Allelopathic Impact of Erigeron canadensis and Erigeron annuus on Major Crop Species

Abstract

This study investigates the allelopathic potential of two invasive plants from the Asteraceae family, Erigeron canadensis L. and Erigeron annuus (L.) Desf., which are prevalent in Heilongjiang Province, China. We systematically examined the effects of water extracts from these plants at various concentrations (25, 50, 75, and 100 g·L⁻¹) on the germination and seedling growth of three major food crops: wheat (Triticum aestivum L.), rice (Oryza sativa L.), and corn (Zea mays L.). Using the Petri dish method and two-way ANOVA with SPSS27 software, we assessed the interaction effects of species and concentration on these crops. The results revealed differential chemosensory effects between E. canadensis and E. annuus extracts. Specifically, the aqueous extract of E. canadensis at 25 g·L⁻¹ promoted wheat root length, while all other growth indicators showed inhibitory effects. The inhibitory effects on wheat, rice, and maize increased with the concentration of the leaching solution. At 100 g·L⁻¹, E. annuus extract completely inhibited the germination of wheat and rice, with an integrated sensitization effect index of −1. The inhibitory effects of the extracts on seed growth indices were in the order of shoot length > root length > biomass. Wheat was the most affected among the three crops, followed by rice, and maize was the least affected. The allelopathic potential of E. annuus was more substantial than that of E. canadensis.

1. Introduction

With the deepening of globalization, the problem of invasive alien plants is becoming increasingly prominent. China has become one of the countries most seriously affected by the invasion of alien organisms in the world [1,2]. The majority of the invasive plant species produce allelochemicals with the potential to affect native plant performance negatively [3]. Invasive plants excel in their new ranges because they make new metabolites to which native species possess little resistance [4]. Invasive plants can affect native plants through competition or allelopathy [5]. To preserve agriculture resources from invasive species and determine the necessary measures, we should identify the biological traits of invasive species and their negative impacts on the native species . To address this challenge, China has implemented a series of integrated prevention and control measures, including institutional construction, investigation and monitoring, integrated prevention and control, and scientific and technological support [6,7,8]. The effects of chemosensory substances on plants are closely related to the type and concentration of the substance and the species of the receptor plant [9,10]. Therefore, an in-depth study of the chemosensory effects of invasive alien plants will not only help to reveal the mechanisms of their invasion and dispersal but also be of great significance for the development of natural herbicides based on phytotoxic compounds [11]. Heilongjiang Province, as a key ecological barrier in the northeastern region of China, plays a crucial role in maintaining the environmental and ecological security of the main grain-producing areas in the northeastern plains [12]. In this critical environmental region, the invasive alien plants E. canadensis and E. annuus, as members of the genus Erigeron in the family Asteraceae, have been widely distributed in Heilongjiang Province and have been categorized as Class I pernicious invasive species [13]. These two plants have successfully invaded not only because they possess strong resistance but also because of their significant chemosensory effects [14]. In a 2023 census of invasive alien species in the forest, grassland, and wetland ecosystems in Heilongjiang Province, E. canadensis and E. annuus were found to have become locally dominant species, with significant negative impacts on species richness, species diversity, evenness, and soil nutrients in native communities. The project team conducted a statistical analysis of the habitat types of the invaded sites of E. canadensis and E. annuus, and the results showed that 52% of E. canadensis and 60 percent of E. annuus were mainly distributed along roadsides, on wasteland, and on farmland. The tiny seeds and long crown hairs of invasive plants in the family Asteraceae enable them to spread over long distances by spreading in the wind or being carried by adherence to animal fur, etc. [15]. Seed production of E. canadensis and E. annuus is abundant. There is a wide range of suitable temperatures for seed germination. The study showed that these seeds in the surface layer of the soil had the highest germination and remained highly active even after a year of burial in the soil [16,17]. Plant seeds growing along roadsides and in agricultural fields are not only capable of dispersing in the wind but may also be mixed with crops and spread, highlighting the urgent need for preventive measures to avoid potential threats to native crops.

Seed germination is a crucial step in the life cycle of seed plants, which not only determines the continuation of plant populations but is also highly susceptible to external environmental conditions. Ma Ruijun et al. [18] showed that Ligularia virgaurea (Maxim.) Mattf. ex Rehder. volatiles had a significant effect on the germination of Festuca sinensis Keng ex E.B.Alexeev, Bromus Magnus Keng, Elymus nutans Griseb, Poa infirma Kunth, and Festuca ovina L; the germination of five forage grass seeds had a significant inhibitory effect. A study by Sturz [19] found that root bacterial secretion of Tagetes erecta L. in Africa and France could inhibit the population of harmful nematodes in the root region of Solanum tuberosum L. Chen et al. [20] showed that the fresh sample of Isodon serra (Maxim.) Kudôextracts significantly inhibited the germination of seeds of Digitaria sanguinalis (L.)Scop. and Hemarthria sibirica (Gand.)Ohwi. The study of Narwal [21] further confirmed that the aqueous extracts of leaves stems, flowers, seeds, and roots of Medicago sativa L. had a self-toxic effect on seed germination, and this self-toxicity was enhanced with the increase in the concentration of extracts. Tong Zhu [22] and Jun Li et al. [23] observed the effects of different colored branches and leaf extracts of woody plants on Pennisetum alopecuroides (L.) Spreng, Lolium perenne L., Trifolium pratense L., etc., germination, and found that the effects of these extracts on the germination of different seeds varied and showed diverse effects. These findings provide valuable information for our understanding of plant interactions and their roles in ecosystems.

