1. Introduction
Weed species constitute a primary constraint on horticultural productivity, leading to significant yield reductions. Characterized by efficient seed dispersal mechanisms and rapid vegetative growth, their proliferation is often correlated with ecological disturbances and anthropogenic disruptions [1]. The species E. plana is an exotic plant from the South of Africa [2] that is widespread and difficult to control, popularly known as South African lovegrass or tough lovegrass. It has characteristics such as fast seed dissemination because it is small (approximately 0.1 mm × 0.5 mm).
The primary concern regarding Eragrostis weed species is their allelopathic potential, which negatively affects the growth and development of coexisting plant species within the same habitat. These effects are driven by species-specific biochemical interactions rather than solely by population density. Further research is required to clarify their role in facilitating the dispersal of introduced plant species, as well as to assess the potential impacts of their foraging behavior on horticultural systems [3]. The main problem caused by weeds is the reduction in crop yields because they compete for water and nutrients. E. plana is widespread and is competitive with other species. This invasive species has been causing problems since 1971 and also infests Lactuca sativa (lettuce), wheat (Triticum spp.), Trifolium repens (white clover), Lolium rigidum (rigid ryegrass), and Lolium multiflorum [4,5]. E. plana is a diploid, C4 grass species that primarily disperses via endozoochory. It exhibits a flowering period spanning late spring to early summer, with optimal seed germination occurring at 35 °C and viability maintained up to 39–40 °C. The allelopathic properties are attributed to the biosynthesis of various phenolic compounds and alkaloids, which inhibit the growth of coexisting plant species [6]. In this sense, it is important to find a way to control this invasive species.
Some plants have been studied to obtain extracts for controlling weeds and phytopathogens. One example is the E. camaldulensis (eucalyptus), which is a large tree belonging to the Myrtaceae family found in several places in the world. It is used in the production of essential oils due to the antimicrobial and antibiotic activities, among other bioactivities of the extracted compounds [7,8]. L. leucocephala is another example, which is a perennial legume belonging to the Fabaceae family. Allelochemicals are found in the extracts, such as mimosine, which is a toxic amino acid in some plants [9]. Also, C. barbatus, a shrub belonging to the Lamiaceae family, is a plant known for its medicinal use due to the presence of antioxidants in its leaves, roots, and stems. As a bioherbicide agent, the extracts obtained by hydrodistillation exhibited herbicidal potential against Raphanus raphanistrum seeds [10].
The selection of appropriate extraction methodologies is crucial for obtaining bioactive plant extracts with high efficacy. Pressurized Liquid Extraction (PLE) is one such technique that utilizes pressurized solvents at moderate to high temperatures to extract bioactive compounds efficiently, offering benefits such as reduced extraction time and minimal or no organic solvent consumption [11]. Similarly, ultrasound-assisted extraction (UAE) relies on the propagation of mechanical waves through the plant matrix, facilitating the rapid release of target compounds while minimizing energy and solvent requirements [12]. These extraction methods, as well as the infusion method, demonstrated to be efficient in extracting bioactive compounds. Through PLE and UAE, it was possible to extract several compounds from the aerial part of Senecio brasiliensis, with a total of 36 compounds by the PLE method and 17 compounds by the UAE method [13]. The infusion method was also able to extract bioactive compounds such as caffeoylquinic acid flavonoids from the leaves of Cynara cardunculus [14]. Therefore, this study assessed the effectiveness of aqueous and ethanolic extracts of E. camaldulensis, C. barbatus, and L.leucocephala obtained via PLE, UAE, and IF in controlling the invasive E. plana.
2. Materials and Methods
2.1. Raw Materials
The extracts were obtained from leaves of E. camaldulensis, L. leucocephala, and C. barbatus. The leaves were collected in Cachoeira do Sul, Brazil (Lat. −30.099370; Long. −52.908315). The leaves were dried in an oven (Luca-80/27, Lucadema, São José do Rio Preto, Brazil) at 40 °C until reaching constant mass. Thereafter, they were ground in a blender and stored in a refrigerator (approximately 7 °C) for 1–2 weeks for preparation of the extracts. For the extractions, ethanol of 99.8% purity (Dinâmica, Indaiatuba, Brazil) and distilled water were used as solvents. The distilled water was produced immediately before the procedures. The E. plana seeds used in the experiment of pre-emergence were collected and donated by “Embrapa Pecuária Sul” (Bagé, Brazil). For pre- and post-emergence assays, the substrate for plants (Turfa Fértil Agro S/A) was acquired in the local market (Cachoeira do Sul, Brazil).
