Tamarindus indica L. leaf is a source of allelopathic substance

The allelopathic potential of the Tamarindus indica L. leaf was investigated through bioassay guided studies using several weed and edible crop species. Both radicle and hypocotyl growth of all the plant species tested was strongly inhibited by the tamarind leaf using a sandwich method. The growth of weed species was reduced more than that of edible crop species. Among the weed species, barnyard grass followed by white clover, and in the edible crop species, lettuce followed by radish ranked top in terms of growth inhibition. Different concentrations of tamarind leaf crude water-soluble extract exhibited a strong inhibition in all the plant species tested and, by contrast, the magnitude of inhibition in the weed species was higher than in edible crop species and ranged from 30–75%. The 10% concentration of the tamarind leaf crude water-soluble extract was most potent against growth of seedlings. The concentrations of the nutrient components were linearly correlated with an increase in the concentration of tamarind leaf crude water-soluble extract. No significant changes in either pH or EC were found in the variations of different concentrations of tamarind leaf crude water-soluble extracts. As compared to control, growth of both radicle and hypocotyl in weed (barnyard grass and white clover) and in edible crop (lettuce and radish) species were significantly reduced when blended tamarind leaves at different concentrations were incorporated into the growth medium. The inhibitory magnitude increased with an increase in the concentration of the tamarind leaf. In terms of growth inhibition, among these tested plants, weed species particularly barnyard grass were most sensitive to the allelochemicals exuded from blended tamarind leaves. When the blended tamarind leaves were removed from the growth medium, all the seedlings grew quickly and the percentage of recovery was between 76–97% of the corresponding controls. Reduction in the fresh and dry weight of these 4 plant species was observed under the experimental conditions, and ranged between 33–42% and 40–53% in the radicle and hypocotyl, respectively. The fresh and dry weight, and total chlorophyll content declined significantly in the incorporated tamarind leaf treatments. Compared to the control, the highest drop in the chlorophyll content of 60% in barnyard grass was observed with the 10% concentration of the leaf treatment. These results clearly indicate that the tamarind leaf contains one or more strong biologically active allelochemical(s) that function as true growth regulator(s) and is involved in plant growth regulation, particularly in weed species.     

Allelopathic competence of Tamarindus indica L. root involved in plant growth regulation

The allelopathic competence of tamarind root was evaluated using several weed and edible crop species under both laboratory and greenhouse conditions. Bio-assay guided studies using agar and soil medium revealed that the growth of both radicle and hypocotyl were strongly inhibited under both conditions. Accelerated root exudation observed with an increase in the age of tamarind seedlings caused a high magnitude of growth inhibition of the plant species tested by the plant-box method. Tamarind seedlings at 21-DAG (days after germination) exerted the strongest inhibitory effect (85.0–95.1%) on the growth of the plant species tested. Root dry weight of tamarind seedlings in the plant-box method experiment was highly correlated (R ² values more than 0.92) with the percentage of growth inhibition. The growth of species grown in the soil under the tamarind tree was inhibited by 85.3–97.1% in the greenhouse. The percentage of growth inhibition declined by 18.4–22.0% (as compared to the natural soil condition) when autoclaved soil of the same trees was used for bio-assay of plant species by the soil-agar sandwich method. This indicates that ca. a 20% increase in response was associated with the allelopathic activity of tamarind root exuded into the natural soil and was due to the effects of soil microbes and soil texture. In terms of growth inhibition of the plant species tested, the root zone soil of the tamarind tree showed stronger inhibitory effects (80.1–94.2%) than the rhizosphere soil, as determined by the soil-agar sandwich method. In all cases, growth inhibition especially in the radicle was higher in the weed species than the edible crop species. Our observations clearly indicate that tamarind root exudate has allelochemical competence and this contributes to a weed free environment around the tamarind tree.     