The study aims to experimentally assess the allelopathic effects of E. canadensis and E. annuus aqueous extracts on significant crops such as wheat, corn, and rice, focusing on key indicators including germination rate, germination potential, shoot length, and root length. By comparing the allelopathic effects of E. canadensis and E. annuus, the study seeks to determine which species has a more substantial allelopathic potential. It also evaluates the potential threats these invasive plants pose to local agricultural production and explores the mechanisms by which they affect crop growth and competitive ability. The study determines whether the aqueous extracts of these plants have inhibitory or promotional effects on crop seed germination, revealing the specific impacts of allelochemicals on the early growth stages of crops. Additionally, it assesses the effect of different concentrations of allelochemicals and the interactions between species and concentrations on crop seed germination [24]. Ultimately, the study provides scientific data on the allelopathic effects of these invasive plants to understand their potential ecological and agricultural implications.

In summary, an in-depth investigation of the effects of E. canadensis and E. annuus on significant crops in Heilongjiang Province is not only crucial for the protection of local agricultural production but also has a positive impact on the maintenance of ecological balance and biodiversity. Through these studies, we can better understand the impacts of invasive plants on agroecosystems and take corresponding management measures to mitigate their negative impacts on agricultural production and ecosystems.

2. Materials and Methods

2.1. Test Materials

The source plants used in the experiments, namely E. canadensis and E. annuus, were collected from several areas in Heilongjiang Province, including Harbin City, Ning’an City, Mudanjiang City, and Fuyu County, Qiqihar City. Random sampling was used to select E. canadensis and E. annuus plants in good developmental condition and bring them back to the laboratory intact for subsequent experiments. For the recipient seeds in the experiment, wheat, corn, and rice seeds circulating in the market were carefully selected, and 200 g of each were acquired to ensure the quality and consistency of the materials used in the experiment and to provide a solid foundation for the accuracy and reliability of the experimental results.

2.2. Test Methods

2.2.1. Preparation of Aqueous Extracts of Erigeron canadensis and Erigeron annuus

The preparation of the extract followed the method described by Hu et al. [25]. Firstly, the collected weeds were thoroughly washed with tap water and then dried naturally in a well-ventilated indoor area. After drying, the weeds were cut into small segments of 1 to 2 cm, followed by maceration by mixing with distilled water at the rate of 100 g per liter. This process was continued for 48 h at 4 °C under refrigeration to ensure that the active ingredients could be fully dissolved. Upon completion of the extraction, the solution was carefully filtered using two layers of fine gauze and filter paper, and the clear liquid obtained was the mother liquor of the extract. Subsequently, the mother liquor was diluted to different concentrations of 25, 50, 75, and 100 g·L⁻¹ according to the experimental requirements and stored in a refrigerated environment at 4 °C for subsequent use. This rigorous preparation process ensured the accuracy of the quality and concentration of the extracts and provided standardized samples.

2.2.2. Seed Germination and Seedling Growth Experiments

Wheat, rice, and corn seeds, which were full-grained, uniform in size, and free of pests and diseases, were randomly selected as experimental materials. Firstly, the seeds were surface sterilized using a 0.5% potassium permanganate solution for 5 min to ensure their sterility. Subsequently, the seeds were thoroughly rinsed three to four times with distilled water to remove the residual disinfectant solution, and then the surface of the seeds was blotted dry with sterile paper.

The experiment was carried out using the Petri dish filter paper germination method. Two layers of filter paper were laid in each Petri dish (10 cm diameter) and 30 seeds of wheat, rice, and corn were placed in each Petri dish. There were three groups, as well as the control group, for each test at 0 g·L⁻¹ and each seed treatment was repeated three times to ensure the reliability of the experimental results. Next, 5 mL of E. canadensis and E. annuus extracts at different concentrations (25, 50, 75, and 100 g·L⁻¹) were added to the Petri dishes. In contrast, an equal amount of distilled water was added to the control.

Petri dishes were placed in an artificial climatic chamber for constant temperature incubation set at 25 ± 0.5 °C, humidity maintained at 70%, and light provided for 12 h daily. The number of seeds germinated was counted daily from the day the seeds were placed in the Petri dish, and germination was measured by the breakthrough of the radicle through the seed coat. The number of seeds germinated was observed and recorded every 24 h. The experiment was terminated when no new seeds germinated for three consecutive days. The whole incubation cycle was 7 days, during which we measured the stem length, root length, and biomass of the seedlings. At the end of the germination experiment, the growth indices of the seedlings were measured. During the germination period, leaching solution or distilled water was periodically added in appropriate amounts to keep the filter paper in a moist state and to provide a suitable growth environment for the seeds.

2.2.3. Measurement of Growth Indicators

In this study, the allelopathic effects of the extracts of E. canadensis and E. annuus were bioassayed based on the methods proposed by Zeng Renshen and Boena [26], with key indices such as germination rate, germination vigor, inhibition rate, and allelopathic effect index being statistically analyzed. In the narrower sense of the word, seed germination (SG) is an estimate of the viability of a population of seeds. Seed germination is based on the number of germinated seeds, which germinate within (usually) 5–21 days. The equation to calculate seed germination is given in Equation (1):

SG (%) = 100% × (number of germinated seeds/total number of seeds)

(1)

Germination potential represents the vitality of seeds, which is the comprehensive manifestation of the activity intensity and characteristics of seeds during germination and seedling emergence. Currently, China has made unified regulations on the number of days for determining the germination potential and germination rate of some crop seeds [27]. Under normal germination conditions, the germination potential is generally defined as the percentage of seeds germinated on the 3rd day of germination (G3) out of the total number of seeds tested, as seen in Equation (2):

Germination Potential (GP) = [(Total Number of Seeds Tested(G3)]/(total
number of seeds) × 100% (G3: the number of seeds germinated within
3 days)

