Introduction
The quest for food security has never been easy, being a war against various biotic and abiotic factors acting in production fields. Evolutionarily, weeds have emerged as one of the major biotic factors posing immense threats to agricultural production, causing economic losses at the tune of US$33 billion in the United States (WSSA 2024), A$3.3 billion in Australia (Llewellyn et al. Reference Llewellyn, Ronning, Ouzman, Walker, Mayfield and Clarke2016), and US$11 billion in India (Gharde et al. Reference Gharde, Singh, Dubey and Gupta2018). Weed phenotypic plasticity coupled with adaptive trait diversity and the development of 533 unique reported cases of herbicide-resistant weeds covering 273 species, including 156 dicots and 117 monocots, in 101 crops across 72 countries have made weeds one of the greatest threats to intensive agricultural production systems globally (Heap Reference Heap2024).
Among several problematic weeds in the United States, horseweed [Erigeron canadensis L.; syn.: Conyza canadensis (L.) Cronquist (2n = 18), family: Asteraceae], also known as marestail or Canadian fleabane, is known as 1 out of 10 most troublesome and most commonly occurring weeds in 12 categories of broadleaf crops, fruits, and vegetables (WSSA 2017). Erigeron canadensis has become one of the predominant weeds in 40 crops across 70 countries (Holm et al. Reference Holm, Doll, Holm, Pancho and Herberger1997), especially under reduced tillage and no-till conditions (Steckel and Culpepper Reference Steckel and Culpepper2006). It can cause yield losses to the tune of 68% to 92% in soybean [Glycine max (L.) Merr.] and cotton (Gossypium hirsutum L.) (Byker et al. Reference Byker, Soltani, Robinson, Tardif, Lawton and Sikkema2013; Silva et al. Reference Silva, Vargas, Agostinetto and Mariani2014; Trezzi et al. Reference Trezzi, Vidal, Patel, Miotto, Debastiani, Balbinot and Mosquen2015) and 28% to 64% in sugar beet (Beta vulgaris L.) and grapes (Vitis vinifera L.) (Holm et al. Reference Holm, Doll, Holm, Pancho and Herberger1997; Shrestha et al. Reference Shrestha, Fidelibus, Alcorta and Cathline2010). Erigeron canadensis is a C3, invasive species native to North America. It exhibits high fecundity (230,000 seeds per plant) (Weaver Reference Weaver2001), efficient long-distance seed dispersal (from 500 m to 772 km from the source plant) (Dauer et al. Reference Dauer, Mortensen and Vangessel2007; Shields et al. Reference Shields, Dauer, VanGessel and Neumann2006), germination under wide range of environmental conditions (Loux et al. Reference Loux, Stachler, Johnson, Nice, Davis and Nordby2006; Waggoner et al. Reference Waggoner, Mueller, Bond and Steckel2011), luxurious growth and adaptability to harsh environments (Tozzi et al. Reference Tozzi, Beckie, Weiss, Gonzalez-Andujar, Storkey, Cici and Acker2014), and propensity for herbicide-resistance development (Heap Reference Heap2014). These adaptive traits may have played a significant role in the spread of E. canadensis to a wide range of geographic landscapes across various countries of Asia, Africa, Europe, and Oceania (Bajwa et al. Reference Bajwa, Sadia, Ali, Jabran, Peerzada and Chauhan2016; Tilley Reference Tilley2012).
Multiple reports have indicated that several invasive plants, including E. canadensis (Bhowmik and Bekech Reference Bhowmik and Bekech1993; Shaukat et al. Reference Shaukat, Munir and Siddiqui2003; Shields et al. Reference Shields, Dauer, VanGessel and Neumann2006), release numerous allelochemicals, the secondary metabolites, in leachates during the process of biomass decomposition or through volatilization from living biomass into the local ecosystem, affecting population dynamics and community composition of native plants (Callaway and Aschehoug Reference Callaway and Aschehoug2000; Dorning and Cipollini Reference Dorning and Cipollini2006; Inderjit et al. Reference Inderjit, Callaway, Pollock and Kaur2008). Although the persistence of these allelochemicals in soil may be short-lived, they can still influence plant succession depending on the continuity of supply of these compounds from live or dead plants (Rice Reference Rice1979).
Erigeron canadensis has been reported to contain various allelochemicals (Queiroz et al. Reference Queiroz, Cantrell, Duke, Wedge, Nandula, Moraes and Cerdeira2012; Shaukat et al. Reference Shaukat, Munir and Siddiqui2003). The distribution of these allelochemicals and their impact on native plant communities vary depending on geographic locations. For example, gallic acid, vanillic acid, catechol, and syringic acid were main constituents of E. canadensis collected from Karachi, Pakistan, and inhibited the growth of tomato (Solanum lycopersicum L.), radish (Raphanus sativus L.), wheat (Triticum aestivum L.), corn (Zea mays L.), millet (Pennisetum spp.), and mungbean [Vigna radiata (L.) R. Wilczek] (Shaukat et al. Reference Shaukat, Munir and Siddiqui2003). Around 42 compounds, including some phenols, ketones, and acids, were identified as allelopathic in E. canadensis collected from China (Zhang Reference Zhang2010); whereas p-coumaric, ferulic, p-hydroxybenzoic, vanillic, and syringic acids were predominant compounds in E. canadensis collected in Belgrade, Serbia, with allelopathic effects on orchardgrass (Dactylis glomerata L.) and red clover (Trifolium pratense L.) (Djurdjević et al. Reference Djurdjević, Mitrović, Gajić, Jarić, Kostić, Oberan and Pavlović2011). Interestingly, (4Z)-lachnophyllum lactone was the major allelopathic compound isolated from E. canadensis in Mississippi, USA, with phytotoxicty on creeping bentgrass (Agrostis stolonifera L.) and lettuce (Lactuca sativa L.) (Queiroz et al. Reference Queiroz, Cantrell, Duke, Wedge, Nandula, Moraes and Cerdeira2012). Shah et al. (Reference Shah, Callaway, Shah, Houseman, Pal, Xiao, Luo, Rosche, Reshi, Khasa and Chen2014) reported negative relationships between abundance of E. canadensis and richness of native species in non-native ranges of Europe, China, and India. But there existed either positive or no relationships in the native North American range which might be due to stronger allelopathic effect in non-native ranges.