2.2. Obtaining the Extracts
The experiments were carried out in a completely randomized design with three replications. Three plants (E. camaldulensis, L. leucocephala, and C. barbatus), three extraction methods (infusion, PLE, and UAE), and two solvents (water and ethanol) were tested. The solvent-to-feed mass ratio (S/F) used for all methods was 10 g solvent/g leaves. All ethanolic extracts were evaporated in a rotary evaporator (SKL-25A, Even, London, UK) under a vacuum at 60 °C.
Infusion (IF): The dried and ground leaves of each one of the three plants were mixed with distilled water or ethanol in a beaker at 80 °C and allowed to infuse for 15 min. Afterward, the extracts were filtered using filter paper (Whatman, grade 1). The extracts were stored in amber glass flasks for further tests.
PLE: Dry milled leaves (15 g) were loaded in a 50 mL stainless steel reactor, and the superior lid was closed. The pump (Series III, RoHS, Teledyne SSI, State College, PA, USA) pressurized the solvent, and the band heater increased the temperature to reach 10 MPa and 60 °C, respectively. A static time of 20 min was used to guarantee temperature uniformity. Afterward, the inlet and outlet valves were opened, and a dynamic extraction occurred with a constant solvent flow rate of 8 mL/min for approximately 19 min. The extracts were stored in amber glass flasks for further tests.
UAE: The dried and milled leaves were inserted in beakers, and the solvent was added. Each beaker was inserted on the support of the equipment (QR 750, Eco Sonics, Indaiatuba, Brazil) of 500 W power, and the ultrasonic probe (4 mm thick microtip) was submerged in the mixture for at least 5 mm. The extraction conditions were the maximum power fraction of 50%, a pulse of 0 (continuous operation), and an extraction time of 20 min. The extracts were stored in amber glass flasks for further tests.
2.3. Pre-Emergence of E. plana in Paper Roll
The extracts from three plants (E. camaldulensis, L. leucocephala, and C. barbatus), three extraction methods (IF, PLE, and UAE), and two solvents (water and ethanol) were tested with an additional control (3 × 3 × 2 + 1) consisting of distilled water. The weed seeds were germinated on filter paper of 10 × 10 cm previously sterilized in an oven at 105 °C for 2 h. The seeds (25) germinated between two sheets of paper, which were wrapped in the form of rolls. The amount of extract applied was 2 times the mass of the filter paper. Three repetitions of each condition were applied.
The rolls were placed in plastic bags to maintain moisture and taken to a climatic chamber (MA 402, Marconi, Piracicaba, Brazil) for germination at 25 °C and a photoperiod of 12 h for 21 days. The application of the extracts was performed once and the antifungal ketoconazole was applied in the first and last weeks to avoid the appearance of mold in the experiment. The germinated seeds were counted (seeds that presented radicle equal to or higher than 2 mm were considered germinated) every day until the end of the experiment. Therefore, the responses indicating germination percentage and germination speed index (GSI), which were calculated from the sum of the number of seeds germinated each day, were divided by the number of days since the seeds were set to germinate.
2.4. Pre-Emergence of E. plana in Substrate
Based on the results obtained with pre-emergence in paper rolls, this step was developed with only two plants (E. camaldulensis and C. barbatus) because L. leucocephala did not present satisfactory results in that stage. The three extraction methods (IF, PLE, and UAE) and two solvents (water and ethanol) were maintained in the tests with an additional control (2 × 3 × 2 + 1) consisting of distilled water. Three repetitions of each condition were applied.