Isolation and identification of a plant growth inhibitor from Tinospora tuberculata Beumee

Tinospora tuberculata Beumee has been used widely as a folk medicine and several bioactive compounds have been isolated. However, no herbicidal compound has been reported in this species. Therefore, we investigated the presence of phytotoxins in T. tuberculata. The aqueous methanol extracts of T. tuberculata inhibited the growth of roots and shoots of cress (Lepidum sativum L.), lettuce (Lactuca sativa L.), timothy (Phleum pratense L.) and barnyard grass (Echinochloa crus-galli (L.) Beauv.). The extract was then purified by several chromatographic runs with monitoring the inhibitory activity and the main phytotoxic substance was isolated. The chemical structure of the compound was determined by spectral data as syringin (4-[(1E)-3-Hydroxyprop-1-en-1-yl]-2,6-dimethoxyphenyl β-d-glucopyranoside). It inhibited the root and shoot growth of all test plant species at concentrations >10 µM. The concentrations required for 50 % inhibition of root and shoot growth of cress and lettuce ranged from 78.2 to 412 μM, and that of timothy and barnyard grass renged from 9.8 to 73.2 µM. Effectiveness of syringin on monocotyledonous (timothy and barnyard grass) plants was greater than that on dicotyledonous (cress and lettuce) plants. These results suggest that syringin may contribute to the allelopathic effect caused by the T. tuberculata extract.     

Phytotoxicity analysis of extracts from compost and their ability to inhibit soil-borne pathogenic fungi and reduce root-knot nematodes

Compost extracts are novel organic amendments, typically applied to suppress soil-borne diseases. This research evaluated the phytotoxicity of compost extracts and analyzed their ability to inhibit pathogenic fungal growth and reduce root-knot nematodes. The physical, chemical and biological characteristics of extracts from a pig manure and straw compost were analyzed. Three types of extracts were tested: direct extracts of compost (DEC), aerated fermentation extracts of compost (AFEC) and non-aerated fermentation extracts of compost (NAFEC). All compost extracts showed low phytotoxicity against lettuce and cress, but AFEC and NAFEC were more phytotoxic than DEC. All compost extracts significantly inhibited pathogenic fungal growth except for the fungus Rhizoctonia solania AG4. For two seasons, tomato root biomass of three compost extracts was 1.25–5.67 times greater than CK (water control), and AFEC and NAFEC showed the best tomato root growth promotion. The reduction ratio of root egg mass and density of soil nematodes were 34.51–87.77% and 30.92–51.37%, when applied with three compost extracts. The microbial population in compost extracts was considered to be the most significant factor of inhibition pathogenic fungal growth. No markedly correlations among bacterial community diversity, the inhibition of pathogenic fungal growth and the reduction of root-knot nematodes were observed. This information adds to the understanding of the growth-promoting and suppression effects of compost extracts and will help to enhance crop production.     

Allelopathic Substance Exuded from a Serious Weed, Germinating Barnyard Grass (Echinochloa crus-galli L.), Roots

The allelopathy of a serious weed, barnyard grass (Echinochloa crus-galli L.), was investigated. Root exudates of young barnyard grass showed allelopathic effects and plant-selective activity and inhibited root elongation of all plants tested. With respect to shoot growth, the exudates did not show inhibition of barnyard grass only. The allelopathic substance was isolated and identified as p-hydroxymandelic acid by NMR. p-Hydroxymandelic acid strongly inhibited shoot growth and root elongation of all plants tested. The effects of three congeners of p-hydroxymandelic acid were tested on rice shoot growth. In the biological activity exhibited in rice, shoot growth was related to the hydroxyl groups. Received October 7, 1998; accepted March 29, 1999     

Allelopathic effects of volatiles from some Cruciferae species on lettuce,barnyard grass and wheat growth

Exposure of germinating seeds of lettuce, barnyard grass and wheat to volatile substances released from pulverised leaves of some Cruciferae species resulted in delay of germination and reduction of overall growth.B. juncea andB. nigra volatiles were the most damaging to all of the tested species. It is suggested that degradation products of glucosinolates occuring in cruciferous plants caused this allelopathic action.     

