Allelopathic effects ofPinus densiflora on undergrowth of red pine forest

Correlation between the distributional frequency of undergrowth species of red pine forest and their germination and growth effected by pine extracts and leachates was found. It was made clear by germination and growth tests that pine toxic substances inhibit the germination and growth of low frequency species more than high frequency species in a red pine forest and that these substances are contained in descending concentration in fresh and fallen leaves, roots, pine forest soil, and pine rain. The concentration of pine toxic substances in extracts or leachates was affected by extracting or leaching within a given period of time, requiring a few hours for extracts or a few days for leachates. The amount of dry weight inhibition of the undergrowth treated by pine leachates was expressed as a growth inhibition index (GII) for the comparison of tolerance in various species. GII is a relative value (%) of the test groups against the control and it is an exponential function of the amount of pine toxic substances affecting the dry weight of the undergrowth. The substances were analyzed by paper and gas chromatography. Benzoic acid and 11 phenolic acids were identifed by gas chromatography. Benzoic acid was considered to be a key factor of allelopathy in the red pine forest.     

Allelopathic effects of alfalfa

Several experiments have been conducted at the University of Illinois, Urbana, Illinois, to determine the role of allelopathic (autotoxicity) effects of alfalfa (Medicago sativa L.) on the establishment of alfalfa. By means of correcting any soil fertility differences and protecting the alfalfa seedlings from fungal attack, results suggest that there is a release of phytotoxic factors from the previous crop on reestablishing alfalfa. Field results indicate that the best preceding crop for alfalfa establishment is corn (Zea mays L.), followed by various small grains, soybeans (Glycine max L.), and the worst preceding crop is alfalfa. Results indicate there were no major genetic differences among cultivars for resistance to autotoxicity. There was no evidence supporting saponins as being phytotoxic to alfalfa. Exudates of different saponin level cultivars indicated that saponin content is not the suspected phytotoxic factor.     

Allelopathic effects ofCitrus aurantium L.

Field observations on undisturbed stands of sour orange revealed thatCynodon dactylon, Chenopodium album, Avena sativa, andAmaranthus retroflexus were not able to grow normally and complete their life cycles under its canopies, although the same species grow well under adjacent trees of date palm. Investigations revealed that the failure of the test species to grow normally under sour orange was not due to competition for light, moisture and minerals or to differences in soil texture or pH. Soil under sour orange trees drastically reduced seed germination and/or seedling growth of test species. Aqueous extracts, decaying materials, and volatile compounds of senescent and nonsenescent sour orange leaves were found to inhibit seed germination and/or seedling growth of test species. Therefore, allelopathy appeared to be the basic factor responsible for the reduction in plant growth with competition propably accentuating its effects.     

Allelopathic dominance ofMiscanthus transmorrisonensis in an alpine grassland community in Taiwan

A study site located at 2600 m elevation in Tartarchia Anpu, Nantou county, Taiwan, exhibits a unique grassland community composed of two principal species,Miscanthus transmorrisonensis andYushinia niitakayamensis, and 35 other species. The relative frequencies of the two species are 12% and 11%, while their relative coverages are 25% and 19.5%, respectively. The values for the remaining 35 species are lower than4% each, while species diversity of the community is –3.04839, indicating great diversity. To elucidate the mechanism of dominance ofM. transmorrisonensis, allelopathic evaluation of the plant was conducted. Aqueous extracts of M.Transmorrisonensis plant parts with two ecotypes were bioassayed. The extracts showed significant phytotoxic effects on seed germination and radicle growth of four tested plants: rye grass, lettuce, and two varieties of Chinese cabbage. In addition, rhizosphere soils underMiscanthus also exhibited significant phytotoxicity, indicating that allelopathic interaction was involved. Some responsible phytotoxic phenolics, namely, p-coumaric, ferulic, vanillic, protocatechuic, o-hydroxyphenylacetic, andm-hydroxyphenylacetic acids, and 4-hydroxycoumarin and phloridzin were identified. Allelopathy thus can play an important role in regulating plant diversity in the field.The study was supported by grants to C.H. Chou awarded by the Academia Sinica, Taipei and the National Science Council (NSC-80-0211-B001-25) of The Republic of China.     

Allelopathic effects ofPolygonum aviculare L.