Plastic containers measuring 2.5 cm in height and 4.3 cm in diameter were filled with plant substrate (Turfa Fértil Agro S/A, Brazil). Twenty-five E. plana seeds were placed to germinate in each of the containers. Therefore, the seeds were covered with substrate, and the containers were taken to the germination chamber at 25 °C and a 12-h photoperiod for 21 days. The extract was applied immediately after sowing at a volume equivalent to 100 L/ha. On every day, the number of germinated seeds was counted (seeds that presented radicle equal to or higher than 2 mm were considered germinated). The variables analyzed at the end of the experiment were the germination percentage and root and shoot lengths. The variables analyzed at the end of the experiment were the germination percentage and root and shoot lengths.
2.5. Post-Emergence of E. plana in Substrate
The E. plana seeds were placed to germinate in plastic recipients with dimensions of 2.5 cm in height and diameter of 4.3 cm (25 seeds per recipient) filled with plant substrate (Turfa Fértil Agro S/A, Criciúma, Brazil). Afterward, the recipients were taken to the germination chamber at 25 °C and a photoperiod of 12 h for 15 days. After 15 days, the plants were thinned to have the same number of plants, leaving 10 plants per recipient. In the sequence, 4 mL of extract was applied three times a week through a spray bottle for 21 days, with the recipients maintained in the germination chamber. At the end of the experiment, the percentage of dead plants and root and shoot lengths were evaluated.
2.6. Statistical Analysis
The results were analyzed using the Sisvar software 5.6. The data were submitted for the analysis of normality and homogeneity using the Shapiro–Wilk test. When they were significant, the data were submitted to analysis of variance (ANOVA), and the averages were compared using Tukey’s test at 5% error probability.
3. Results
3.1. Pre-Emergence of E. plana in Paper Roll
When using C. barbatus extracts obtained by UAE, the percentage of germination of E. plana seeds was reduced by 89% compared to the control. When E. camaldulensis extracts were used, all methods were demonstrated to be efficient, reducing the percentage of germination of E. plana seeds by 96% to the control. Extracts obtained from C. barbatus and E. camaldulensis were efficient in controlling E. plana regardless of the method used, but when the extracts were obtained from L. leucocephala leaves, only the IF and UAE methods were efficient (Figure 1).
Regarding the solvents, for the C. barbatus extracts, lower percentages of E. plana seed germination occurred when the ethanol solvent was used. For L. leucocephala extracts, using the ethanol solvent extracted compounds that did not inhibit the germination of L. leucocephala seeds, unlike when water was used (reduction in 26.53% of germination to the control). For E. camaldulensis extracts, the germination percentage was not influenced by extracts obtained with different solvents, and both solvents with the extracts of E. camaldulensis and C. barbatus were efficient in reducing the number of germinated seeds (Figure 1). The germination of E. plana seeds was reduced by 96.6% with aqueous extracts.
The GSI of the South African lovegrass was also reduced by the tested extracts (Figure 2). For the three methods (IF, PLE, and UAE), extracts from C. barbatus and E. camaldulensis reduced the GSI of E. plana seeds in relation to the control. However, the reduction in GSI was higher when extracts from E. camaldulensis leaves were used. Regarding the solvents used, the most efficient plant was E. camaldulensis when distilled water was used in the preparation of extracts. When ethanol was used as an extraction solvent, the most efficient plants to reduce the GSI of E. plana were E. camaldulensis and C. barbatus. For L. leucocephala, the use of ethanol increased the GSI of the seeds by 38.53% than the control (Figure 2).
3.2. Pre-Emergence of E. plana in Substrate
As in the pre-emergence experiment with paper roll, the extracts from C. barbatus and E. camaldulensis reduced the germination of E. plana seeds in the assays of emergence in the substrate. This behavior occurred when using the UAE method, providing reductions of 56.86 and 88.63% when using extracts from C. barbatus and E. camaldulensis, respectively, when compared to the control. Regarding the solvent, extracts obtained with distilled water were efficient in reducing the germination percentage when used for C. barbatus and E. camaldulensis leaves in the preparation of extracts, while when ethanol was used, only the E. camaldulensis extracts were efficient (Figure 3).