Measuring the ecological preference for growth of 150 of the most influential weeds in weed community structure associated with agronomic and horticultural crops

The phytosociological researches which intent for studying the performance of weeds and the structure of weed assemblages associated with different crops derives their importance mainly from the adverse effect of weeds on crop productivity. Consequently, it is worth questioning about the ecological preferences of the weed growth in response to three main drivers for weed community structure associated with agronomic, and horticultural crops: crop diversification, crop seasonality, and soil type. A study area was selected comprising farmland of Nile Delta and its adjoining east and west territories, Egypt. A total of 555 species were recorded in 30 agroecosystems monitored and depending on species frequency/abundance values, 150 species were designated as the most influential weeds in weed community structure associated with agronomic and horticultural crops. The ecological preference of species for crop seasonality was evident through the results of Agglomerative hierarchical clustering. Three weed assemblage groups (WAG) identified: WAG A associated with winter agronomic crops, WAG B associated with summer agronomic crops, and WAG C associated with perennial agronomic crops and horticultural crops (orchards). Their diversity evaluated at different levels. The growth preference of the 150 species which were assigned as most influential weeds was gauged in response to the three environmental variables. 61 species were faithful to WAG A, 45 to WAG B, and 44 to WAG C. Concerning crop diversification, 34-species were significantly affected and scored coefficient of variation ≥ 100%. As for soil type, indicator species analysis revealed that 66-species show growth preference in fine grained soil while 84-species prefer coarse grained soil. In the three vegetation units (WAG A – C), 12 within-group associations (alliances) were specified of less-common (differential) species. The record of these alliances match to a specific environmental condition (ecological niche) and in them 29 strong indicators are identified. Redundancy analysis was used to extract and summarize the variation in species records in the response matrix (species vs. sites) that can be explained by the three different types of growth preference (explanatory variables), and the partial linear effect of them was evaluated by variation partitioning.     

Barnyard grass-induced rice allelopathy and momilactone B

Here, we investigated chemical-mediated interaction between crop and weeds. Allelopathic activity of rice seedlings exhibited 5.3-6.3-fold increases when rice and barnyard grass seedlings were grown together, where there may be the competitive interference between rice and barnyard grass for nutrients. Barnyard grass is one of the most noxious weeds in rice cultivation. The momilactone B concentration in rice seedlings incubated with barnyard grass seedlings was 6.9-fold greater than that in rice seedlings incubated independently. Low nutrient growth conditions also increased allelopathic activity and momilactone B concentrations in rice seedlings. However, the increases in the low nutrient-induced allelopathic activity and momilactone B concentration were much lower than those in barnyard grass-induced allelopathic activity and momilactone B concentration. Root exudates of barnyard grass seedlings increased allelopathic activity and momilactone B concentration in rice seedlings at concentrations greater than 30 mg/L of the root exudates, and increasing the exudate concentration increased the activity and momilactone B concentration. Therefore, barnyard grass-induced allelopathic activity of rice seedlings may be caused not only by nutrient competition between two species, but also by components in barnyard grass root exudates. As momilactone B shows strong allelopathic activities, barnyard grass-induced allelopathic activity of rice may be due to the increased concentration of momilactone B in rice seedlings. The present research suggests that rice may respond to the presence of neighboring barnyard grass by sensing the components in barnyard grass root exudates and increasing allelopathic activity by production of elevated concentration of momilactone B. Thus, rice allelopathy may be one of the inducible defense mechanisms by chemical-mediated plant interaction between rice and barnyard grass, and the induced-allelopathy may provide a competitive advantage for rice through suppression of the growth of barnyard grass.     

An allelopathic substance exuded from germinating watermelon seeds

When watermelon seeds were cultured in a Petri dish together with amaranth, barnyard grass, cockscomb, lettuce or tomato seeds, the shoot growth of amaranth and cockscomb was markedly promoted, whereas the shoot growth of lettuce and tomato was inhibited. The shoot growth of barnyard grass was not affected. These results suggest that plant-selective allelopathic substance(s) affecting the shoot growth of other plant seedlings were exuded from watermelon seeds. An allelopathic substance was isolated from the exudates of germinating watermelon seeds and identified as vanillic acid by its spectral analysis and Rf value on TLC. Vanillic acid promoted the shoot growth of cockscomb at the concentrations of 300 to 10 mg/l and that of amaranth at the concentrations of 30 to 3 mg/l, although the shoot growth of amaranth was inhibited by 300 mg/l of vanillic acid. The shoot growth of lettuce and tomato was inhibited at the concentrations higher than 30 mg/l by vanillic acid. However, the shoot growth of barnyard grass was not affected at the concentrations used. All these results suggest that vanillic acid may play as a major component of allelopathic substance(s), which shows plant-selective activity, in the exudates of germinating watermelon seeds.     