Erigeron canadensis is present in 2,540 counties out of 3,244 counties in the United States, covering 78.29% of the total geographic area (Swearingen and Bargeron Reference Swearingen and Bargeron2016 ). Although previous reports indicated wide variation in the allelopathic compositions among E. canadensis populations and their effects on native or non-native plants, there is no information available on the allelopathy of native E. canadensis on the dominant southern U.S. weeds, including four native weeds (Palmer amaranth [Amaranthus palmeri S. Watson], smooth pigweed [Amaranthus hybridus L.], prickly sida [Sida spinosa L.], and pitted morningglory [Ipomoea lacunosa L.]) and three non-native weeds (lambsquarters [Chenopodium album L.], curly dock [Rumex crispus L.], and barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.]). A. palmeri, I. lacunosa, C. album, and E. crus-galli rank among the top 10 most troublesome and widespread weeds across the United States (WSSA 2017). In addition, A. hybridus, S. spinosa, and R. crispus present significant threats to agricultural lands in the U.S. Southeast, particularly in states like Alabama (Buchanan et al. Reference Buchanan, Crowley and McLaughlin1977; da Silva et al. Reference da Silva, Oliveira, Jones and Li2022; Jones and Davis Reference Jones and Davis1963). The objective of this study was to evaluate the allelopathic potential of E. canadensis on various parameters of seed germination and seedling growth of seven selected common native and non-native weed species of the southern United States. This study also aimed to identify the allelochemicals present in the E. canadensis aqueous extract and discuss its allelopathic effects on studied weed species.
Materials and Methods
Collection of Erigeron canadensis Biomass
Aboveground parts of mature but green E. canadensis plants were collected from natural areas in Auburn, AL, USA (32.6442°N, 85.52265°W). The fresh leaves were separated from the stems, followed by washing under tap water to remove adhered dirt, and then removing excess water by soaking onto tissue paper/blotting paper. The cleaned leaves were stored in a freezer maintained at −80 °C and used to prepare E. canadensis aqueous extract for different experiments within the present study.
Preparation of Aqueous Extract from Erigeron canadensis Biomass
One hundred grams of E. canadensis leaf was weighed and macerated with a porcelain mortar and pestle. The resulting paste was mixed with 400 ml of double-distilled water in a 1,000-ml Erlenmeyer flask and was agitated over an orbital shaker (Innova 4000, New Brunswick Scientific, Hauppauge, New York, USA) at 150 rpm for 48 h under 25 ± 1 °C. The primary leachate was collected by filtering the materials of the flask with a double-layer of cheesecloth, followed by centrifugation (Megafuge ST4R Plus-MD, Thermo Fisher Scientific, Langenselbold, Hesse, Germany) at 3,000 rpm for 30 min at 25 ± 1 °C. The supernatant was collected in a glass bottle, marked as “25% w/v basis” (100 g of leaves in 400 ml of water), and stored at 4 ± 1 °C for use in various experiments. Further, this primary stock solution was diluted to prepare a 10% aqueous extract with double-distilled water and used in various studies.
Seed Germination Assay
Screening studies were conducted to understand the effect of E. canadensis extracts (0% and 10%) on seed germination of seven common weed species, including broadleaves, namely, A. palmeri, A. hybridus, S. spinosa, R. crispus, I. lacunosa, and C. album, and one grass, E. crus-galli. The seeds (procured from Azlin Seed Service, Leland, MS, USA) were collected in 2022 and placed in permeable paper bags for storage under laboratory conditions at 20 ± 2 °C in the dark until commencement of the experiment. A preliminary viability test was conducted to ensure adequate seed viability before the experiment.
Twenty-five seeds in triplicates per population were placed on two layers of Whatman No.1 filter paper (supplierd by VWR International, Radnor, Pennsylvania, USA) in a series of 9-cm-diameter petri dishes. Twelve milliliters of 10% E. canadensis extract was added to each petri dish to evaluate the allelopathic effect of E. canadensis extract on various weed seeds. Preliminary studies indicated that a 12-ml volume of either water or E. canadensis extract was sufficient for conducting 21-d germination studies under present incubation conditions of light/dark (12/12-h, 25/18 °C) at 60% relative humidity. Water levels (for control) or E. canadensis extract levels (for treatments) were regularly monitored throughout the 21-d experiment, and only double-distilled water was added to petri dishes as needed to maintain the initial solution levels for both control and E. canadensis treatments, compensating for evaporation and concentration changes in the solutions. Petri dishes with control and E. canadensis treatment were incubated for 21 d under a controlled environment of light/dark (12/12-h, 25/18 °C) and relative humidity of 60% by randomly distributing the petri dishes between the shelves of a growth chamber (ISTA 2024). Seed germination was recorded every 2 d up to 10 d and thereafter at 14 and 21 d (Maity et al. Reference Maity, Paul, Rocha, Bagavathiannan, Beckie and Ashworth2025). All experiments were repeated three times under the same experimental conditions. At the conclusion of the germination test, the viability of nongerminated seeds that appeared intact was assessed by gently tapping the seeds with forceps to check for the presence of a turgid embryo. The seeds that exhibited blackened, decayed tissues or were empty were classified as dead. The healthy nongerminated seeds were longitudinally dissected and immersed in a 1% solution of 2,3,5-triphenyl tetrazolium chloride for 24 h at 25 ± 1 °C. Seeds with red-stained embryos were considered viable. All viable but nongerminated seeds were categorized as dormant (Supplementary Figure 1). Germination-associated parameters, such as gemination percentage (G%), inhibited germination (IG%), relative inhibited germination (RIG%), speed of germination (SG), and mean germination time (MGT) were calculated using the following equations.