For the shoot length of E. plana seedlings, extracts from C. barbatus and E. camaldulensis reduced this response when compared to the control (distilled water) by 29.46 and 52.23%, respectively. However, extracts from E. camaldulensis leaves were more efficient when compared to those from C. barbatus leaves. The extracts obtained by PLE were not statistically efficient in reducing the shoot length of seedlings. No statistical difference between the solvents used in the preparation of the extracts was found. Therefore, the extracts of E. camaldulensis and C. barbatus, both distilled water and ethanol, were demonstrated to be efficient in reducing the shoot length of E. plana seedlings (Figure 4).
The root length of the seedlings was also reduced by the applied extracts. When distilled water was used as an extraction solvent, extracts from C. barbatus leaves reduced the root length of E. plana seedlings by 40.84%. When the ethanol solvent was used, the E. camaldulensis leaf extracts reduced the root length of the seedlings by 45.55% to the control (Figure 5).
3.3. Post-Emergence of E. plana with Substrate
The number of senescent E. plana plants was high when E. camaldulensis leaves were used in the preparation of extracts obtained by PLE with water and by UAE with ethanol, causing a 100% senescence. However, for false boldo, no assay caused senescence of E. plana in comparison to the control (Table 1). The extracts from eucalyptus reduced the shoot length (Table 2 and Table 3).
4. Discussion
In most cases, the plant extracts reduced the percentage of germination of the E. plana. When the IF method was used to obtain the extracts, all plants were efficient, reducing the percentage of germination to the control. This result is relevant considering that the IF method consists of placing the plant material in contact with a solvent and leaving it to infuse for a certain time until the active compounds pass into the solvent, becoming an infusion. This brings advantages in relation to the other methods tested, such as the ease of obtaining extracts when compared to the PLE and UAE methods and lower cost and extraction time [15]. For UAE and PLE methods, the emergence using extracts from only C. barbatus and E. camaldulensis differed from the control. Overall, extracts obtained by UAE presented the best results in controlling E. plana, reducing the number of germinated seeds regardless of the plant used to obtain the extracts. This method presented better results compared to the other methods due to its ability to break the cell walls of the plant material by cavitation, allowing higher penetration of the solvent and facilitating the extraction of bioactive compounds. Elsewhere, the UAE method was efficient in extracting compounds from Senecio brasiliensis for weed control, showing better results than the microwave hydrodiffusion and gravity method [13]. In a study comparing the UAE method with the maceration method for obtaining extracts of Hibiscus sabdariffa, the UAE method was demonstrated to be more efficient in obtaining compounds than the maceration method. In addition, the 1:10 biomass/solvent ratio used in the extraction was more efficient than the 1:5 ratio. When comparing the type of solvent used in the extraction, water was more efficient [16]. The UAE method also stood out in relation to the other methods tested for inhibiting the germination and growth of the weed E. plana, and the ratio that had the most effective effect on the extraction of compounds from H. sabdariffa was the same as that one used in the current work, but the extraction time used in the two studies was different: 40 min in the study with H. sabdariffa [16] and 20 min in the current work.
Ethanol-extracted compounds that did not inhibit the germination of E. plana seeds for L. leucocephala extracts using the PLE method, which indicates that extract obtained from L. leucocephala using this solvent is not active against the germination of E. plana seeds. Distilled water extracted compounds with higher bioactivities than ethanol did. Considering water availability and cost, it is an important result when compared to the ethanol solvent, even though they are considered GRAS (Generally Recognized As Safe). In the same trend, water and other solvents such as methanol, n-hexane, acetone, and ethanol were tested in the extraction of phenolic compounds present in Pistacia vera bark (pistachio). The maximum extraction of these compounds was obtained when water was used as a solvent [17].
Among the three plants used in the preparation of extracts, E. camaldulensis was the one that had the highest influence on the invasive species, reducing the number of germinated seeds and GSI. The phytotoxicity of the E. camaldulensis extracts on the invasive grass is explained by the bioactive compounds found in this plant, causing allelopathy in the invasive species. They can be used as soil correctives to improve soil microbial quality [18] and as a biotic control of some fungal pathogens, such as Exserohilum ssp., Fusarium ssp., and Nigrospora ssp., along with improving the growth and productivity performance of zucchini (Cucurbita pepo) [19].
