How Do the Extracts From the Invasive Plant Acmella radicans Inhibit Germination and Growth of Brassica rapa (Field Mustard) and Chrysanthemum coronarium (Garland Daisy)? (2025)

1. Introduction

Alien plant species invasions pose significant threats to global biodiversity leading to various environmental and economic challenges [1, 2]. Their dominance in ecosystems arises from rapid growth, physiological and ecological adaptability, and often the use of allelopathic effects [3–6]. Such allelopathic impacts tend to increase the competitiveness of invasive plants [6, 7]. Plant allelopathy is the chemical interaction of one plant on other plants through the production of chemicals, which affects the seed germination, plant growth, and population establishment of the neighboring plants [8]. The majority (51.4%) of 524 invasive plant species could produce allelochemicals with the potential to negatively affect native plant performance [9], indicating that allelopathy could have a large impact on native species across the globe. Moreover, allelopathy is usually considered as one of the mechanisms by which such exotics may become successful invaders [6–9]. Hence, investigating the allelopathic potential of invasive plants is critical for understanding the potential competitive mechanisms employed by particular invasive plants against crops or native plant species.

The annual herbaceous plant Acmella radicans (Jacquin) R. K. Jansen originated in Central America [10]. Currently, the nonnative range of A. radicans includes Bangladesh, China, Cuba, Curaçao, India, Tanzania, and Thailand [10–14]. In 2014, A. radicans was first documented as a naturalized species in China, specifically in Anhui Province [10]. This species thrives in moist weedy environments, including roadside ditches, riparian zones, and relatively damp agricultural fields like rice paddies. It is a relatively tall plant (up to 155 cm), with broad leaves and long sexual reproduction period (November through March), and produces a large number of seeds. Acmella radicans also has several medicinal applications, such as alleviating toothaches and treating infections [12, 15, 16].

Acmella radicans was identified as a potential concern in Yunnan Province, Southwestern China in 2017. By that time, it was already widely established in the vicinity of both Baoshan and Lincang, where it primarily invaded anthropogenically disturbed habitats like farm fields, tea plantations, orchards, roadsides, and ditches. The plant has rapidly become dominant in local habitats, adversely impacting plant species diversity and soil nutrients [17]. In an additional field survey, it has been found that A. radicans appeared to inhibit the growth of some vegetable species, especially two local major vegetable crops Brassica rapa (field mustard) and Chrysanthemum coronarium (garland daisy). Acmella radicans was widely distributed in fields of B. rapa and C. coronarium, with the crops exhibiting reduced seed germination rates and delayed germination compared to fields without A. radicans. Moreover, the yields of the two vegetable crops were also reduced when A. radicans was present. However, it was not known whether the impact of A. radicans on these and other species was related to allelopathic effects.

This study hypothesized that some of the observed impacts of A. radicans may be related to allelopathic effects. In order to explore the allelopathic potential of A. radicans against the two major vegetable crops B. rapa and C. coronarium, investigations were conducted to assess the allelopathic potential of aqueous extracts from root, stem, leaf, and flower and fruit heads of A. radicans to inhibit seed germination and growth of these vegetable crops.

2. Materials and Methods

3. Results

4. Discussion

The invasiveness of many plant species is enhanced by allelopathy [9, 21]. Release of allelochemicals by invasive plants has been shown to inhibit native plants physiologically, even to the extent of causing local extinctions [7, 22–24]. Recent studies have indicated that many invasive plants can suppress growth of major crops via allelopathy [3, 6, 22–24]. However, with A. radicans​ being a recently recorded invasive species for Yunnan Province, China, its allelopathic effects on neighboring plants, especially major vegetable crops, were largely unknown prior to the current study.

The current study demonstrated various impacts of A. radicans extracts on the early life stages of B. rapa and C. coronarium based on the plant part, concentration, and interaction level. These findings were similar to the patterns seen in some previous studies [25–28]. Acmella radicans extracts impacted B. rapa and C. coronarium at their most vulnerable life stages through allelopathy. Root length was most severely inhibited in both species, followed by biomass, shoot length, and germination index, with the germination rate experiencing the least inhibition. The successful establishment of invasive plant species in new habitats often requires high rates of seed germination [22]. Reduced root length, extended germination time, and delayed seedling emergence caused by invasive alien plants can significantly impact how well native plants compete for both aboveground and belowground resources [22]. Methanol extracts from the invasive alien plant Ipomoea cairica delayed the seed germination of two vegetable species, Lactuca sativa and L. sativa var. ramosa, and the roots treated with the extract displayed evident shortness and stoutness, reduction of root hairs or no root hairs, and brownness or death [28]. Allelopathic inhibition of germination and root length often leads to decreased water and nutrient absorption, reducing effective resource utilization and negatively impacting growth, species viability, and ultimately resulting in reduced plant populations [22, 28].