Does endophyte symbiosis resist allelopathic effects of an invasive plant in degraded grassland?

Allelopathic effects and plant associated systemic endophytic fungi are often thought to play a role in the invasion of exotic plant species. Here, we tested the inhibitory effects of the aqueous extracts of the hemiparasitic weed Pedicularis kansuensis on seed germination and seedling growth of endophyte-free (E−) and -infected (E+) grass species, Stipa purpurea and Elymus tangutorum. The weed extracts significantly inhibited both seed germination and seedling growth of the target grass species. Extracts from the inflorescences gave greater inhibition than those from the stems or roots, while the concentration of the extract had a direct effect on the extent of inhibition. The E+ target plants were less susceptible to the extracts than their E-counterparts. Our results suggest that the allelopathic potential of P. kansuensis will lead to increased frequencies of endophyte infected plants in grass populations.     

Isolation and identification of two potential phytotoxic substances from the aquatic fern <Emphasis Type="Italic">Marsilea crenata</Emphasis>

Organic agriculture emphasized using of biologically originated herbicides and phytotoxic substances are being considered as a replacement to chemical herbicides. Marsilea crenata is an aquatic perennial fern distributed in the South-East Asian countries and is well known for various biological properties. However, to date, there has been no report that addresses the phytotoxicity of Marsilea crenata. Therefore, we explored phytotoxic properties and phytotoxic substances from Marsilea crenata. An aqueous methanol extracts of Marsilea crenata showed inhibition on the seedling growth of cress, lettuce, alfalfa, barnyard grass, Italian ryegrass, and foxtail fescue. Inhibition increased with increasing extract concentration. The extract was purified by several chromatographic steps and two phytotoxic substances were isolated and identified by spectroscopic analysis as loliolide and isololiolide. At the concentration of 30 μM, loliolide and isololiolide inhibited seedling growth of cress and barnyard grass by 41.3 to 51.1%, and 58.15 to 87.5% of control seedlings, respectively. The concentrations required for 50% inhibition of cress and barnyard grass seedlings ranged from 32.1 to 128.5 μM for loliolide, 37.0 to 176.2 μM for isololiolide. These results suggest these compounds may be responsible for phytotoxic effects of Marsilea crenata extract and could be an important part of organic agriculture.     

Inhibition of germination and α-amylase induction by 6-methoxy-2-benzoxazolinone in twelve plant species

6-Methoxy-2-benzoxazolinone (MBOA) inhibited germination of rice (Oryza sativa L.), wheat (Triticum aethiopicum Jakubz), rye (Secale cereale L.), onion (Allium cepa L.), wild oat (Avena fatua L.), barnyard grass [Echinochloa crus-galli (L.) Beauv.], ryegrass (Lolium rigidum Gaudin), cress (Lepidium sativum L.), lettuce (Lactuca sativa L.), tomato (Lycopersicum esculentum Mill.), carrot (Daucus carota L.) and amaranth (Amaranthus retroflexus L) and the inhibition increased with increasing MBOA concentrations. MBOA also inhibited the induction of α-amylase in these plant seeds and the inhibition increased with increasing MBOA concentrations. There were variations in sensitivity of these plant species to MBOA, and species of family Poaceae (barnyard grass, wild oat, rice, rye, ryegrass, and wheat) were less sensitive to MBOA than the other plant species.     

Weeds as a source of plant growth promoting rhizobacteria in agricultural soils.