(2)

Other valuable parameters expressing the vitality of the seeds include the germination index (GI). This dimensionless parameter (index) can be successfully applied to botanical plant species as well [28]. The GI appears to be the most comprehensive measurement parameter, combining both seed germination and time of germination (speed) [29]. Therefore, the faster the batch of seeds germinated, the higher the germination index. The equation for the calculation is as follows (Equation (3)):

GI = [(number of germinated seeds on 1st count)/(day of the 1st count)] + …
+ [(number of germinated seeds on the final count)/(day of the final count)]

(3)

The inhibition rate (IR) is a quantitative indicator that measures the inhibitory effect of a specific treatment (such as a drug, chemical substance, or biological factor) on a target object (such as cells, organisms, or physiological processes). It is commonly used to assess the degree of inhibition in the treated group relative to the control group (Equation (4)):

Inhibitor rate [IR] = [C − T/C] × 100% (C is the control value, T is the
treatment value)

(4)

To measure the allelopathic potential changes in a plant at different growth stages and analyze the allelopathic effects of plant residue decomposition over various periods, corresponding controls are used in each test. Typically, the ratio of the treated value to the control value (T/C) is used as an indicator. Williamson [30] proposed using the sensitivity index (RI) as a measure (Equation (5)):

RI = 1 − TC

(5)

When T > C, RI = CT − 1. When T < C, RI = 1 − TC. Here, C is the control value, T is the treated value, and RI represents the allelopathic effect. When RI > 0, it indicates a promoting effect; when RI < 0, it indicates an inhibitory effect. The absolute value of RI represents the strength of the allelopathic effect.

Synthetical allelopathic index [AI] = (germination rate RI + germination index RI + shoot length RI + root length RI + germination potential RI)/5.

(In the formula, AI > 0 means that the overall performance is facilitating and AI < 0 means that the overall performance is inhibiting).

2.2.4. Statistics and Analysis of Data

All datasets were tested for normality using the Shapiro–Wilk test and homogeneity of variances via Levene’s test. Non-parametric data (Shapiro–Wilk p ≤ 0.05) underwent log transformation to meet parametric assumptions. Parametric data were analyzed using two-way ANOVA (SPSS 26) to evaluate the main effects of extract concentration (25–100 g·L^{− 1}) and plant species (wheat/rice/corn), along with their interaction. Significant interactions (p ≤ 0.05) were resolved using Tukey’s HSD post hoc tests (α = 0.05). Fixed factors (concentration, species) represented experimental manipulations, while random factors (replicates) assessed result reliability. All statistical workflows were performed in SPSS 26, and graphs were generated using GraphPad Prism 10.1 with Tukey HSD comparison letters (p < 0.05) annotated.

3. Results and Analyses

3.1. Effect of Aqueous Extracts of Erigeron canadensis and Erigeron annuus on Seed Germination of Three Major Crops

The findings of this study provide compelling evidence that aqueous extracts of E. canadensis and E. annuus significantly hinder the germination of seeds from wheat, rice, and corn. Notably, the percentage of germination, germination index, and overall germination potential of these seeds declined markedly with increasing concentrations of the leachate treatments. Furthermore, the variability observed in the impacts of different leachate treatments underscores the specific nature of the chemosensory effects on a range of crops. These critical insights reveal the substantial inhibitory effects that invasive alien plants can have on the germination of native crops through chemosensitization. This serves as a powerful reminder of the urgent need for effective management and control strategies to combat these invasive species and protect agricultural biodiversity. The results of the present study revealed that the aqueous extracts of E. canadensis and E. annuus significantly affected the germination of wheat, rice, and corn seeds. Specifically, the germination percentage, germination index, and germination potential of the seeds decreased after these leachate treatments, and this decreasing trend became more pronounced as the concentration of the leachate increased. In addition, the effects of different leachate treatments on seed germination showed variability, suggesting that the chemosensory effects of E. canadensis and E. annuus are specific to the seeds of other crops. These findings reveal the potential inhibitory effects of invasive alien plants on seed germination of native crops through chemosensitization, further emphasizing the importance of management and control of these invasive plants.

3.1.1. Germination Rate

This study systematically evaluated the effects of E. canadensis and E. annuus extracts on the germination rates of wheat, rice, and corn using two-way ANOVA and Tukey’s HSD post hoc test. The results (Figure 1) showed that both weed extracts significantly inhibited the germination rates of the three crops, with the inhibitory effects increasing with higher concentrations. E. canadensis extract had the most significant inhibitory effect on corn germination, reducing the germination rate from 95.67% to 17.67% at 100 g·L⁻¹ (mean difference = 75.67, q = 38.53, p < 0.001), while the germination rates of wheat and rice decreased to 4.333% (mean difference = 93.44, q = 68.18, p < 0.001) and 15.56% (mean difference = 77.78, q = 37.42, p < 0.001), respectively. In contrast, E. annuus extract exhibited a more pronounced inhibitory effect on wheat germination, completely suppressing the germination rate to 0.000% at 100 g·L⁻¹ (mean difference = 97.78, q = 62.19, p < 0.001), while the germination rates of rice and corn decreased to 0.000% (mean difference = 93.33, q = 68.55, p < 0.001) and 20.00% (mean difference = 75.67, q = 38.53, p < 0.001), respectively. These findings suggest that E. annuus extract may have a stronger inhibitory effect on crop germination than E. canadensis, particularly at higher concentrations.