$${\rm{G}}\% = \left( {{\rm{Number \,\,of \,\,normal \,\,seedling/number \,\,of \,\,seeds}}} \right) \times 100\;$$
$${\rm{IG}}\% = 100 - {\rm{G}}\% $$
$$\begin{align}{\rm{RIG}}\% = &[({\rm{IG}}\% \,\,{\rm{ at \,\,treatment}} - {\rm{IG}}\% \,\,{\rm{ at \,\,control}})/(100 \\&- {\rm{IG}}\% \,\,{\rm{ at \,\,control}})] \times 100\end{align}$$
$${\rm{SG}} = {n_1}/{d_1} + {n_2}/{d_2} + {n_3}/{d_3} + \ldots \;\;\;$$
$$\begin{align}{\rm{MGT}} = &({n_1} \times {d_1} + {n_2} \times {d_2} + {n_3} \times {d_3} \\&+ \ldots )/{\rm{total \,\,number \,\,of \,\,days}}\end{align}$$
where n represents the number of germinated seeds on the dth day.
At the end of the germination test (21 d), seedling shoot and root lengths were measured, which served as an indicator of seed vigor. The allelopathic effects of E. canadensis extracts were measured by calculating the allelopathic response index (RI) as described by Williamson and Richardson (Reference Williamson and Richardson1988).
$${\rm{RI}} = 1 - \left( {{\rm{C}}/{\rm{T}}} \right) \left( {{\rm{T}} \gt {\rm{C}}} \right){\rm{ \;or\; RI}} = \left( {{\rm{T/C}}} \right) - 1\left( {{\rm{T}} \lt {\rm{C}}} \right)$$
where C and T represent the corresponding index values for control and treatment. The RI ranges from +1 to −1. If RI > 0, it indicated there was a promoting effect, otherwise RI < 0 indicated an inhibiting effect, and the absolute value of RI depicted the strength of the allelopathy. The synthetical allelopathic effects (SE) were assessed based on the average RI value of five parameters including gemination percentage (G%), speed of germination (SG), mean germination time (MGT), shoot height (S), and root length (R) (Dai et al. Reference Dai, Wu, Zhou, Jian, Meng and Xu2022; Wang et al. Reference Wang, Liu, Wang, Pang, Zhang, Wen, Zhao, Sun, Wang and Yang2022). SE was calculated using the following equation:
$${\rm{SE}} = \left[ {{\rm{RI}}\left( {{{\rm{P}}_1}} \right) + {\rm{RI}}\left( {{{\rm{P}}_2}} \right) + {\rm{RI}}\left( {{{\rm{P}}_3}} \right) + \ldots {\rm{ RI}}\left( {{{\rm{P}}_{n}}} \right)} \right]/n$$
where RI(P1), RI(P2), RI(P3), and RI(P n ) represent response index (RI) values until the nth parameter (P). The SE ranges from +1 to −1, with positive values indicating treatment-induced stimulation and negative values indicating inhibition relative to the controls. SE values close to zero indicate no or low effect of the treatment. All measurements were taken from the same receptor seeds subjected to the same treatment.
Identification of Compounds in Erigeron canadensis Extract with Reverse-Phase Liquid Chromatography–Mass Spectrometry (LC–MS)
For reverse-phase analysis, 100 µl of sample was mixed with 500 µl ice-cold methanol with 15 min of freezing time followed by centrifugation at 16,000 × g (g denotes the relative centrifugal force or g-force) for 5 min to precipitate protein. The supernatant was concentrated on a Thermo Savant DNA 120 vacuum centrifuge at 300 × g on medium heat for 2 h. The sample was re-dissolved with 100 µl of water and analyzed. Analysis was performed on a Vanquish UHPLC system (Thermo Fisher Scientific, Waltham, Massachusetts, USA) coupled with a quadrupole orbitrap mass spectrometer (Orbitrap Exploris 120, Thermo Fisher Scientific, Waltham, Massachusetts, USA) with electrospray ionization (H-ESI) switching between positive or negative modes using Xcalibur software (v. 4.4.16.14). Injection of 10 µl of the sample was made on a C18 column (Accucore RP-MS 100 × 2.1 mm with 2.6-µm particles; Thermo) held at 40 °C with a 200 μl min−1 flow rate of mobile phase solution A (99.9% water with 0.1% formic acid) and solution B (100% acetonitrile). The gradient began at 0% B, was held for 2 min, followed by a linear ramp to 95% B in 11 min, was held at 95% B for 1 min, and decreased to 0%B in 1 min, then was held for 5 min for a total analysis time of 20 min. The flow was diverted to waste for the first minute and a half of analysis and after 15 min.
For mass spectrometry, the scan range was 50 to 500 m/z with resolution of 120,000, 70% radio frequency (RF) lens, maximum injection time auto, with EASY-IC run-start on. The spray voltage was 3,300 V in positive and 2,100 V in negative mode, the ion transfer tube temperature was 320 °C, and the vaporizer temperature was 275 °C. Data-dependent acquisition on singly charged precursors only was used with dynamic exclusion on auto, with an intensity threshold of 50,000, the window was 2 Da, the higher-energy collisional dissociation (HCD) was set to 40% normalized, the tandem mass spectrometry (MS/MS) resolution was 15,000, and the automatic gain control (AGC) was set to standard for the four dependent scans. A targeted mass exclusion list was created based on a blank injection and apex detection was set to 30%.