The influence of plant growth promoting (PGP) activity of bacterial communities recovered from each of six weed species (barnyard grass (Echinochloa crusfalli (L.) Beauv.), corn spurrey (Spergula arvensis L.), goldenrod (Sonchus sp.), Italian ryegrass (Lolium multiflorum L.), lamb's-quarters (Chenopodium album L.), and quack grass (Agropyron repens (L.) Beauv.)) was examined in relation to the effect it had on the growth of the potato cultivar Russet Burbank. Bacterial species composition and community structure were compared, species-abundance relationships were determined, and those members conferring positive benefits for potato growth and development were identified. Of the genera identified, Bacillus, Arthrobacter, Stenotrophomonas, Acinetobacter, and Pseudomonas were the most common, and Stenotrophomonas maltophilia was the most frequent species recovered across all sources. Significantly higher population densities were found in the root zones of quack grass, compared with Italian ryegrass and lamb's-quarters. There were no significant differences in species richness among the root zones; however, evenness indices (species distribution) were significantly lower in corn spurrey (P = 0.05). Significantly higher diversity indices (Hill-1 and Hill-2 numbers) (P = 0.05) were found in the root zone soil communities of potato and goldenrod, indicating a decrease in the proportional abundance of common and very abundant species, respectively, while in barnyard grass, corn spurrey, and Italian ryegrass the reverse was the case. In both years of the study, Italian ryegrass and corn spurrey were consistently better sources of PGP rhizobacteria for potatoes, significantly (P < 0.001) increasing the mean wet weight of shoots and roots in in vitro bacterization studies. Barnyard grass was a consistently poor source of such isolates. Species-abundance measures of root zone bacterial biodiversity were not found, in this instance, to be a particularly good predictor of the presence or absence of PGP rhizobacteria. We consider that the study of complementary crops and soil-conditioning treatments should not preclude the examination of weed species as possible beneficials, as alterations in rhizobacterial biodiversity and functional versatility can influence the numbers and types of PGP bacterial strains, and consequently may serve to improve soil quality.     

Bacterial Communities Associated with <Emphasis Type="Italic">Chenopodium album</Emphasis> and <Emphasis Type="Italic">Stellaria media</Emphasis> Seeds from Arable Soils

The bacterial community compositions in Chenopodium album and Stellaria media seeds recovered from soil (soil weed seedbank), from bulk soil, and from seeds harvested from plants grown in the same soils were compared. It was hypothesized that bacterial communities in soil weed seedbanks are distinct from the ones present in bulk soils. For that purpose, bacterial polymerase chain reaction denaturing gradient gel electrophoresis (PCR–DGGE) fingerprints, made from DNA extracts of different soils and seed fractions, were analyzed by principal component analysis. Bacterial fingerprints from C. album and S. media seeds differed from each other and from soil. Further, it revealed that bacterial fingerprints from soil-recovered and plant-harvested seeds from the same species clustered together. Hence, it was concluded that microbial communities associated with seeds in soil mostly originated from the mother plant and not from soil. In addition, the results indicated that the presence of a weed seedbank in arable soils can increase soil microbial diversity. Thus, a change in species composition or size of the soil weed seedbank, for instance, as a result of a change in crop management, could affect soil microbial diversity. The consequence of increased diversity is yet unknown, but by virtue of identification of dominant bands in PCR–DGGE fingerprints as Lysobacter oryzae (among four other species), it became clear that bacteria potentially antagonizing phytopathogens dominate in C. album seeds in soil. The role of these potential antagonists on weed and crop plant growth was discussed.     