The allelopathy of eucalyptus (Eucalyptus erythrocorys) was also tested on two weed species: Sinapis arvensis, belonging to the Brassicaceae family, and Phalaris canariensis, belonging to the Poaceae family. The extracts inhibited the germination and growth of the tested weeds, with the highest inhibition observed at the highest concentration (30 μL/mL) of applied extracts [20]. Leaves from the Eucalyptus globulus were tested as a soil fertilizer to assess their influence on weed emergence in maize cultivation. The potential of the leaves as a fertilizer was confirmed, as they reduced the appearance of dicotyledonous weeds in maize cultivation [21]. The species Eucalyptus maculata was used in a trial to test its bioherbicidal potential on the weeds Sinapis arvensis and Phalaris canariensis, both pre- and post-emergence, for which the essential oil from the leaves of E. maculata was extracted by hydrodistillation. As in the current work, eucalyptus caused inhibition of the germination and growth of the weeds in which it was tested, although the method used to obtain the extracts was different. In addition, an analysis of the compounds present in the oil was carried out, identifying 46 compounds, with 1,8 cineol as the major one [22]. This demonstrates the potential of Eucalyptus species in weed control, as observed in the current study.
Some other evaluations against the germination and development of E. plana have been performed. For example, the effect of aqueous extracts of Luehea divaricata prepared by infusion was tested on the germination and initial growth of E. plana. However, the aqueous extracts did not interfere with the germination percentage and GSI of this invasive species [23]. The results obtained in that study were different from those presented in the current work, where the extracts obtained by infusion with E. camaldulensis and C. barbatus significantly reduced the germination percentage and GSI of E. plana.
Aqueous and ethanolic extracts of cultivated thistle (Cynara cardunculus) were applied against four weeds commonly infesting horticulture: Amaranthus retroflexus, Portulaca oleracea, Stellaria media, and Anagallis arvensis. The inhibitory effect of the extracts on the weeds was tested, concluding that the ethanolic extracts completely inhibited the germination of the seeds of all weeds [14]. Also, the essential oil from Foeniculum vulgare had a significant (p < 0.05) inhibition effect on germination and early growth of field bindweed seedlings of Convolvulus arvensis when compared to the control [24]. In this work performed with extracts from C. barbatus, L. leucocephala, and E. camaldulensis, ethanolic extracts were also efficient against E. plana seeds when C. barbatus and E. camaldulensis were used, thus demonstrating another option of extracts for weed control. Otherwise, for L. leucocephala, extracts obtained with ethanol were not efficient in reducing the germination of E. plana seeds.
Extracts from E. camaldulensis showed a higher reduction in the GSI of E. plana. These results are explained due to the compounds that the Eucalyptus species have in their composition, such as the terpenes eucalyptol, camphenol, pinocarveol, pinocarvone, exo-2-hydroxycineole, and α-terpineol, which have bioactivities to inhibit the development of another species through bioherbicide effects [25]. Extracts from E. camaldulensis were applied to weed cover in corn (Zea mays), showing satisfactory results in reducing 21.21% the density of weeds and increasing corn productivity [26]. The leaves of E. globulus were used to obtain aqueous extracts for testing in pre- and post-emergence of a weed grass (Agrostis stolonifera). In this case, the inhibition from 60 to 68% to control and reductions in the length of A. stolonifera was reported when the concentration of extract of 33.33 g/L was used, in addition to affecting the photochemical state of the plants in the post-emergence [27].
Ethanol extracts from E. erythocorys were tested on the germination and growth of field mustard (Sinapis arvensis), canary seed (Phalaris canarienses), and wheat (Triticum durum). The extracts showed an inhibitory effect on the germination and growth of all species [20]. In a study carried out to control weeds found in peas, extracts from Eucalyptus citriodora leaves were tested, demonstrating that the extracts were efficient in controlling weeds and increasing the yield of pea seeds [28]. As in the current work, these findings demonstrate that Eucalyptus extracts from several species show allelopathy in weeds, having the potential to be used as bioherbicides. The evaluation of bioherbicides has been increasingly explored, seeking their replacement for synthetic herbicides, using microorganisms and plant extracts to obtain these products [29,30].