We previously reported thatPolygonum aviculare has a strong allelopathic action againstCynodon dactylon (L.) Pers. and other test species. Moreover, we found that several phenolic compounds appeared to be important allelochemics in this activity. We have now isolated other potential inhibitors fromP. aviculare residues and soil underPolygonum stands, and none of these occurred in soil underC. dactylon stands. GC-MS analysis revealed that these additional inhibitors are long-chain fatty acids with 14–22 carbons. Nine were identified inP. aviculare residues and seven in soil underP. aviculare. Sodium salts of all the identified fatty acids inhibited seedling growth ofC. dactylon and at least some test strains of the nitrogen-fixing bacteria,Azotobacter andRhizobium.     

Allelopathic and autotoxic effects ofAnastatica hierochuntica L.

Laboratory experiments were undertaken to investigate the autotoxic effects ofAnastatica hierochuntica and possible effects on five other desert plants:Rumex cyprius, Trigonella stellata, Diplotaxis harra, Cleome droserifolia, andFarsetia aegyptia. Seed germination, seedling growth, and cell division of all species tested were inhibited by the shoot aqueous extract ofA. hierochuntica. A gradual increase in the percentage of prophase and decrease in the other mitotic stages as well as the mitotic index were observed with increasing extract concentration. At an extract concentration of 8% the mitotic index was reduced from the control by 55% inC. droserifolia, 54% inT. stellata, 45% inF. aegyptia, 43% inA. hierochuntica, and 35% inR. cyprius. The inhibitory substances are apparently released onto soil by repeated washing of the standing plants by rain and dew interception.     

Allelopathic effects of fescue on the growth of sweetgum

The height growth of field-planted sweetgum (Liquidambar styraciflua L.) seedlings was reduced where residual fescue (Festuca arundinacea Shreb. Var. Ky. 31) from previous agricultural experimentation was present. Interference (competition+allelopathy) effects of fescue on the growth of sweetgum were tested in the greenhouse. Fescue seeded into pots containing sweetgum seedlings resulted in dry weight reduction of sweetgum from 29 to 95%. Elimination of the competitive effect through the use of a stairstep apparatus implicated an allelopathic mechanism in the interference of sweetgum growth by fescue. Leachates from the rhizosphere of live fescue, dead fescue roots, and dead fescue leaves resulted in reduction up to 60% in dry matter production of sweetgum seedlings. Chemical analysis of sweetgum seedlings from the stairstep experiment suggested impaired adsorption of phosphorus and nitrogen by seedlings treated with fescue leachates.     

Allelopathic effects of black walnut on european black alder coplanted as a nurse species

Black alder trees were interplanted with black walnut on a 2.4 × 3.7-m spacing at four locations in Illinois and Missouri. In every plantation, the alders suddenly declined and died after 8–13 years. Alternative causes for the mortality were investigated–competition from walnut, adverse soil properties, frost, insects, disease, unsuitable seed source, and allelopathy—but only allelopathy could be substantiated. The decline in alder survival and diameter growth was strongly related to walnut above-ground dry biomass and walnut crown competition factor. We propose that the black walnut allelopathy most likely results from a combination of (1) sufficient walnut biomass to contribute substantial amounts of juglone to the environment, and (2) wet soil that greatly restricts aerobic metabolism by soil microorganisms, allowing juglone to build up to toxic levels.     

Allelopathic effects ofPolygonum aviculare L. I. Vegetational patterning

Polygonum aviculare was observed to spread rapidly into heavy stands ofCynodon dactylon (L.) Pers. resulting in death of the latter. This indicated a strong interference againstCynodon dactylon. Measurements of selected soil minerals and physical factors indicated that competition was probably not the chief cause of that interference. Soil collected under deadPolygonum was very inhibitory to all test species exceptSporobolus pyramidatus (Lam.) Hitchc., suggesting the presence of inhibitory compounds. Tops and roots ofPolygonum, root exudates, and leachate of the tops inhibited seed germination and seedling growth of most test species. Therefore, allelopathy apeared to be the dominant component of the interference, with competition probably accentuating its effects.Polygonum aviculare was inhibitory toGossypium barbadense L. andSorghum bicolor (L.) Moench, indicating that allelopathy is an important component of the interference byPolygonum against crop yields.     

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.     

Allelopathic effects ofChrysanthemum morifolium on germination and growth of several herbaceous plants

Aqueous extracts obtained from young green tops ofChrysanthemum morifolium inhibited the germination of six flowering plants, including chrysanthemum itself, provided for experiments. The same phenomenon was also clearly observed when powder made from young green tops and old leaves of chrysanthemum was used. Moreover, the growth of seedlings planted again in garden soil which was once used for the culture of chrysanthemum was greatly interrupted. Chrysanthemum cultured in used garden soil showed far less dry weight than that cultured in fresh garden soil. The weight of chrysanthemum cultured using its root exudates was also less than that cultured with water leachate of fresh garden soil, and therefore these results may be considered to indicate allelopathic effects. In order to find the allelochemicals related to this phenomenon, benzoic acid and phenolic acids such as salicylic,p-hydroxybenzoic, vanillic, gentisic, protocatechuic, syringic, gallic, ferulic, and caffeic acids were identified by gas chromatography.     