The LC-MS results were used in Compound Discoverer v. 3.2 to align retention times; detect compounds; merge features; group compounds; search mzCloud; search ChemSpider with BioCyc, ChEBI, and ChEMBL databases with tolerance of 5 ppm; search mass lists, including the Arita Lab Flavinoid Structure Database, EFS HRAM compound Database, and the Endogenous Metabolites database; and predict compositions automatically. The LC-MS analysis indicated the presence of several compounds in the E. canadensis extract. The “top 38” compounds were selected based on the relative abundance or % area contribution of a particular compound in the total ion chromatogram (TIC) of the LC-MS analysis. It was determined as follows:
$$\begin{align}\%\,{\rm{Area}} = &\left( \matrix{{\rm{Area \,under \,the \,peak \,of \,a\, particular \,compound/}} \hfill \cr {\rm{total\,area\,of\,all\,peaks\,in\,the\,total\,ion \,chromatogram}} \hfill \cr} \right) \\&\times 100\end{align}$$
Data Analysis
For all germination and seedling growth data, deviations from normality and the homogeneity of the variances were evaluated in RStudio (v. 3.0.1; Ritz et al. Reference Ritz, Baty, Streibig and Gerhard2015) by using a Shapiro-Wilk test and Bartlett’s test, respectively. Differences in the values of various parameters of seed germination and seedling growth for all studied weed species were measured using a one-way ANOVA with Student’s t-test JMP PRO v. 18 (1989–2023, SAS Institute, Cary, NC, USA). Means were separated using Tukey’s honest significant difference (HSD) for comparing among treatments, whereas Student’s t-test was used to compare between two treatments at a significance level of α = 0.05. Data presented in this paper indicated mean value ± SE of various parameters for different weed species. For developing the regression curves for germination percentage recorded over the entire period of germination test, a third-order polynomial function was used based on the best R2 value using the LINEST function in Microsoft® Excel (Redmond, WA, USA).
Results and Discussion
Effect of Erigeron canadensis Aqueous Extract on Seed Germination of Weed Species
The effect of E. canadensis aqueous extract (10%) on studied weed species varied significantly (Table 1). At the end of a 21-d germination study, it was observed that application of E. canadensis aqueous extract resulted in 33.33%, 41.03%, 60.71%, and 65.22% decrease in germination for S. spinosa, A. palmeri, A. hybridus, and R. crispus, respectively, with respect to the control treatments (P < 0.05) (Figure 1). SG values decreased by 39.35%, 48.39%, 64.78%, and 82.01% for S. spinosa, A. palmeri, R. crispus, and A. hybridus, respectively, in response to E. canadensis aqueous extract treatment over the control; whereas MGT values were reduced by 25.65%, 41.44%, 65.26% and 69.58% for S. spinosa, A. palmeri, R. crispus, and A. hybridus, respectively, under E. canadensis aqueous extract treatment over the control. However, there was no significant difference (P < 0.05) in seed germination of C. album, I. lacunosa, and E. crus-galli with E. canadensis aqueous extract over the control (Figure 2), and SG and MGT values remained lower (<4.04%) or not affected with E. canadensis aqueous extract.
Table 1. Effect of aqueous extract of Erigeron canadensis (HW) on the germination and growth parameters of various weeds a
a At the end of a 21-d germination test. G(%), % germination; RI(G%), response index for G(%); SG, speed of germination; RI(SG), response index for SG; MGT, mean germination time; RI(MGT), response index for MG. Different lowercase letters following mean values within a species in columns G (%), SG, and MGT indicate significant differences between the treatments within a species at P < 0.05. Different uppercase letters following mean values in columns RI(G), RI(SG), and RI(MGT) indicate significant differences in the respective parameter among the species at P < 0.05.
Figure 1. Seed germination of (A) Sida spinosa (PS), (B) Rumex crispus (CD), (C) Amaranthus palmeri (PA), and (D) Amaranthus hybridus (SPW), in response to 10% aqueous extracts of Erigeron canadensis (HW) at the end of a 21-d germination test. In panels, different letters on the data points indicate significant difference (P < 0.05) among observation timings in response to a specific treatment (0% or control in brown and 10% HW in other colors). Asterisks (*) indicate significant difference (P < 0.05) between two treatments for a given day. Third-order regression curves were based on the best R2 value using the LINEST function in Microsoft® Excel.
Figure 2. Seed germination patterns of (A) Chenopodium album (LQ), (B) Ipomoea lacunosa (PMG), and (C) Echinochloa crus-galli (BYG) in response to 10% aqueous extracts of Erigeron canadensis (HW) at the end of a 21-d germination test. In panels, different letters on the data points indicate significant difference (P < 0.05) among observation timings in response to a specific treatment (0% or control in brown and 10% HW in other colors). Asterisks (*) indicate significant difference (P < 0.05) between two treatments for a given day. Third-order regression curves were based on the best R2 value using the LINEST function in Microsoft® Excel.
