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.     

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.     

Abscisic Acid in Soil Facilitates Community Succession in Three Forests in China

Plants release secondary metabolites into the soil that change the chemical environment around them. Exogenous abscisic acid (ABA) is an important allelochemical whose role in successional trajectories has not been examined. We hypothesized that ABA can accumulate in the soil through successional processes and have an influence on forest dynamics. To this end, we investigated the distribution of ABA in forest communities from early to late successional stages and the response of dominant species to the gradient of ABA concentrations in three types of forests from northern to southern China. Concentrations of ABA in the soils of three forest types increased from early to late successional stages. Pioneer species’ litters had the lowest ABA content, and their seed germination and seedling early growth were the most sensitive to the inhibitory effect of ABA. Mid- and late-successional species had a much higher ABA content in fallen leaves than pioneer species, and their seed germination and seedling early growth were inhibited by higher concentrations of ABA than pioneers. Late-successional species showed little response to the highest ABA concentration, possibly due to their large seed size. The results suggest that ABA accumulates in the soil as community succession proceeds. Sensitivity to ABA in the early stages, associated with other characteristics, may result in pioneer species losing their advantage in competition with late-successional species in an increasingly high ABA concentration environment, and being replaced by ABA-tolerant, late-successional species.     

Forest understory biomass heterogeneity is “moisture complex” or associated litter the cause?

Understory biomass heterogeneity underPlatanus occidentalis L.,Quercus alba L.,Celtis occidentalis Pursh., andUlmus americana L. was studied. Soil pH, Ca, Mg, Mn, Cu, NH₄ nitrogen, total N, and soil moisture were significantly different under all dominant species. The pattern of soil moisture levels under different tree species was not consistent from one sampling period to the next. The aboveground biomass of understory vegetation also varied significantly under different tree species and was not correlated with variation in any of the soil properties. Maximum understory biomass gain (340%) from May to September occurred under oak trees, where soil moisture and most nutrients were the lowest. On the other hand, sycamore and hackberry had continuous release of allelochemics and the smallest understory biomass gain (103%) during the growing season, even though soil under both species had more moisture and nutrients than the soils under white oak. Eliminating all the factors studied leads to the conclusion that organic substances released in the immediate environment of dominant trees and their litter influenced soils and associated herb growth. In each dominant niche system, understory species may have to develop their own system (microniche) to minimize the stresses of competition and allelochemics.     

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 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.     

Allelochemicals of Pinus halepensis as Drivers of Biodiversity in Mediterranean Open Mosaic Habitats During the Colonization Stage of Secondary Succession

The Mediterranean region is recognized as a global biodiversity hotspot. However, over the last 50 years or so, the cessation of traditional farming has given way to strong afforestation at the expense of open habitats. Pinus halepensis Miller, known to synthesize a wide range of secondary metabolites, is a pioneer expansionist species colonizing abandoned agricultural land that present high species richness. Here, laboratory bioassays were used to study the potential impact of P. halepensis on plant diversity through allelopathy, and the role of microorganisms in these interactions. Germination and growth of 12 target species naturally present in fallow farmlands were tested according to concentration of aqueous extracts obtained from shoots of young pines (aged about 5 years), with or without the presence of soil microorganisms (autoclaved or natural soil). Under the highest concentrations and autoclaved soil, more than 80 % of target species were germination and/or growth-inhibited, and only two species were non-sensitive. Under more natural conditions (lower extracts concentrations and natural soil with microorganisms), only 50 % of species were still inhibited, one was non-sensitive, and five were stimulated. Thus, microorganisms alter the expression of allelochemicals released into the ecosystem, which highlights their key role in chemical plant-plant interactions. The results of allelopathic experiments conducted in the lab are consistent with the community patterns observed in the field. These findings suggest that allelopathy is likely to shape vegetation composition and participate to the control of biodiversity in Mediterranean open mosaic habitats.     