Further analysis revealed significant differences in the inhibitory patterns of the two weed extracts on crop germination. E. canadensis extract showed a milder inhibitory effect on wheat and rice germination at lower concentrations (25 g·L⁻¹) (wheat: mean difference = 15.78, q = 3.252, p < 0.05; rice: mean difference = 11.33, q = 2.561, p < 0.05), but a significant inhibitory effect on corn germination (mean difference = 13.67, q = 5.145, p < 0.05). In contrast, E. annuus extract already exhibited a noticeable inhibitory effect on wheat germination at lower concentrations (mean difference = 23.33, q = 9.899, p < 0.01), and the inhibition intensified rapidly with increasing concentrations (50 g·L⁻¹: mean difference = 45.55, q = 16.08, p < 0.001; 75 g·L⁻¹: mean difference = 92.22, q = 44.32, p < 0.001). Additionally, pairwise comparisons between concentrations showed that E. canadensis extract had a significant difference in wheat germination inhibition between 75 g·L⁻¹ and 100 g·L⁻¹ (mean difference = 44.67, q = 16.07, p < 0.001), while E. annuus extract exhibited a more significant difference in wheat germination inhibition between 50 g·L⁻¹ and 75 g·L⁻¹ (mean difference = 46.67, q = 34.33, p < 0.001). These differences may be due to the biology of the species, e.g., differences in the chemical composition of different plants, the sensitivity of different crops to these compounds, etc. The inhibitory effect on germination rate was slightly more substantial in E. annuus than in E. canadensis, especially at higher concentrations. An explanation of this can be provided by comparison with essential oils, which are usually more biologically active and may have a more substantial inhibitory effect on crop germination, whereas E. canadensis may have a lower content of volatile compounds and, therefore, its inhibitory effect on crop germination is relatively weak [31]. There was a notable variation in the sensitivity of different crops. Rice was the most sensitive to the inhibitory effects of these two plants, followed by wheat and corn. The reasons for the differences in the sensitivity of different crops to these compounds are numerous and may be related to the physiological characteristics and metabolic pathways of the crops. These findings significantly affect crop management and plant selection in agricultural practices.

The two-factor ANOVA results indicate that concentration is the main factor influencing the inhibitory effects of E. canadensis and E. annuus on crop germination rates. For E. canadensis, the effect of concentration on germination rate is significant (F = 143.5, df = 4, p = 0.0050), while for E. annuus, the effect of concentration is even more significant (F = 800.9, df = 4, p < 0.0001). The species factor does not significantly affect the germination rate of E. canadensis (F = 0.5323, df = 2, p = 0.5434), but it has a significant effect on the germination rate of E. annuus (F = 68.88, df = 2, p = 0.0142). Additionally, the interaction between species and concentration has a marginal effect on the germination rate of E. canadensis (F = 8.513, df = 4, p = 0.0549), while it is significant for E. annuus (F = 14.79, df = 8, p = 0.0248).

These results (

Table 1. Two-way ANOVA results: effects of species and concentration on seed germination rate.

Parametric	Species	Factor	p-Value	df		ms	f (dfn, dfd)
Germination Rate	Erigeron canadensis	species	0.5434	2		11.85	f (1.017, 2.034) = 0.5323
		concentration	0.0050	4		11,084	f (1.085, 2.171) = 143.5
		species×concentration	0.0549	8		100.6	f (1.580, 3.160) = 8.513
	Erigeron annuus	species	0.0142	2		1512	f (1.000, 2.000) = 68.88
		concentration	<0.0001	4		12,222	f (1.494, 2.988) = 800.9
		species×concentration	0.0248	8		267.9	f (1.609, 3.218) = 14.79

) suggest that for E. annuus, the combination of different species and different concentrations had a more complex effect on the germination rate. This may be due to the chemical composition of different species interacting with different concentrations. For example, the extract of E. annuus was analyzed by GC-MS, and 28 compounds were identified, including acids, ketones, esters, and terpenes, as the main chemosensory components. Among these, 5-butyl-3-yloxy-2,3-dihydrofuran-2-acyl is a germination-inhibitory component of E. annuus [32]. Since the species itself contains germination-inhibitory components, a specific pattern of interactions may have arisen when co-acting with concentrations.

3.1.2. Germination Index

In this study, the effects of extracts from E. canadensis and E. annuus on the germination indices of wheat, rice, and corn were systematically evaluated using two-way ANOVA. The results (Figure 2) showed that the extracts from both weed species significantly inhibited the germination indices of the three crops, with the inhibitory effects increasing as the concentration increased. For wheat, the extracts from E. canadensis and E. annuus significantly reduced the germination index at concentrations of 25 g·L⁻¹, 50 g·L⁻¹, 75 g·L⁻¹, and 100 g·L⁻¹ (p < 0.05), with the most significant inhibition observed at 100 g/L. For example, the E. canadensis extract reduced the germination index of wheat from 12.55 to 0.000 (mean difference = 12.55, q = 43.45, p < 0.001). Similarly, the E. annuus extract reduced the germination index of corn from 11.47 to 2.007 (mean difference = 9.460, q = 33.55, p < 0.001). Both E. canadensis and E. annuus extracts significantly reduced the germination index of rice (p = 0.000), with the inhibitory effect of E. annuus being stronger than that of E. canadensis (mean difference = −0.1356, p = 0.001). These findings demonstrate that the effects of E. canadensis and E. annuus extracts on crop germination indices are concentration-dependent, and wheat and rice exhibit higher sensitivity to the extracts compared to corn.

The results (

Table 2. Two-way ANOVA results: effects of species and concentration on seed germination index.