Allelopathic substances in ecosystems

Recovery studies were conducted with ferulic acid, a common allelopathic agent, using various soils and soil components. Ferulic acid was placed into sterile soil components (gibbsite, geothite, Georgia kaolin, and Utah bentonite), and different sterile soil materials (from different horizons in the same profile) varying in mineralogy and in organic matter content. The initial concentration of ferulic acid added to the soil materials was 1000 μg/g (5.149 mmol/g). The pH of the soil materials was adjusted and maintained at approximately 4.5 or 7.5. Samples were extracted with 0.03 M EDTA at days, 1, 4, 7, 10, and 13 after addition of ferulic acid. Concentrations of ferulic acid in the extracts were determined with a high performance liquid chromatograph. No breakdown products were detected. Models were developed to describe the recovery of ferulic acid from each soil material and soil component over time. Organic matter was the most active soil component involved in the irreversible retention of ferulic acid. The inorganic soil components were much less active than organic matter but appeared to be similar to each other in activity. Irreversible retention of ferulic acid by soil and soil components was greatest as pH 7.5.     

Allelopathic interactions in crop-weed mixtures

A very important component of the aggressive nature of weeds is allelopathic interference, the full potential of which is just being realized in the management of agroecosystems. Research results are presented which demonstrate the allelopathic interactions involved in a wide range of cropweed combinations occurring in a great variety of habitats. This includes crops planted in weed control, crops with allelopathic potential, and noncrop plants of beneficial use for weed control as a result of allelopathic interference. Allelopathy can play a beneficial role in multiple cropping systems, crop rotations, and cover cropping. The potential role for allelopathic interactions in the design of biological weed control is proposed.     

Allelopathic effects of western ragweed on seed germination and seedling growth of selected plants

Western ragweed (Ambrosia psilostachya DC.) top growth and rhizome extracts were inhibitory to germination and growth of seedling shoots and roots of test plants in almost all cases. Germination of all plants tested was reduced an average of 19.5% by the ragweed extracts. Shoot and root growth of plants tested was reduced an average of 56.8% by the ragweed extracts. The combined effects of germination plus growth reductions resulted in the tested plants producing only 34.8% in the extracts compared to production in distilled water.     

Allelopathic effects of juglone on germination and growth of several herbaceous and woody species

Laboratory experiments were conducted to determine juglone sensitivity of 16 species (Trifolium incarnatum, Coronilla varia, Vicia villosa, Lespedeza stipulacea, L. cuneata, Acer ginnala, Caragana arbor-escens, Elaegnus angustifolia, E. umbellata, Lonicera maackii, Quercus alba, Fraxinus americana, Liriodendron tulipifera, Alnus glutinosa, Pinus strobus, andP. sylvestris) being considered for mixed plantings withJugions nigra (black walnut). All species were sensitive to juglone, but seed germination and radicle elongation were less affected than shoot elongation and dry weight accumulation. Seed germination and radicle elongation were affected by juglone in 6 and 11 species, respectively, mainly by the higher concentrations (10^?3 M and 10^?4 M). Shoot elongation and dry weight accumulation of all species were affected by juglone; many species were sensitive to concentrations as low as 10^?6 M. Seedlings of all species were severely wilted and eventually killed by 10^?3 M juglone, and most were chlorotic and severely retarded by 10^?4 M juglone. Seedlings inhibited by 10^?6 M and 10^?5 M juglone did not showany visible signs of injury. Based on the effects on seedling shoot elongation and dry weight accumulation, the five species found to be most sensitive to juglone were:Lonicera maackii, Lespedeza cuneata, Trifolium incarnatum, Alnus glutinosa, and Elaeagnus umbellata.     

Allelopathic effects of giant foxtail on germination and radicle elongation of loblolly pine seed

Water extracts of giant foxtail grass inhibited germination and radicle elongation of loblolly pine seed in a laboratory test. Most of the toxic effects came from extracts of dried foxtail tops, with lesser amounts from fresh tops and roots. Soil texture affected the phytotoxicity of the extracts. Eight chemical compounds known to be phytotoxins inSetaria were identified in extracts of foxtail leaves but could not be found in soil samples under the leaves.     

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.     

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.     

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.