The allelopathic mechanisms of dominance in bracken (Pteridium aquilinum) in Southern California

Bracken,Pteridium aquilinum, exerts a strong dominance over associated plants throughout much of its worldwide range. Associated plants are often severely inhibited or even excluded from dense stands of the fern. This study investigated the various aspects of herb suppression in bracken stands and assessed the contribution of the various forms of interference between plants to the establishment and maintenance of bracken dominance. It was shown that competition for soil moisture, light, and nutrients could not account for the lack of herbs in bracken stands. Further, uniformity of soil pH, texture, water-holding capacity, and organic matter content ruled out variability in physical factors as a cause. Baiting and trapping experiments showed that the higher concentration of animal activity inside the bracken stands contributed significantly to the pattern of herb suppression, but only against select species. The maintenance of this pattern in the animal-free Santa Cruz Island stands indicates the importance of another factor, allelopathy. It was found that phytotoxins leached from the dead, standing bracken fronds with the first few rains of the wet season were largely responsible for herb suppression. These toxins were isolated in raindrip and from soil inside the fern stands. Removal of the fronds from the stand before the rains could leach them resulted in reinvasion by the herbs after several seasons, and, conversely, placing fronds over the herbs in the grassland brought about herb inhibition. A number of known allelopathic chemicals were tentatively identified from bracken leachates. The importance of the interaction of allelopathy with other factors of plant interference is illustrated by bracken.     

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 properties ofPolygonella myriophylla

Polygonella myriophylla is a perennial shrub endemic to the Florida scrub. Striking bare zones surround maturePolygonella stands. Quantitative measurements of root distribution show that fewPolygonella roots extend into the bare zones, supporting the hypothesis that the bare zones result from chemical interference byPolygonella with the growth of other species. Bioassays of soils collected biweekly for one year from beneathPolygonella, the bare zone, and adjacent grassed areas confirm that the germination and growth of grasses is reduced inPolygonella soil and soil from the surrounding bare zone. Compared to adjacent grassed area soil, the average germination of bahiagrass (Paspalum notatum) was 71% inPolygonella soil and 81% in bare zone soil, and average shoot dry weight was 48% inPolygonella soil and 81% in bare zone soil. Seasonal variation in the inhibition of grass germination and growth was not pronounced.     

How much nitrogen is fixed by biological symbiosis in tropical dry forests? 1. Trees and shrubs

Despite the recognized importance of the process, estimates of the amount of nitrogen fixed by biological symbiosis in tropical dry forests are almost nonexistent. We estimated the nitrogen fixed annually by the leaves of trees and shrubs at sites regenerating for 16 and 38?years and in an old-growth dry forest using ¹⁵N abundance methodology. The total leaf biomass (1,824?C3,036?kg?ha^?1) and nitrogen contents (62?C90?kg?ha^?1) did not differ among the areas. In all of the areas, most of the leaf biomass belonged to legume plants, but the proportion of the N₂-fixing legumes decreased with increasing regeneration time. In the 16-year regenerating area, almost all of the N was in the leaves of the N-fixing Mimosa tenuiflora plants, but fixation was absent or very low as it was in the N-fixing species present in the 38-year regenerating area. In the old-growth Caatinga, all of the N-fixing species (M. tenuiflora, Piptadenia stipulacea and Anadenanthera colubrina) had large proportions (47?C62?%) of their N derived from atmospheric N₂, but the amount of fixed N (6?kg?ha^?1) was a small proportion of the total leaf N because these plant species were a small part of the vegetation. The total input of biologically fixed N to the old-growth forest was similar in magnitude to an estimate made for a humid tropical forest in Amazonia.     

Allelopathic inhibition ofCynodon dactylon (L.) pers. and other plant species byEuphorbia prostrata L.

Field observations indicated thatEuphorbia prostrata strongly interferes withCynodon dactylon (L.) Pers. Analysis of some physical and chemical soil factors indicated that competition was not the dominant factor of that interference. Soil collected from underE. prostrata stands was very inhibitory to seed germination and seeding growth of some of the test species including C.Dactylon. This suggests the presence of inhibitory compounds in soil ofE. prostrata stands. Subsequent experiments showed that aqueous extract, decaying residues, and root exudates ofE. prostrata were inhibitory to most of the test species including C.Dactylon. Thus, it appears that allelopathy is the major component of the interference, with competition probably accentuating its effect. It also was found that allelopathy is an important component of the interference byE. prostrata againstAmaranthus retroflexus, Medicago sativa, andGossypium hirsutum.     

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.     

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.     