Allelopathic aquatic plants for aquatic weed management

This report presents, results of a feasibility study of use of allelopathic aquatic plants for aquatic weed management. In order to establish a list of potential allelopathic plants, we selected 16 aquatic plants native to the southeastern United States and subjected them to two bioassays — one involving lettuce seedlings and one involving the aquatic plantLemna minor as the target species. The lettuce seedling bioassay was selected because it is a widely used, experimentally simple assay to determine allelopathic activity. However, it uses lettuce, a terrestrial plant, as the target species, and thus may be less appropriate for use with aquatic plants. TheL. minor assay involves an aquatic plant as the target species and so is more appropriate for our goals, but it is experimentally much more complex and time-consuming. The plants selected for study wereBrasenia schreberi, Cabomba caroliniana, Ceratophyllum demersum, Eleocharis acicuiaris, Eleocharis obf usa, Hydrilla verticillata, Juncus repens, Limnobium spongia, Myriophyllum aquaticum, Myriophyllum spicatum, Najas guadalupensis, Nymphaea odorata, Nymphoides cordata, Potamogeton foliosus, Sparganium americanum, and Val/isneria americana.Nymphaea odorata (leaves and petioles) inhibited 78 % of lettuce seedling radicle growth and 98 % ofL. minor frond production. Brasenia schreberi inhibited 82 % of lettuce seedling radicle growth and 68 % of L. minor frond production. These results suggest thatN. odorata andB. schreberi are both highly inhibitory and are therefore candidates for use in aquatic weed management. Results also suggest that the simple lettuce seedling assay is a reasonable first “easy” one to use in an attempt to determine allelopathic potential of aquatic plants.     

Assessment of interspecific hybridization between transgenic oilseed rape and wild radish under normal agronomic conditions

In order to assess the hybridization rate between oilseed rape and wild radish under normal agronomic conditions, three 1-ha field experiments were performed. In each case, wild radish plants were transplanted at different densities in the middle, the border, or the margin of the herbicide-tolerant oilseed rape field. Among the 189084 seedlings obtained from seeds harvested on wild radish plants, only one herbicide-tolerant interspecific hybrid (RrRrAC, 2n = 37) was characterized from seeds harvested on an isolated plant growing in the margin of the field. Thus, for the wild radish total harvest, with a 95% confidence limit, the frequency of interspecific hybrids was assessed to range from 10^–7 to 3.10^–5. Interspecific hybrids were detected in all cases among the smallest seeds with a diameter less than 1.6 mm harvested on oilseed rape, but the highest frequency was obtained from oilseed rape close to wild radish plants growing as clusters in the border or the margin of the field. Most hybrids had the expected triploid genomic structure (ACRr, 2n = 28) except for four amphidiploids (AACCRrRr, 2n = 56) and one hybrid from a wild radish unreduced gamete (ACRrRr, 2n = 37). Among the 73847 seedlings observed on the oilseed rape total harvest, the frequency of interspecific hybrids was assessed to range from 2.10^–5to 5.10^–4, with a 95% confidence limit. The results are discussed with regard to the type of oilseed rape variety used and the characteristics of the interspecific hybrids. Received: 5 October 1999 / Accepted: 11 November 1999     

喜马拉雅紫茉莉根水浸提液对五种植物的化感作用研究

以采自西藏林芝地区的藏药植物喜马拉雅紫茉莉为试材,探讨其根部不同浓度(0.0125、0.025、0.05、0.1 g·mL-1)水浸提液对莴苣、绿豆、小麦、萝卜、油菜等5种植物的化感效应。结果表明,随着浸提液浓度的增加,受试植物的种子萌发及幼苗生长均受到不同程度的抑制,植株体内丙二醛含量持续升高,抗氧化酶活性及叶绿素含量逐渐下降,受试植物不同部位对浸提液敏感程度为地下部分>地上部分。另外,5种植物对浸提液敏感程度为莴苣>绿豆>小麦>萝卜>油菜。     

Isolation and identification of potent allelopathic substances in rattail fescue

Aqueous methanol extracts of rattail fescue (Vulpia myuros) inhibited the growth of roots and shoots of cress (Lepidium sativum), lettuce (Lactuca sativa), alfalfa (Medicago sativa), timothy (Phleum pratense), Digitaria sanguinalis and Lolium multiflorum. Increasing the extract concentration increased the inhibition, suggesting that rattail fescue may have growth inhibitory substances and possess allelopathic potential. The aqueous methanol extract of rattail fescue was purified and two main inhibitory substances were isolated and identified by spectral data as (−)-3-hydroxy-β-ionone and (+)-3-oxo-α-ionol. Both substances inhibited root and shoot growth of cress at concentrations greater than 0.3 μM. The concentrations required for 50% growth inhibition on root and shoot growth of cress, lettuce, alfalfa, timothy, D. sanguinalis and L. multiflorum were 2.7–19.7 μM for (−)-3-hydroxy-β-ionone, and 2.1–34.5 μM for (+)-3-oxo-α-ionol. The concentration of (−)-3-hydroxy-β-ionone and (+)-3-oxo-α-ionol, respectively, in rattail fescue was 7.8 and 3.7 μg g⁻¹ fresh weight. Considering the endogenous level and the inhibitory activity, (−)-3-hydroxy-β-ionone and (+)-3-oxo-α-ionol may work as allelopathic substances in rattail fescue through the growth inhibition of neighboring plant species.     