Parametric	Species	Factor	p-Value	df	ms	f (dfn, dfd)
Germination Index	Erigeron canadensis	species	0.0230	2	7.079	f (1.097, 2.194) = 33.41
		concentration	<0.0001	4	200.8	f (1.806, 3.612) = 684.4
		species×concentration	0.0587	8	2.399	f (1.131, 2.263) = 12.70
	Erigeron annuus	species	0.0003	2	46.33	f (1.529, 3.058) = 290.8
		concentration	0.0008	4	188.5	f (1.302, 2.604) = 326.6
		species×concentration	0.0149	8	7.864	f (1.483, 2.966) = 24.74

) of the two-way ANOVA indicate that both species and concentration have significant effects on the germination rate for E. canadensis and E. annuus. For E. canadensis, the species effect is significant (p = 0.0230, F = 33.41), and the concentration effect is highly significant (p < 0.0001, F = 684.4), while the interaction between species and concentration is marginally significant (p = 0.0587, F = 12.70). For E. annuus, both the species effect (p = 0.0003, F = 290.8) and the concentration effect (p = 0.0008, F = 326.6) are significant, with a significant interaction between species and concentration (p = 0.0149, F = 24.74). These results demonstrate that concentration is a major factor influencing germination rates, and E. annuus exhibits more significant responses to different concentrations.

3.1.3. Germination Potential

The analysis of the effects of E. canadensis and E. annuus extracts on the germination potential of wheat, rice, and corn shows that both weed extracts significantly inhibit the germination potential of the crops, with the inhibitory effects increasing as the concentration increases. Wheat exhibits higher sensitivity to both extracts compared to rice and corn, as reflected by a greater reduction in germination potential. For example (Figure 3), at a concentration of 100 g·L⁻¹, the extracts of E. canadensis and E. annuus reduced the germination potential of wheat from 86.67 to 0.000 (mean difference = 86.67, q = 63.65, p < 0.05). The germination potential of rice was significantly inhibited only at 100 g·L⁻¹ (p = 0.008), while the germination potential of corn also showed a significant reduction at 100 g·L⁻¹, with E. annuus extract decreasing from 75.56 to 13.33 (mean difference = 62.22, q = 12.08, p < 0.05). These results indicate that the effects of E. canadensis and E. annuus extracts on crop germination potential are concentration-dependent, and wheat is more sensitive to their effects. This provides important insights for developing weed management strategies in agricultural practices.

The results (

Table 3. Two-way ANOVA results: effects of species and concentration on seed germination potential.

Parametric	Species	Factor	p-Value	df	ms	f (dfn, dfd)
Germination Potential	Erigeron canadensis	species	0.0260	2	81.73	f (1.047, 2.093) = 33.09
		concentration	0.0004	4	9650	f (1.225, 2.449) = 744.1
		species×concentration	0.2583	8	88.51	f (1.098, 2.195) = 2.360
	Erigeron annuus	species	0.0051	2	2042	f (1.038, 2.077) = 166.9
		concentration	0.0032	4	7381	f (1.134, 2.267) = 183.4
		species×concentration	0.0387	8	461.3	f (1.503, 3.006) = 11.90

) of the two-way ANOVA indicate that both species and concentration have significant effects on the seed germination rate for E. canadensis and E. annuus. For E. canadensis, the species effect is significant (p = 0.0260, F = 33.09), and the concentration effect is highly significant (p = 0.0004, F = 744.1), while the interaction between species and concentration is not significant (p = 0.2583, F = 2.360). For E. annuus, both the species effect (p = 0.0051, F = 166.9) and the concentration effect (p = 0.0032, F = 183.4) are significant, with a significant interaction between species and concentration (p = 0.0387, F = 11.90). These results demonstrate that concentration is a major factor influencing seed germination rates, and E. annuus exhibits more significant responses to different concentrations.

3.2. Effect of Aqueous Extracts of Erigeron canadensis and Erigeron annuus on Seedling Growth of Three Major Grain Seeds

Under the treatment of E. canadensis, the root length, shoot length, and fresh weight of rice and maize significantly decreased with increasing treatment concentrations. For example (Figure 4), the mean difference in rice root length at 100 g·L⁻¹ treatment was 4.293 (q = 910.8, p < 0.0001), and the mean difference in maize fresh weight at 100 g·L⁻¹ treatment was 0.3517 (q = 40.87, p = 0.0030), both demonstrating strong inhibitory effects. Similarly, E. annuus also showed significant inhibitory effects, but its inhibitory effects on the root length of rice and maize were slightly weaker than those of E. canadensis. For instance, the mean difference in maize root length at 100 g·L⁻¹ treatment was 3.780 (q = 212.4, p = 0.0001), and the mean difference in maize fresh weight at 100 g·L⁻¹ treatment was 0.3463 (q = 31.70, p = 0.0065). Additionally, the inhibitory effects of E. canadensis on the shoot length of rice and maize were slightly stronger than those of E. annuus, indicating that the allelopathic effects of E. canadensis were more pronounced.

From the perspective of species specificity, the inhibitory effects of Erigeron annuus on root length, shoot length, and fresh weight of rice and maize were generally stronger than those of E. canadensis, especially at high treatment concentrations (e.g., 75 g·L⁻¹ and 100 g·L⁻¹). This suggests that Erigeron annuus may contain stronger allelopathic substances or have a more efficient mechanism of action. However, E. canadensis also exhibited significant inhibition.