In the experiment carried out in pre-emergence with the substrate, the extracts obtained by UAE caused the highest reduction in the germination of E. plana seeds. The UAE was also more efficient than other methods, as compared to the maceration method aimed at extracting compounds such as chlorophylls and carotenoids from spinach [31]. The same behavior occurred in a study conducted by Aryanti et al. [32]. The UAE method was more effective in the extraction of saponins from Sapindus rarak than the macerated method, obtaining the maximum extraction of saponins (27.87 mg/100 mg feed) at an extraction time of 40 min. It is three times faster than the time applied in the maceration method to obtain 23.74 mg saponins/100 mg feed. The efficiency of the UAE method was also reported for the extraction of a bioactive compound from the pomegranate bark: the ellagic acid. This compound is known for its antioxidant activity, which is commonly found in fruits and vegetables [33].
The results of root and shoot length measurements indicate that E. camaldulensis exhibited the highest inhibitory efficacy among the tested plant species when phytochemicals were extracted using UAE with ethanol. The treated seedlings showed a 2.2-fold and 2.6-fold reduction in root and shoot lengths, respectively, compared to the control. Anchieta et al. [34] studied the phytotoxic effect of extracts obtained from Capsicum baccatum in the control of the weed Hovenia dulcis. When using the same extraction method (UAE), they found satisfactory results, with the root and shoot lengths 3.0 and 2.6 smaller, respectively, than the control. In this sense, the reduction in root and shoot lengths of E. plana seedlings is explained by the ability of the UAE method to extract the desirable phytotoxic compounds from the plants. According to Hassan et al. [30], phytochemical compounds found in plants are inhibitors of the morphological properties of weeds, reducing their development capacity, which interferes with the root and shoot lengths.
Leaves and flowers from Ocimum basilicum were used in the preparation of extracts to test their allelopathic potential in weeds. The experiment in pots showed the allelopathy of the extracts on the weeds Portulaca oleracea and Amaranthus spp. when applied in the post-emergence of the species [35]. Also, the effect of extracts from bark, wood, and leaves of Haldina cordifolia was evaluated on the germination and initial growth of two weeds: Mimosa pigra and Cenchrus echinatus. The extracts presented a phytotoxic effect on the weeds, highlighting that the extracts from the bark have the highest phytotoxicity [36]. In some cases, it is possible to formulate a powder containing biomolecules. It allows for a longer shelf life and storage facilities (low need for storage spaces).
For C. barbatus, no motality of E. plana was seen in comparison to the control. The C. barbatus leaves were not efficient at this stage of the E. plana but showed good results in the pre-emergence of the species. This confirms that C. barbatus has compounds capable only of interfering with the pre-emergence of E. plana, but these phytotoxic compounds are not enough to control the species after it is already developed [10].
Peumus boldus Mol. also demonstrated its bioherbicide potential in the pre-emergence of weeds, such as P. oleracea, commonly known as purslane. The compounds identified as responsible for inhibiting the germination of P. oleracea were p-cymene, 1–8 cineole, and ascaridole [37]. Otherwise, E. camaldulensis extracts were efficient in the post-emergence, which demonstrated to be efficient in all tested conditions of E. plana development. When analyzing the effect of the toxicity of essential oils of Eucalyptus citriodora and Cymbopogon nardus on the post-emergence of the weeds Digitaria horizontalis and Cenchrus echinatus, it was found that 12 h after application of the oil signs of toxicity began to be noticed, with the greatest signs being noted in the plants treated with E. citriodora oils. In addition, there was a reduction of more than 50% in the dry mass of the aerial part and root of the weeds treated with this same oil. E. citriodora was more efficient in terms of plant toxicity than C. nardus [38], as in the current work, which E. camaldulensis was more efficient in terms of phytotoxicity in a post-emergence test with E. plana plants than C. barbatus.