Growth of hybrid poplar as affected by dandelion and quackgrass competition

A pot experiment was conducted in a growth chamber to investigate the effects of dandelion (Taraxacum officinale) and quackgrass (Elymus repens) on the growth of hybrid poplar (Populus deltoides × Populus × petrowskyana var. Walker). Single hybrid poplar seedlings were grown in pots either alone (SHP) or with four or eight dandelion plants per pot or with one or three quackgrass plants per pot in two soils collected from sites previously managed for alfalfa and pasture near Meadow Lake, Saskatchewan, Canada. Hybrid poplar and weed species were harvested approximately 7 and 14 weeks after planting. Approximately 14 weeks after planting, hybrid poplar shoot biomass in the SHP treatment was 28 g for the pasture soil and 22 g for the alfalfa soil. Corresponding shoot biomass for hybrid poplar grown with the dandelion and quackgrass treatments varied from 0.54 to 0.81 g and 0.3 to 3.66 g, respectively. Other hybrid poplar growth parameters including stem height, root collar diameter and fresh root biomass were similarly reduced by competition with the weed species. During the growing period, soil solution N and K concentrations decreased several-fold in both soils; however, the magnitude of decrease was comparatively higher in the weed treatments. Nitrogen, P and K uptake by hybrid poplar was greater in the SHP treatment in both the soils (337–425, 38–49 and 396–463 mg pot⁻¹, respectively) compared to the weed treatments (4–28, 0.4–6.2 and 0.6–54.0 mg pot⁻¹, respectively) by the end of the experiment. The presence of quackgrass and dandelion severely affects the growth of hybrid poplar by causing intense below-ground competition for nutrients. Responsible Editor: Ismael Cakmak.     

Disturbance-mediated competition: the interacting roles of inundation regime and mechanical and herbicidal control in determining native and invasive plant abundance

Disturbance is a key component of many successful plant invasions. However, interactions among natural and anthropogenic disturbances and effects of these interacting disturbances on invasive plants and desired vegetation are rarely examined. We investigated the effect of anthropogenic disturbance (herbicidal and mechanical) along a natural inundation gradient (20–282 days) on the biomass and resource allocation of the invasive wetland plant, alligator weed (Alternanthera philoxeroides), and two co-occurring competitor plants, the introduced grass, kikuyu (Pennisetum clandestinum), and the native grass, couch (Cynodon dactylon), over a 2-year period. In the absence of additional disturbance, kikuyu biomass was negatively affected, alligator weed biomass was positively affected, and couch biomass was not affected by inundation disturbance. In addition, kikuyu was not affected by the selective removal of alligator weed, while couch increased in wetter habitats where kikuyu was absent due to inundation stress. This suggests that kikuyu is a superior competitor in drier habitats and inundation facilitates the invasion of alligator weed, while couch is an inferior competitor to both kikuyu and alligator weed and is therefore suppressed across its entire niche by these two introduced species. Mowing alone had no effect on the biomass of the species, suggesting the plants are equally tolerant of shoot removal. Selective herbicide reduced alligator weed biomass by 97.5% and the combination of selective herbicide and mowing reduced the biomass of alligator weed significantly more than herbicide alone, by 98.6% compared with un-manipulated controls. To predict community change and prevent sequential exotic plant invasions after weed removal, it is necessary to consider the interacting effects of disturbance and the niche space of invasive species in the local propagule pool.