Tukey’s HSD test results revealed that both E. canadensis and E. annuus exhibited significant dose-dependent inhibitory effects on wheat root length, shoot length, and fresh weight. As the treatment concentration increased, the inhibitory effects became more pronounced, with the 100 g·L⁻¹ treatment showing the most significant inhibition on wheat root length, shoot length, and fresh weight (standard test statistics = 3.125, 3.768, and 4.011, respectively, p < 0.001). At lower concentrations (e.g., 25 g·L⁻¹ and 50 g·L⁻¹), the differences in wheat root length, shoot length, and fresh weight were also significant (p ≤ 0.05), indicating that even lower concentrations of treatment had a noticeable inhibitory effect on wheat growth. Furthermore, the differences between 75 g·L⁻¹ and 25 g·L⁻¹ were significant (standard test statistics = 3.076, 2.519, and 2.383, respectively, p ≤ 0.017), demonstrating that the inhibitory effect of the 75 g·L⁻¹ treatment was stronger than that of the 25 g·L⁻¹ treatment. Comprehensive analysis indicates that both E. canadensis and E. annuus significantly inhibit wheat growth through allelopathic effects, with the highest inhibitory effects observed at high concentrations.

The study results indicate differences in the inhibitory effects of E. annuus and E. canadensis across crop species. The allelopathic effects of E. annuus showed marked inhibition on seed germination, root length, and seedling height at high concentrations. At the same time, other studies report that low concentrations of E. annuus extracts promoted shoot growth in crops like Chinese cabbage and tomato [33]. Additionally, chloroplast genome analysis of E. annuus revealed genes encoding allelopathy-related proteins, which may serve as a molecular basis for its effects on neighboring plants [34]. For E. canadensis, studies have demonstrated its allelopathic potential and mechanisms in maize [35]. Although its specific effects on wheat, rice, and maize were not explicitly detailed in prior research, the current experiment suggests similar inhibitory patterns. Despite variations in their impacts on different crops, both plant extracts share a consistent trend of concentration-dependent inhibition.

3.3. Index of the Chemosensory Effect of Erigeron canadensis and Erigeron annuus Extracts on Seedling Growth of Different Grain Seeds

The chemosensitivity index (RI) serves as a crucial measure of the type and intensity of allelopathic effects. Its upbeat, downbeat, negative, and large values indicate the nature and intensity of chemosensitivity, respectively; a positive value indicates a facilitating effect, while a negative value indicates an inhibiting effect. The more significant the absolute value of the RI is, the stronger the chemosensitivity is [36].

This study employed one-way analysis of variance (one-way ANOVA) to investigate the differences in allelopathic effect indices of E. canadensis and E. annuus on wheat, rice, and maize. Under the premise that the data met the assumptions of normal distribution and homogeneity of variance, one-way ANOVA was used to compare the significant differences in allelopathic effect indices among different treatment groups (E. canadensis and E. annuus) on the three crops. The analysis results indicated that there were significant differences in the allelopathic effect indices of E. canadensis and E. annuus on wheat, rice, and maize (p < 0.05). Further multiple comparisons (Tukey HSD test) revealed specific differences between the treatment groups.

We can deduce that the strength of allelopathic effects is related to the concentration. As the concentration of extracts from E. canadensis and E. annuus increases, the inhibitory effects on germination rate, germination index, germination potential, root length, shoot length, and the comprehensive allelopathic index of the three crops (wheat, rice, and corn) are generally enhanced. For instance, when the concentration of E. annuus extract is 25 g·L⁻¹, the allelopathic response index (RI) for wheat germination rate is −0.159, and it becomes −0.955 when the concentration rises to 100 g·L⁻¹. Notably, at a concentration of 100 g·L⁻¹, E. annuus exerts a complete inhibitory effect (−1.000) on all parameters. Under the same concentration, wheat exhibits larger absolute values of the allelopathic response index across several indicators, suggesting it may be more sensitive to the allelopathic effects of both plants. For example, at a concentration of 100 g·L⁻¹ of E. annuus extract, the RI for wheat germination rate is −1.000, −1.000 for rice, and −0.813 for corn.

Based on the data in

Table 4. Allelopathic response index of different grains to the Erigeron canadensis and Erigeron annuus radicals (mean ± SD).