Allelopathy on Bark of Downed Logs of <Emphasis Type="Italic">Chamaecyparis Obtusa</Emphasis> sieb. and Zucc. var. <Emphasis Type="Italic">formosana</Emphasis> (Hayata) Rehder

Chamaecyparis obtusa Sieb. and Zucc. var. formosana (Hayata) Rehder is the dominant species in the temperate forest of Yuanyang Lake Nature Reserve (YYL), Taiwan. Although downed logs of C. obstusa var. formosana occupy only a small percentage of the forest floor area in YYL, they are important regeneration substrates. Seedlings of this species often grow without competition on the new downed logs, and a few broadleaf trees grow with them. We hypothesized that the bark of the newly fallen logs possesses allelopathic potential that provides a habitat especially suitable for seedling establishment. Eight different seeds including those from Lactuca sativa L. (lettuce), Bidens pilosa (an invasive weed), and six species in YYL were planted on the bark of the downed logs in an incubator for germination tests. Two dominant species in the forest of YYL, C. obtusa var. formosana and Rhododendron formosanum, were able to grow normally, but the others, Pieris taiwanensis, Barthea formosana, Chamaecyparis formosensis, Miscanthus transmorrisonensis, lettuce, and B. pilosa were growth inhibited. A bioactivity-guided isolation was designed to isolate allelochemicals from the bark. Salicylic acid, one of the inhibiting substances, was isolated and identified by gas chromatography/mass spectroscopy (GC/MS), proton nuclear magnetic resonance (¹H NMR), and infrared (IR). Bioassay of salicylic acid confirmed a phytotoxic effect. The results suggest that the dominance of C. obtusa var. formosana seedlings on bark could be partly due to allelopathy.     

Response of Poplar and Associated Fungal Endophytic Communities to a PAH Contamination Gradient

Microbial populations associated to poplar are well described in non-contaminated and metal-contaminated environments but more poorly in the context of polycyclic aromatic hydrocarbon (PAH) contamination. This study aimed to understand how a gradient of phenanthrene (PHE) contamination affects poplar growth and the fungal microbiome in both soil and plant endosphere (roots, stems and leaves). Plant growth and fitness parameters indicated that the growth of Populus canadensis was impaired when PHE concentration increased above 400 mg kg⁻¹. Values of alpha-diversity indicators of fungal diversity and richness were not affected by the PHE gradient. The PHE contamination had a stronger impact on the fungal community composition in the soil and root compartments compared to that of the aboveground organs. Most of the indicator species whose relative abundance was correlated with PHE contamination decreased along the gradient indicating a toxic effect of PHE on these fungal OTUs (Operational Taxonomic Units). However, the relative abundance of some OTUs such as Cadophora, Alternaria and Aspergillus, potentially linked to PHE degradation or being plant-beneficial taxa, increased along the gradient. Finally, this study allowed a deeper understanding of the dual response of plant and fungal communities in the case of a soil PAH contamination gradient leading to new perspectives on fungal assisted phytoremediation.     

Field Testing for Pollen Allelopathy: A Review

Field testing for pollen allelopathy comprises several approaches. In vitro tests must be done first to establish the existence of a pollen allelopathic effect. The number of pollen grains required to elicit an in vitro pollen allelopathic effect then is compared to observations of the number of pollen grains of pollen allelopathic species adhering to stigmas of heterospecifics. In animal-pollinated species, the amount of potential heterospecifics may first be assessed by observing pollinator behavior, but actual enumeration of grains is necessary. In all species, enumeration of the amount of heterospecific pollen transfer requires that stigmas be clipped before they senescence, carefully preserved in small test tubes, and assessed immediately thereafter with aid of a compound microscope. Stigmas may be examined for relative amounts of allelopathic and conspecific pollen to determine if the number of conspecific pollen tubes declines hyperbolically with increasing amounts of allelopathic pollen. Enumerating the number of heterospecific pollen grains on stigmas is used to support testing of the ecological impact of pollen allelopathy by direct manipulation. In habitats (e.g., old fields surrounded by forest tracts) where atmospheric conditions prevent export or import of other pollen from surrounding areas, the influorescences of a pollen allelopathic species can be clipped within a large area and the subsequent effect on seed set of target species can be assessed. The hypothesis that pollen allelopathy causes an ecological impact (reduced sexual reproductive success) is not rejected under two conditions. An absence of pollen from the allelopathic species must result in an increase in seed set in target species and the amount of pollen transferred from allelopathic species to heterospecifics is similar to that causing in vitro reductions in sexual reproduction in the targets. This experimental approach must be repeated over several sites and years in order to provide an unequivocal test for ecological effects of pollen allelopathy. This methodological approach has allowed me to demonstrate that pollen allelopathy is an important ecological interaction between Hieracium pratense and five sympatric Asteraceae species and between Phleum pratense and three sympatric Poaceae species.     