Production of allelochemicals varies greatly in terms of timing and plant anatomical sources [25–27]. Allelopathic compounds may be released in various ways, including via volatile compounds, shoot or root leachates, root exudates, and during decomposition in the soil [8]. Numerous studies have indicated that leaf material typically exhibits stronger allelopathic effects and may possess greater concentrations of allelochemical compounds [25, 27]. Fernandez et al. [27] found that the concentrations and diversity of allelochemical compounds were more in the leaves than in roots of Pinus halepensis. The current study found that the inhibition of seed germination and seedling growth of B. rapa and C. coronarium by A. radicans extracts was generally higher for extracts from flowers and fruit heads compared to leaves and stems, with the lowest rates observed for roots. Additionally, the vast majority of allelopathic response indices for A. radicans extracts consisted of negative values that significantly decreased with increasing concentration.

Jirovetz et al. [16] identified more than 70 constituents in the essential oil produced by A. radicans in Southern India. It is difficult to determine which of these are active against plants without further testing. The major compounds they identified were 1-pentadecene, 2-tridecanone, pentadecane, sesquiterpenes, and tridecane [16]. Interestingly, they determined through GC analysis that palmitic acid comprised 16.6% of the volatiles from A. radicans [16]. A study of inhibition of invasive plants by various compounds in sweet potato (Ipomoea batatas) found that palmitic acid produced the second highest levels of inhibition in the 4 invasive alien plants tested [29]. Thus, the allelochemicals produced by A. radicans are likely volatile materials primarily released from aboveground plant parts.

Kato-Noguchi et al. [30] were the first investigators to identify allelopathic activity in the congeneric species, Acmella oleracea, demonstrating inhibition of four plant species by A. oleracea substances. They isolated two substances (E, E)-2,4-undecadien-8,10-diynoic acid isobutylamide and nona-(2Z)-en-6,8-diynoic acid 2-phenylethylamide from A. oleracea responsible for the inhibitory effects [30]. Moreover, the phytotoxic potential of A. oleracea extracts and fractions against three plants such as L. sativa, Calopogonium mucunoides, and Ipomoea purpurea was explored and some allelochemicals were identified through GC/MS analysis [31]. The inhibitory effects of these compounds from A. oleracea may be very useful to determine if these or similar compounds produced by A. radicans also show phytotoxic activity. Some active allelochemicals such as (E,E)-2,4-decadienal, 2-tridecanone, γ-cadinene, δ-cadinene, (E)-α-cadinol, spathulenol, caryophyllene oxide, and widdrol in essential oil from A. radicans were further explored in another study [32].

Acmella radicans has been considered as a noxious invasive alien species in north Yunnan Province, China [17]. In a separate study, the MaxEnt model indicated that in China the potential suitable area for A. radicans is primarily located in southern regions of the country. This plant species has demonstrated strong competitive ability when grown with other plants as it can reduce population densities and importance values of many plant species and absorb more nutrients in invaded habitats [17]. Moreover, the seed germination and seeding growth of some associated weeds were markedly inhibited by aqueous extracts of A. radicans [33]. The strong allelopathic potential of A. radicans against B. rapa and C. coronarium has implications for managing A. radicans, in that it appears the negative impacts are not just due to resource competition. Thus, in order to reduce or avoid the allelopathic effects of A. radicans on crop growth, some measures such as utilizing alternative crop species, variation in crop planting times, and removal of A. radicans plant residues should be considered. In addition, positive uses of A. radicans could be explored such as inhibiting neighboring weed species or developing other ways to make use of its herbicidal properties.

5. Conclusion

The findings of this study demonstrated that aqueous extracts from A. radicans significantly inhibited growth of B. rapa (field mustard) and C. coronarium (garland daisy). The germination rate, germination index, root length, shoot length, and biomass were markedly reduced as the concentration of A. radicans extracts increased. The highest inhibition rates on the two bioassay species were observed for root length and biomass, followed by shoot length and germination index, with the seed germination rate experiencing the least inhibition. Among the allelopathic response indices derived from the aqueous extracts of A. radicans, flowers and fruit heads exhibited the strongest inhibitory effects, followed by leaves and stems, while root extracts had the least impact. Additionally, inhibition by A. radicans was generally greater for B. rapa compared to C. coronarium. Most allelopathic response indices for aqueous extracts of A. radicans significantly decreased with increasing concentration, providing strong evidence of its allelopathic inhibition. To gain a more comprehensive understanding of the allelopathic mechanisms of A. radicans, additional investigations are required to identify its allelochemicals and elucidate the biochemical mechanisms involved in its allelopathic activity.

Conflicts of Interest

The authors declare no conflicts of interest.

Author Contributions

S.S. and F.Z. conceived and designed the experiments; G.X., S.Y., G.J., K.Y., X.W., and Z.F. performed the experiments; S.S. and D.R.C. wrote the draft. The final manuscript was read and approved by all authors.

Funding

This research was supported by the National Key R&D Program of China (2021YFC2600400), Ten Thousand Talent Program (Young Top-Notch Talent) of Yunnan Province (YNWR-QNBJ-2018-201), and Key Research and Development Program of Yunnan Province (202103AF140007, 202203AE140008, and 2019IB007).