Species	Items	Concentration g·L−1	Test Seeds
			Wheat	Rice	Corn
Erigeron canadensis	Germination rate RI	25	−0.159 ± 0.049	−0.128 ± 0.032	−0.076 ± 0.054
		50	−0.364 ± 0.030	−0.403 ± 0.032	−0.378 ± 0.094
		75	−0.500 ± 0.049	−0.642 ± 0.021	−0.609 ± 0.063
		100	−0.955 ± 0.011	−0.833 ± 0.032	−0.769 ± 0.047
	Germination index RI	25	−0.36 ± 0.008	−0.043 ± 0.023	−0.067 ± 0.017
		50	−0.60 ± 0.012	−0.392 ± 0.034	−0.464 ± 0.104
		75	−0.70 ± 0.012	−0.663 ± 0.008	−0.757 ± 0.045
		100	−0.98 ± 0.006	−0.935 ± 0.011	−0.859 ± 0.037
	Germination potential RI	25	−0.295 ± 0.012	−0.273 ± 0.026	−0.067 ± 0.026
		50	−0.705 ± 0.012	−0.576 ± 0.040	−0.456 ± 0.118
		75	−0.897 ± 0.025	−0.818 ± 0.026	−0.822 ± 0.031
		100	−1.000 ± 0.000	−1.000 ± 0.000	−0.900 ± 0.042
	Root length RI	25	0.188 ± 0.011	−0.200 ± 0.013	−0.431 ± 0.044
		50	−0.436 ± 0.008	−0.381 ± 0.009	−0.576 ± 0.020
		75	−0.547 ± 0.014	−0.445 ± 0.009	−0.871 ± 0.006
		100	−0.719 ± 0.021	−1.000 ± 0.000	−0.877 ± 0.003
	Stem length RI	25	−0.218 ± 0.038	−0.414 ± 0.013	−0.412 ± 0.006
		50	−0.427 ± 0.022	−0.627 ± 0.021	−0.691 ± 0.009
		75	−0.542 ± 0.008	−0.816 ± 0.010	−0.730 ± 0.013
		100	−0.708 ± 0.006	−0.963 ± 0.011	−0.886 ± 0.010
	Synthetical allelopathic index RI	25	−0.160 ± 0.008	−0.095 ± 0.016	−0.096 ± 0.023
		50	−0.553 ± 0.006	−0.437 ± 0.021	−0.378 ± 0.089
		75	−0.711 ± 0.004	−0.687 ± 0.002	−0.681 ± 0.052
		100	−0.920 ± 0.008	−0.927 ± 0.009	−0.796 ± 0.034
Erigeron annuus	Germination rate RI	25	−0.239 ± 0.030	−0.068 ± 0.021	−0.022 ± 0.023
		50	−0.466 ± 0.030	−0.379 ± 0.043	−0.245 ± 0.030
		75	−0.943 ± 0.011	−0.618 ± 0.032	−0.422 ± 0.039
		100	−1.000 ± 0.000	−1.000 ± 0.000	−0.813 ± 0.039
	Germination index RI	25	−0.549 ± 0.015	−0.057 ± 0.031	−0.135 ± 0.037
		50	−0.761 ± 0.011	−0.311 ± 0.035	−0.295 ± 0.021
		75	−0.973 ± 0.004	−0.625 ± 0.026	−0.506 ± 0.042
		100	−1.000 ± 0.000	−1.000 ± 0.000	−0.843 ± 0.029
	Germination potential RI	25	−0.590 ± 0.013	−0.303 ± 0.066	−0.122 ± 0.049
		50	−0.910 ± 0.013	−0.424 ± 0.015	−0.267 ± 0.046
		75	−1.000 ± 0.000	−0.727 ± 0.045	−0.500 ± 0.049
		100	−1.000 ± 0.000	−1.000 ± 0.000	−0.833 ± 0.033
	Root length RI	25	−0.284 ± 0.009	−0.687 ± 0.014	−0.439 ± 0.035
		50	−0.558 ± 0.034	−0.911 ± 0.014	−0.625 ± 0.009
		75	−0.817 ± 0.008	−1.000 ± 0.000	−0.853 ± 0.006
		100	−1.000 ± 0.000	−1.000 ± 0.000	−0.926 ± 0.004
	Stem length RI	25	−0.403 ± 0.027	−0.524 ± 0.015	−0.263 ± 0.040
		50	−0.645 ± 0.024	−0.634 ± 0.016	−0.732 ± 0.006
		75	−0.819 ± 0.024	−0.946 ± 0.009	−0.796 ± 0.009
		100	−1.000 ± 0.000	−1.000 ± 0.000	−0.915 ± 0.017
	Synthetical allelopathic index RI	25	−0.398 ± 0.014	−0.194 ± 0.024	−0.072 ± 0.025
		50	−0.647 ± 0.017	−0.397 ± 0.022	−0.359 ± 0.016
		75	−0.930 ± 0.002	−0.702 ± 0.021	−0.582 ± 0.024
		100	−1.000 ± 0.000	−1.000 ± 0.000	−0.850 ± 0.018

, significant differences are observed between various concentrations of the extracts and the control group in most cases, indicating substantial allelopathic effects. For instance, with E. canadensis affecting wheat, most concentration treatments show substantial differences in various indicators (GR, GI, GP, etc.). As the concentration increases, the allelopathic response index increases in the negative direction, indicating an enhanced inhibitory effect. A comprehensive comparison of the combined chemosensory indices of E. canadensis and E. annuus extracts on the seeds of three common crops showed that with E. annuus, the intensity of such effects was in descending order of wheat, rice, and corn. Although the degree and mechanism of impact of the two plant extracts on different crops may vary, the overall trend is similar, showing concentration-dependent inhibitory effects.

4. Discussion

Heilongjiang, as a vital agricultural region in China, faces significant challenges from the invasive species E. canadensis and E. annuus, which exhibit pronounced inhibitory effects on the germination and early growth of crops. The potential threat to local crop production cannot be overlooked. This study aims to provide a scientific foundation for agricultural ecological conservation and invasive species management. However, limitations persist in bridging experimental findings with practical ecological applications, particularly in translating these results into actionable strategies for agricultural production and ecosystem management. The discussion will thus focus on four key issues: (1) identification and characterization of allelochemicals; (2) integration of research outcomes with local ecological and agricultural practices; (3) exploration of the ecological significance of allelopathic effects and potential management measures; and (4) in-depth analysis of allelopathic mechanisms, particularly interactions with soil microorganisms and nutrient competition.