Inhibitory Potential of Naphthoquinones Leached from Leaves and Exuded from Roots of the Invasive Plant Impatiens glandulifera

Exploring the effects of allelopathic plant chemicals on the growth of native vegetation is essential to understand their ecological roles and importance in exotic plant invasion. Naphthoquinones have been identified as potential growth inhibitors produced by Impatiens glandulifera, an exotic annual plant that recently invaded temperate forests in Europe. However, naphthoquinone release and inhibitory potential have not been examined. We quantified the naphthoquinone content in cotyledons, leaves, stems, and roots from plants of different ages of both the invasive I. glandulifera and native Impatiens noli-tangere as well as in soil extracts and rainwater rinsed from leaves of either plant species by using ultra-high pressure liquid chromatography-mass spectrometry (UHPLC-MS). We identified the compound 2-methoxy-1,4-naphthoquinone (2-MNQ) exclusively in plant organs of I. glandulifera, in resin bags buried into the soil of patches invaded by I. glandulifera, and in rainwater rinsed from its leaves. This indicates that 2-MNQ is released from the roots of I. glandulifera and leached from its leaves by rain. Specific bioassays using aqueous shoot and root extracts revealed a strong inhibitory effect on the germination of two native forest herbs and on the mycelium growth of three ectomycorrhiza fungi. These findings suggest that the release of 2-MNQ may contribute to the invasion success of I. glandulifera and support the novel weapons hypothesis.     

Use of a comparative approach to identify allelopathic potential and relationship between allelopathy bioassays and “competition” experiments for ten grassland and plant species

Various allelopathy bioassays were used to evaluate the allelopathic potential of 10 grassland forage species against a common test (phytometer) species,Carduus nutans L. Aqueous extracts did not influenceC. nutans germination, although radicle elongation was often severely inhibited.C. nutans was strongly affected by shoot, but not root, leachates. Decomposing ground tissue had mixed effects, and often stimulated shoot production ofC. nutans. Calculation ofR ² (coefficient of determination) values between these results, and the results of previous experiments investigating the effects of the same 10 species onC. nutans emergence and development in field plots and glasshouse competition experiments frequently revealed strong, statistically significant relationships. Our results therefore provide correlative evidence for the importance of allelopathy in field conditions.     

Allelopathic substances and interactions ofDelonix regia (Boj) Raf

A unique pattern of weed exclusion was found under the canopy ofDelonix regia, which was planted in many places as an ornamental tree in the south of Taiwan. A quadrat method was employed to examine the botanical composition between the area underneathD. regia and its adjacent control grassland. The number of species and coverage of understory species were significantly lower in the area of theD. regia than that of the grassland, indicating the growth of understory species was suppressed byD. regia. A series of aqueous extracts of leaves, flowers, and twigs ofD. regia were bioassayed against three species to determine their phytotoxicity, and the results showed highest inhibition in the flowers. A water-culture experiment indicated the aqueous extract of flowers ofD. regia on two local understory species (Isachne nipponensis andCentella asiatica) inhibited growth of both species by more than 70%. The phytotoxicities of fallen leaves and flowers ofD. regia were not significantly affected by temperature. When the plant material was subjected to temperatures above 70°C, however, phytotoxicity was decreased, indicating that the allelopathic nature ofD. regia could easily be decomposed by fire. By means of paper, thin-layer, and high-performance liquid chromatography, and UV-visible spectrophotometry, responsible phytotoxins present in leaves, flowers, and twigs ofD. regia were identified as 4-hydroxybenzoic, chlorogenic, 3,4-dihydroxybenzoic, gallic, 3,4-dihydroxycinnamic, 3,5-dinitrobenzoic, and L-azetidine-2-carboxylic acids, and 3,4-dihydroxybenzaldehyde. The findings of bioassays and the number and amount of responsible allelopathic compounds found inD. regia are well correlated, thus permitting the conclusion that the exclusion of understory plants under the canopy ofD. regia trees was due primarily to the allelopathic effect of the fallen flower, leaves, and twigs of theD. regia. A possible mechanism of action is discussed.The study was supported by grants to C.H. Chou awarded by the Academia Sinica, Taipei, and the National Science Council (NSC-80 & 81-0211-BOO 1-25) of The Republic of China.