Hu and Zhang (Reference Hu and Zhang2013) reported 79.8%, 85.5%, and 93.8% reduction in germination of broadleaf plantain [Plantago major L.; syn.: Plantago asiatica L.], large crabgrass [Digitaria sanguinalis (L.) Scop.], and Oriental false hawksbeard [Youngia japonica (L.) DC.] in response to 20% aqueous extract of aboveground parts of E. canadensis without any autotoxicity in southern China. Other studies also reported negative impacts of E. canadensis extract alone (Wang et al. Reference Wang, Jiang, Zhou and Liu2017) or in combination of other plants like Canada goldenrod [Solidago canadensis L.] (Wei et al. Reference Wei, Wang, Wu, Cheng and Wang2020) on germination and growth of allelochemical-sensitive lettuce [L. sativa] in China. Shah et al. (Reference Shah, Callaway, Shah, Houseman, Pal, Xiao, Luo, Rosche, Reshi, Khasa and Chen2014) reported mostly negative relationships between abundance of E. canadensis and richness of native species in non-native ranges of Europe (Queen Anne’s lace [Daucus carota L.], heath false brome [Brachypodium pinnatum (L.) P. Beauv.], orchardgrass [D. glomerata], purple moor-grass [Molina caerulea (L.) Moench], and codlins-and-cream [Epilobium hirsutum L.]), China (dwarf daylily [Hemerocallis minor L.], Chinese motherwort [Leonurus tataricus L.], tidal marsh flat sedge [Cyperus serotinus Rottb.], yellow avens [Geum aleppicum Jacq.], redroot pigweed [A. retroflexus], and patience dock [Rumex patientia L.], and India (Himalayan rhubarb [Rheum australe D. Don], prickly lettuce [Lectuca serriola L.], and nodding Carpesium [Carpesium cernuum L.]). But they observed either positive or no relationships in the native North American range (large-leaved avens [Geum macrophyllum Willd.], Great Basin wildrye [Elymus cinereus Scribn. & Merr.], slender cinquefoil [Potentilla gracilis Douglas ex Hook.], and nettleleaf giant hyssop [Agastache urticifolia (Benth.) Kuntze]), suggesting the presence of biogeographic differences. Differential inhibition of weed germination in the current study resonates this biogeographic difference. Wang et al. (Reference Wang, Jiang, Zhou and Liu2017) reported higher inhibitory effect of E. canadensis on lettuce [L. sativa] at higher latitudes in Shenyang, China (41.82°N, 123.46°E) due to production of more allelochemicals in a cold temperate climate compared with lower latitudes in Zhenjiang, China (32.20°N, 119.51°E) with a subtropical monsoon climate. Production of higher concentrations of allelochemicals in E. canadensis collected from higher-latitude areas in Jiangsu (31°57′N to 32.15°N, 118°54′E to 120°53′E) compared with lower-latitude areas in Hubei (30°21′N to 30°6′N, 114°21′E to 115°21′E) and Anhui (30°37′N to 31°22′N, 117°32′E to 118°23′E) in China was also reported by Cheng et al. (Reference Cheng, Wu, Yu, Wang, Wei, Wang and Du2021). Similar reports of production of higher concentrations of allelochemicals at higher latitudes were also made earlier for other plants like goat weed [Ageratum conyzoides L.] (Hu and Kong Reference Hu and Kong2002) and western waterweed [Elodea nuttallii (Planch.) H. St. John] (Erhard and Gross Reference Erhard and Gross2005).
Effect of Erigeron canadensis Aqueous Extract on Seedling Growth
The effect of E. canadensis aqueous extract on root and shoot lengths of seedlings varied across the studied weed species (Table 2; Figure 3). The root lengths decreased significantly (P < 0.05) at 34.6%, 48.77%, and 68.35% for C. album, A. hybridus, and A. palmeri, respectively, over the control treatments, but no significant reduction in root lengths was observed in S. spinosa, R. crispus, E. crus-galli, and I. lacunosa. A similar trend was observed with a significant reduction (P < 0.05) in shoot length at 29.57%, 35.90%, and 37.50% for A. hybridus, A. palmeri, and C. album, respectively, in E. canadensis aqueous extract treatments over the respective control, while other weed seedlings’ shoot length remained less or not affected. Shaukat et al. (Reference Shaukat, Munir and Siddiqui2003) reported significant reduction of root and shoot lengths of tomato [S. lycopersicum], radish [R. sativus], wheat [T. aestivum], corn [Z. mays], millet [Pennisetum spp.], and mungbean [V. radiata] with aqueous extract of E. canadensis in Karachi, Pakistan. Djurdjević et al. (Reference Djurdjević, Mitrović, Gajić, Jarić, Kostić, Oberan and Pavlović2011) reported 68.06% to 77.53% and 72.95% to 95.37% reduction in root and shoot lengths of orchardgrass [D. glomerata] and white clover [Trifolium repens L.], respectively, under 25% aqueous extract of areal parts of E. canadensis in Belgrade, Serbia. The allelopathic chemicals released by plants can influence the physiological processes of neighboring plants in several ways (Dadkhah Reference Dadkhah2015; Macías et al. Reference Macías, Mejías and Molinillo2019; Rice Reference Rice1984), with growth retardation being the most observed response (Cordeau et al. Reference Cordeau, Triolet, Wayman, Steinberg and Guillemin2016; Rice Reference Rice1974, Reference Rice1979; Singh et al. Reference Singh, Segbefia, Fuller, Shankle, Morris, Meyers and Tseng2022).
Table 2. Effect of aqueous extract of Erigeron canadensis on the seedling growth parameters of various weeds a
a At the end of a 21-d germination test. R, root length (mm); RI(R), response index for R; S, shoot length (mm); RI(S), response index for S. Different lowercase letters following mean values within a species in columns R (mm), and S (mm) indicate significant differences between the treatments within a species at P < 0.05. Different uppercase letters following values in columns RI(R), and RI(S) indicate significant differences in respective parameter among the species at P < 0.05.
Figure 3. Seedling growth of (A) Sida spinosa, (B) Rumex crispus, (C) Amaranthus palmeri, (D) Amaranthus hybridus, (E) Chenopodium album, (F) Ipomoea lacunosa, and (G) Echinochloa crus-galli in response to 10% aqueous extracts of Erigeron canadensis (HW) at the end of a 21-d germination test.