This study reveals that extracts from E. canadensis and E. annuus inhibit the germination of major crops, though the specific chemical compounds responsible remain unidentified. Previous allelopathy studies have implicated phenolic compounds, flavonoids, and terpenoids as key allelochemicals. Mohamed [37] demonstrated that Erigeron bonariensis L. exhibits higher invasiveness than Bidens pilosa L., which is linked to its substantial allelopathic effects on plant species and soil properties. Future research should employ techniques such as high-performance liquid chromatography (HPLC) and gas chromatography–mass spectrometry (GC-MS) to identify potential allelochemicals (e.g., phenolics, flavonoids) and refine biological hypotheses. Bioassay-guided isolation and purification could further confirm their bioactivity. These compounds have been widely hypothesized as critical allelochemicals in prior studies [38]. The results confirm that Erigeron species significantly inhibit the germination and growth of crops. Additionally, studies have shown that Xanthium strumarium subsp. Italicum (Moretti)Löve, Xanthium strumarium L., and Galinsoga parviflora (Cav.), which are invasive plants in the Asteraceae family, also exhibit allelopathic effects. For instance, the volatile oils from the stems, leaves, and seeds of Xanthium strumarium exhibit strong allelopathic effects and can alter the soil nutrient balance, enhancing its competitive advantage over native plants [39]. Different parts of Cyclachaena xanthiifolia (Nutt.) Fresen. have varying degrees of allelopathic effects when extracted with water, with the leaf extracts showing the strongest inhibitory effects [40]. Cyclachaena xanthiifolia also has certain allelopathic effects on soybeans and peanuts, affecting seed germination and radicle elongation. Future research should consider including plants from these genera and delve deeper into the mechanisms and ecological impacts of their allelopathic effects. By comparing the allelopathic effects of different genera, we can better understand their competitive dynamics within ecosystems and how to use this knowledge to control invasive species or promote the growth of native plants. Furthermore, studying the differences in allelopathic effects among various genera can help reveal the complexity of plant interactions and provide a scientific basis for the management of agricultural ecosystems. Conducted in Heilongjiang Province—a central agricultural hub for rice, wheat, and corn—this study highlights China’s current reliance on chemical control for invasive weed management, such as pre-sowing soil treatments with triazine and methoxuron to suppress weed growth in wheat fields [41,42]. Results confirm that Erigeron species significantly inhibit crop germination and growth.

For instance, leguminous cover crops can suppress Erigeron proliferation. Vang [43] suggests that NTCT (novel tillage and cover crop techniques) may serve as a sustainable weed management tool, particularly for herbicide-resistant weeds in diverse ecosystems. Mechanical weeding and organic mulching also mitigate the spread of these invasives [44,45]. Agricultural practices like tillage, soil cover, and manual weeding could further aid in managing invasive weeds in the region.

While this study primarily documents experimental results, its implications extend to biodiversity in invaded ecosystems. Invasive weeds may alter plant community composition and structure, threatening ecosystem stability and biodiversity [46,47]. Suppressing allelopathic effects could promote native species recovery and enhance ecosystem richness. Protective measures, such as restoring native plant communities and bolstering ecological resilience, are critical. Agriculturally, farmers may adopt practices like regular invasive species removal to reduce allelochemical accumulation, coupled with biological controls (e.g., integrated pest management [IPM] strategies, pathogen/herbivorous insect biocontrol agents) to minimize crop threats.

The allelopathic impacts of Erigeron species likely involve multiple pathways. Although this study focuses on direct germination inhibition, interactions with soil microbial communities and nutrient competition warrant further investigation. Soil microorganisms significantly influence allelochemical bioavailability [48]. For example, allelochemicals can reshape bacterial and fungal community structure and function, with phenolic acids exerting notable effects on soil microbiota [49]. Climate and soil factors also modulate microbial communities, necessitating allelopathy assessments across diverse soil types. Future research should explore how Erigeron species alter soil microbial diversity function and how microbial adaptation to allelopathic stress may alleviate native plant suppression [50].

This study offers valuable insights into the allelopathic effects of E. canadensis and E. annuus on crops. However, future work must clarify the allelochemicals involved, assess broader ecological impacts, and develop effective management strategies. Integrating allelopathic mechanisms, soil microbial interactions, and nutrient competition will deepen understanding of invasive species dynamics within agroecosystems. Current limitations include unexplored interactions with soil microbiomes and nutrient competition. Planned experiments will investigate how extracts influence soil microbial community structure and nutrient competition between invasives and crops, strengthening the theoretical framework for allelopathy research. Furthermore, future research should also consider including other invasive plant genera with potential allelopathic effects, such as Xanthium, Iva, and Galinsoga, for a more comprehensive comparative analysis. This will help to reveal differences in allelopathic effects among plants from different genera and provide a more comprehensive scientific basis for the management of agricultural ecosystems. By delving deeper into the mechanisms and ecological impacts of allelopathy, we can better understand the competitive dynamics between invasive and native plants and develop more effective control strategies to manage these invasive species, thereby protecting local biodiversity and agricultural productivity.

5. Conclusions

This study thoroughly investigated the allelopathic effects of two invasive Asteraceae plants, E. canadensis and E. annuus, on major crop species. The results clearly showed that the aqueous extracts of these plants significantly inhibited the germination rate and seedling growth of wheat, rice, and maize, with the inhibitory effects intensifying as the concentration of the extracts increased. Notably, E. annuus had a more pronounced inhibitory effect on crop germination and growth, indicating a greater allelopathic potential.

Furthermore, our study found differences in the sensitivity of different crops to these extracts, with wheat being the most sensitive, followed by rice, and maize being the least sensitive. These differences may be related to the physiological characteristics and metabolic pathways of the crops, which are of significant importance for agricultural management and crop selection.

Although this study primarily focused on direct germination inhibition, the interactions involving soil microbial communities and nutrient competition also warrant further investigation. The results of this study highlight the importance of identifying and characterizing the allelopathic compounds involved. Previous allelopathy studies have suggested that phenolic compounds, flavonoids, and terpenoids may be key allelopathic compounds. Future research should employ techniques such as high-performance liquid chromatography (HPLC) and gas chromatography–mass spectrometry (GC-MS) to identify potential allelopathic compounds and further confirm their bioactivity.

In addition to the Erigeron genera, other genera of invasive plants with potential allelopathic effects, such as Xanthium, Iva, and Galinsoga, should also be included in future research. In conclusion, this study provides new scientific data on the impact of invasive plants on crop growth and emphasizes the necessity for further research into the allelopathic mechanisms and ecological impacts of these invasive plants.