been evaluated by testing leachates of the plant and the compound on the germination and radicle growth ofAmaranthus hypochondriacus, Echinochloa crusgalli, Senna uniflora, I. tricolor, andI. purpurea. The allelopathic potential ofI. tricolor was evaluated in a greenhouse experiment with dryI. tricolor mixed with sterile and nonsterile soil in pots.A. hypochondriacus was sown in pots containingI. tricolor, 2-chloro-4-(ethylamino)-6-(isopropylamino)-1,3,5 triazine (Gesaprim) or 1-glyphosphate, and the glyphosphate salt of isopropylamine (Faena), two different commercial herbicides used as a comparison toI. tricolor. Number and dry weights of different monocotyledonous and dicotyledonous weeds andA. hypochondriacus growing in the different treatments were measured.Ipomoea and Faena herbicide had a similar inhibitory effect on monocots.     

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.