Allelopathy of Erigeron canadensis Extract
The allelopathic potential of E. canadensis aqueous extract was evaluated based on various seed germination parameters (G%, SG, and MGT) and seedling growth measures (R and S) of studied weed species. The RI was calculated for each parameter (Dai et al. Reference Dai, Wu, Zhou, Jian, Meng and Xu2022; Williamson and Richardson Reference Williamson and Richardson1988) (Tables 1 and 2). Generally, RI ranges from −1 to +1, where positive values indicate stimulation by the treatments, and negative values signify inhibition in comparison with the controls. The absolute value indicates the strength of allelopathic effect. RI close to zero indicates no or low effect from the treatment. The order for germination inhibition [RI(G)] with E. canadensis extract was R. crispus (−0.655) ≥ A. hybridus (−0.606) > A. palmeri (−0.406) > S. spinosa (−0.223); whereas low/no inhibition of germination was noted for C. album (−0.094), I. lacunosa (−0.046), and E. crus-galli (0.083) with RI(G) values close to zero. The order for inhibition of speed of germination [RI(SG)] was A. hybridus (−0.817) > R. crispus (−0.65) > A. palmeri (−0.481) ≥ S. spinosa (−0.394), with low/no effect on I. lacunosa (−0.002), E. crus-galli (−0.046), and C. album (0.127). The order for inhibition of mean germination time [RI(MGT)] was A. hybridus (−0.696) ≥ R. crispus (−0.655) > A. palmeri (−0.412) > S. spinosa (−0.258), with low/no effect on I. lacunosa (−0.039), E. crus-galli (0.015), and C. album (0.039). The order for inhibition of root [RI(R)] was A. hybridus (−0.49) > A. palmeri (−0.403) > C. album (−0.345) > R. crispus (−0.167), with no effect on S. spinosa (−0.090), I. lacunosa (−0.021), and E. crus-galli (−0.012). The order for inhibition of shoot [RI(S)] was C. album (−0.364) > A. palmeri (−0.344) > A. hybridus (−0.292) > R. crispus (−0.194) ≥ S. spinosa (−0.192), with low/no effect on I. lacunosa (−0.066) and E. crus-galli (0.002). The study indicated that the allelopathic effect of E. canadensis extract impacted different parameters of seed germination and seedling growth of studied weed species. Overall germination parameters like G%, SG, and MGT were inhibited with 10% E. canadensis extract in A. hybridus, R. crispus, A. palmeri, and S. spinosa, while other weeds like C. album, I. lacunosa, and E. crus-galli remained low/not affected. Weed species like A. hybridus and A. palmeri that showed inhibition of various germination parameters with E. canadensis extract also showed inhibition of seedling growth parameters. Although E. canadensis extract produced low/no inhibition of germination parameters (as indicated by low RI values, ranging from −0.094 to 0.127) for C. album, seedling root and shoot lengths were inhibited (RI values ranged from −0.364 to −0.345). Erigeron canadensis extract had no inhibitory effects on seed germination and seedling growth of I. lacunosa and E. crus-galli. Similar reports on differential RI values for various germination and growth parameters of sensitive plants were also reported for extracts of wolf poison [Stellera chamaejasme L.] (Liu et al. Reference Liu, Meng, Dang, Song and Zhai2019), coastal plain yellowtops [Flaveria bidentis (L.) Kuntze] (Dai et al. Reference Dai, Wu, Zhou, Jian, Meng and Xu2022), and barrelclover [Medicago truncatula Gaertn.] (Wang et al. Reference Wang, Liu, Wang, Pang, Zhang, Wen, Zhao, Sun, Wang and Yang2022).
Figure 4 presents the synthetical allelopathic effect (SE) of E. canadensis aqueous extract on the weed species studied. The key distinction between RI and SE is that the RI value represents the effect of a treatment on a specific parameter (either inhibition or stimulation), whereas the SE value reflects the overall inhibitory or stimulatory effect of the treatment in comparison to the control. Therefore, SE value represents the ultimate impact of treatment. All RI values of five parameters (%G, SG, MGT, S, and R) were negative, with higher absolute values for A. hybridus, S. spinosa, A. palmeri, and R. crispus (Tables 1 and 2), which resulted in higher negative SE values for A. hybridus (−0.580 ± 0.023), R. crispus (−0.464 ± 0.013), A. palmeri (−0.409 ± 0.006) and S. spinosa (−0.248 ± 0.066). In C. album, RI values for RI(R) and RI(S) were moderately negative, with other parameters (G, SG, MGT) close to zero, resulting in a low SE of −0.143 (±0.088) compared with A. hybridus, S. spinosa, A. palmeri, and R. crispus. For I. lacunosa, treatment with E. canadensis extract led to numerically smaller, but statistically nonsignificant at P < 0.05, values for all parameters studied. Interestingly, E. canadensis treatment in E. crus-galli resulted in statistically nonsignificant at P < 0.05, but numerically higher values for three parameters (G%, MTG, and S) and lower values for two parameters (R and S). This resulted in variations in the SE values for E. crus-galli in the range of −0.046 to 0.0782, with a positive average of 0.009 (±0.036), which was close to zero. Hence, in terms of overall allelopathic impact, 10% E. canadensis extract showed high inhibitory effects (higher negative SE values) on A. hybridus and R. crispus, while it did not show any inhibitory effect on I. lacunosa and E. crus-galli (low SE values).
Figure 4. The synthetical allelopathic effects (SE) of aqueous extract of Erigeron canadensis (HW) on different weed species: Amaranthus hybridus (SPW), Chenopodium album (LQ), Sida spinosa (PS), Amaranthus palmeri (PA), Rumex crispus (CD), Echinochloa crus-galli (BYG), and Ipomoea lacunosa (PMG). Different letters below bars indicate significant difference (P < 0.05) in SE values among weed species treated with 10% HW aqueous extract.
Variations in the inhibition of seed germination and seedling growth with 12.5% extract of wolf poison [S. chamaejasme] were reported on alfalfa (Medicago sativa L.) (SE = −0.35), Dahurian wildrye [Elymus dahuricusTurcz. ex Griseb.] (SE = −0.42), and crested wheatgrass [Agropyron cristatum (L.) Gaertn.] (SE = −0.24) (Liu et al. Reference Liu, Meng, Dang, Song and Zhai2019). Dai et al. (Reference Dai, Wu, Zhou, Jian, Meng and Xu2022) reported variations in SE values for field mustard (Brassica rapa L.) (SE= −0.70), wheat [T. aestivum] (SE = −0.40), and barnyardgrass [E. crus-galli] (SE = −0.65) with 5% aqueous extract of coastal plain yellowtops [F. bidentis]. Erigeron canadensis shoot extracts and volatile compounds extracted from the inflorescence of the invasive flaxleaf fleabane [Erigeron sumatrensis (Retz.) L.; syn.: Conyza albidaL.], a sister species of horseweed, in Athens, Greece, were reported to significantly inhibit the germination and growth of several crop species (Economou et al. Reference Economou, Tzakou, Gani, Yannitsaros and Bilalis2002; Shaukat et al. Reference Shaukat, Munir and Siddiqui2003). Similar allelopathic effects were reported for water extracts of coastal plain yellowtops [F. bidentis] (Dai et al. Reference Dai, Wu, Zhou, Jian, Meng and Xu2022), leaf extracts of Scots pine [Pinus sylvestris L.] and paper mulberry [Broussonetia papyrifera L.] (Wang et al. Reference Wang, Zhang, Wang, Di, Wang and Sikdar2021) and root exudates of alfalfa [M. sativa] (Wang et al. Reference Wang, Liu, Wang, Pang, Zhang, Wen, Zhao, Sun, Wang and Yang2022) on other native plants.
Shajib et al. (Reference Shajib, Pedersen, Mortensen, Kudsk and Fomsgaard2012) reported differential uptake and transformation of biochanin A, a major allelochemical present in red clover [Trifolium pratense L.] and white clover [T. repens], in two broadleaf weeds, namely, dove’s-foot crane’s-bill [Geranium molle L.] and night-flowering catchfly [Silene noctiflora L.], and a grass weed, barnyardgrass [E. crus-galli]. This study reported resistance of E. crus-galli to biochanin A due to lack of uptake, while comparatively higher uptakes were observed in broadleaf weeds studied. Within the category of broadleaf weeds, although the uptake of biochanin A was higher in G. molle compared with S. noctiflora, biotransformation of toxic biochanin A to nontoxic compounds was greater in G. molle, making it less susceptible to biochanin A compared with S. noctiflora. In the present study, the reason for differential effects of E. canadensis aqueous extract on various weed species could be due to differential uptake and transformation of allelochemicals by weed species. This might have resulted in higher susceptibility of A. hybridus, R. crispus, and A. palmeri toward E. canadensis aqueous extract, while E. crus-galli and I. lacunosa showed no effect for E. canadensis treatment.
Identification of Allelopathic Compounds in Erigeron canadensis Aqueous Extract
The present study indicated allelopathic effects of E. canadensis aqueous extract on the native weed species of the southern United States, although the allelopathic effect varied among weed species. Allelopathy is a process whereby plants release phytotoxins into the environment to gain a competitive edge, significantly influencing plant dominance and succession within communities (Lambers et al. Reference Lambers, Chapin and Pons1998). Allelopathic compounds with phytotoxic or fitness-reducing effects may be novel to plant competitors lacking a coevolutionary history, making them more susceptible to these chemicals in the native range (Callaway and Aschehoug Reference Callaway and Aschehoug2000; Callaway and Ridenour Reference Callaway and Ridenour2004; Inderjit et al. Reference Inderjit, Callaway, Pollock and Kaur2008).
The LC-MS analysis indicated the presence of numerous compounds in the aqueous extract of E. canadensis aerial biomass, and the top 38 compounds are presented in Table 3. It was found that the aqueous extract of E. canadensis contained some natural compounds with previously known allelopathic potential. Based on the % area of a peak in the TIC of LC-MS analysis, the primary allelopathic compound was piperidine (5.322%).
Table 3. Liquid chromatography–mass spectrometry (LC–MS) analysis of Erigeron canadensis aqeous extract (10%) a
a Sl. no., serial number; RT (min), retention time (in minutes) of the coumpound in the total ion chromatocgram of LC-MS analysis; % area, area-wise contribution of a particular compound in the total ion chromatogram (TIC) of LC-MS analysis.
The aqueous extract of castor bean [Ricinus communis L.] containing gallic acid along with other phenolic compounds was noted to inhibit germination and growth of Spanish needles [Bidens bipinnata L.] (Lopes et al. Reference Lopes, Morais, Lacerda and Araújo2022). The aqueous extract of dragon spurge [Euphorbia dracunculoides L.], which contained 2-furoic acid, was allelopathic on germination and seedling growth of chickpea [Cicer arietinum L.] and wheat [T. aestivum] (Tanveer et al. Reference Tanveer, Jabbar, Kahliq, Matloob, Abbas and Javaid2012). Other phenolic acids like genistein present in various plants (Shajib et al. Reference Shajib, Pedersen, Mortensen, Kudsk and Fomsgaard2012) and gentisic acid present in buffalograss [Buchloe dactyloides (Nutt.) J.T. Columbus] (Wu et al. Reference Wu, Guo and Harivandi1998) were reported to have growth inhibitory activities on E. crus-galli and annual bluegrass (Poa annua L.), respectively. The phytotoxicity of E. canadensis aqueous extract is probably not due to the presence of any one compound; rather it could be due to the combined action of all compounds present in the aqueous extract. Similar postulations regarding the allelopathic effects of the aqueous extracts from E. dracunculoides on chickpea [C. arietinum] and wheat [T. aestivum] (Tanveer et al. Reference Tanveer, Jabbar, Kahliq, Matloob, Abbas and Javaid2012), as well as aqueous extracts from Common gorse [Ulex europaeus L.] and Scotch broom [Cytisus scoparius (L.) Link] on A. retroflexus and D. sanguinalis (Pardo-Muras et al. Reference Pardo-Muras, Puig, Souto and Pedrol2020) have also been reported.
The present study indicated that the mechanism behind the inhibiton of germination and seedling growth of selected weeds species by E. canadensis aqueous extract could be a combination of several mechanisms associated with compounds present in the extract (Blum et al. Reference Blum, Shafer and Lehman1999; Reigosa et al. Reference Reigosa, Sanchez-Moreiras and Gonzalez1999). For example, the aqueous extract of E. canadensis aerial parts collected in Karachi, Pakistan, contained gallic acid, vanillic acid, catechol, and syringic acid and exhibited allelopathic effects on crops such as tomato [S. lycopersicum], radish [R. sativus], wheat [T. aestivum], corn [Z. mays], millet [Pennisetum spp.], and mungbean [V. radiata] (Shaukat et al. Reference Shaukat, Munir and Siddiqui2003). The aqueous extract of E. canadensis aerial parts, collected in Belgrade, Serbia, contained p-coumaric, ferulic, p-hydroxybenzoic, vanillic and syringic acids and demonstrated allelopathic effects on D. glomerata and T. repens (Djurdjević et al. Reference Djurdjević, Mitrović, Gajić, Jarić, Kostić, Oberan and Pavlović2011). The dichloromethane extract of E. canadensis aerial parts, collected in Mississippi, USA, contained (4Z)-lachnophyllum lactone, which inhibited growth of creeping bentgrass [A. stolonifera] and lettuce [L. sativa] (Queiroz et al. Reference Queiroz, Cantrell, Duke, Wedge, Nandula, Moraes and Cerdeira2012).
Earlier reports showed that the allelopathic potential and nature of allelopathic compounds in E. canadensis biomass varied with geographic location. This could be the reason for the negative relationships between abundance of E. canadensis and richness of native species in non-native ranges of Europe, China, and India, but either positive or no relationships in native North American range (Shah et al. Reference Shah, Callaway, Shah, Houseman, Pal, Xiao, Luo, Rosche, Reshi, Khasa and Chen2014). However, the present study did not show any trend in respect to inhibition of seed germination and seedling growth of native (A. palmeri, A. hybridus, S. spinosa, and I. lacunosa) and non-native weed species (C. album, R. crispus, and E. crus-galli) by the native E. canadensis from the southern United States. Accumulation of allelochemicals in soil needs continuous supply due to the shorter life span of these compounds; however, gradual release from decomposing biomass could cause inhibition of various physiological processes, including reduced germination, poor seedling growth, low photosynthetic efficiency, and decreased water and nutrient uptake in receptor plants, leading to a decrease in their abundance or complete elimination (Cheng et al. Reference Cheng, Wu, Yu, Wang, Wei, Wang and Du2021; Djurdjević et al. Reference Djurdjević, Popović, Mitrović, Pavlović, Jarić, Oberan and Gajić2008, Reference Djurdjević, Mitrović, Gajić, Jarić, Kostić, Oberan and Pavlović2011; Rice Reference Rice1974, Reference Rice1979).
A recent study by Cheng et al. (Reference Cheng, Wu, Yu, Wang, Wei, Wang and Du2021) examined the allelopathic effects of E. canadensis with varying levels of invasion in Jiangsu, Hubei, and Anhui provinces along the Yangtze River in China. The results indicated that E. canadensis populations from higher-latitude regions in Jiangsu produced higher concentrations of allelochemicals that had a stronger inhibitory effect on the growth of native lettuces compared with those from lower-latitude areas. Erigeron canadensis’s ability to survive in extreme environmental conditions (Waggoner et al. Reference Waggoner, Mueller, Bond and Steckel2011), along with its dense foliage production (Tozzi et al. Reference Tozzi, Beckie, Weiss, Gonzalez-Andujar, Storkey, Cici and Acker2014), contributes to its ecological resilience. Additionally, the widespread distribution of E. canadensis across North America, along with its allelopathic impacts on both native and non-native plant species, poses a growing ecological concern. This is especially significant given the rise in herbicide resistance (Heap Reference Heap2024), which complicates weed management. This research provides a foundation for further exploring the allelopathic impacts of both herbicide-resistant and herbicide-susceptible E. canadensis populations on various plant species and for understanding their role in the ecological process of invasion.
In conclusion, the present study indicated differential allelopathic effects of E. canadensis aqueous extract on the seven weed species studied. The E. canadensis aqueous extract showed the highest inhibitory effects on seed germination and seedling growth of A. hybridus, followed by R. crispus, A. palmeri, and S. spinosa, without affecting I. lacunosa, C. album, or E. crus-galli. The E. canadensis aqueous extract contained several allelopathic compounds, including piperidine, choline, 4-hydroxybenzaldehyde, acetonecyanohydrin, gallic acid, 2-furoic acid, genistein, and gentisic acid. The observed variations in inhibition could be due to differential uptake and transformation of allelochemicals by weed species. The results from this preliminary laboratory study provide a robust foundation for elucidating the mechanisms driving the successful invasion of E. canadensis and offer a valuable theoretical framework for assessments of ecological risks.
Supplementary material
To view supplementary material for this article, please visit https://doi.org/10.1017/wsc.2025.10034
Acknowledgments
We are grateful to Melissa Boersma, director, Mass Spectrometry Lab, Department of Chemistry & Biochemistry, Auburn University, Auburn, AL 36849, for chromatographic analysis.
Funding statement
Startup funds provided to AM by Auburn University are gratefully acknowledged.
Competing interests
The authors declare that they have no conflicts of interest.
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