Generally, both distilled water and ethanol solvents were efficient in extracting compounds that caused senescence in E. plana plants, but the efficiency or inefficiency of each solvent depended on the extraction method used in the preparation of the extracts. The leaves, roots, and stems from Croton bonplandianum were used in the preparation of aqueous extracts to be tested on the germination and initial growth of cultivated plants (Triticum aestivum, Brassica oleracea, and Brassica rapa) and weeds (Melilotus alba, Vicia sativa, and Medicago hispida). The extracts from all botanical parts did not have an allelopathic effect on plant germination, but extracts from the roots and leaves inhibited the root length of the tested species [39].
The UAE method, as well as in the other experimental tests, was also efficient when tested in the post-emergence of E. plana, causing total senescence of the seedlings. This demonstrates that, regardless of the stage of E. plana, the UAE method can extract compounds that are efficient in controlling E. plana both in the pre- and post-emergence of the species, such as phenolic compounds and lactones [14]. The essential oils of lavender (Lavandula angustifolia), lemongrass (Cymbopogon citratus), and citronella (Cymbopogon winterianus), obtained by hydrodistillation, also showed satisfactory results in controlling E. plana [40]. However, in comparison with extracts obtained by the UAE method using E. camaldulensis leaves in the current work, they present a disadvantage due to their lengthy extraction time and low yield of essential oils. The extracts obtained by the UAE method offer rapid extraction and enable obtaining larger quantities of extract. Considering the results obtained, as well as the fact that E. plana is considered an invasive plant that is difficult to control, it is stated that the extracts showed satisfactory effects in controlling the species.
The ability of plant extracts to interfere with weed germination and development is primarily attributed to the presence of bioactive secondary metabolites. Compounds such as steroids, tannins, ketones, aldehydes, quinones, phenolics, flavonoids, terpenoids, and lactones exhibit allelopathic activity, potentially serving as bioherbicidal agents against invasive weed species [41,42]. The allelopathic effects of E. camaldulensis and C. barbatus extract on E. plana can be explained by the presence of these bioactive compounds. In Eucalyptus spp., several compounds with bioherbicidal potential have been identified, including 1,8-cineole, limonene, α-pinene, and β-pinene [43]. Additionally, in Eucalyptus flocktoniae, essential oil analyses have reported the presence of 1,8-cineole, trans-pinocarveol, and α-pinene, further supporting the allelopathic potential of these species of Eucalyptus [44].
Terpenes, such as carvone, have been identified in plant species belonging to the Lamiaceae family and are associated with inhibitory effects on seed germination, radicle elongation, and pedicel growth in weed species [45]. Also, monoterpenes such as carvacrol, α-terpinene, and thymol have been reported to interfere with photosynthesis, cellular respiration, and mitotic activity, disrupting key physiological processes in plants [46].
A study on Piper nigrum fruit extracts utilized maceration, UAE, and Soxhlet extraction to assess the recovery of bioactive compounds, particularly phenols and flavonoids. The results demonstrated that UAE achieved the highest phenolic compound yield, highlighting its efficiency in extracting bioactive metabolites with allelopathic properties [47]. Also, eco-friendly elicitors produced from rosemary and eucalyptus essential oils demonstrated success in preserving cucumber fruit quality [8]. In the present study, extracts obtained by UAE effectively induced allelopathic effects on E. plana, likely due to the enhanced ability of the method to extract key secondary metabolites such as phenols and flavonoids, which are known to contribute to plant–plant interactions and bioherbicidal activity.
5. Conclusions
Extracts from leaves of E. camaldulensis and C. barbatus showed a significant allelopathic effect on germination, germination speed index, root and shoot lengths, and senescence of E. plana weed. Extracts from E. camaldulensis were the most efficient in controlling the weed because the extracts reduced the germination by 96.9% and the GSI by 97.5%, in addition to causing total senescence of seedlings of E. plana Ness. Regarding the extraction methods, ultrasound-assisted extraction was the most efficient one in extracting bioactive compounds for controlling this invasive species. Regarding the type of solvent, the extracts from E. camaldulensis and C. barbatus obtained with either water or ethanol showed positive results in the control of E. plana. Thus, based on an overall assessment of the results, the extracts of E. camaldulensis leaves obtained by the ultrasound-assisted extraction method can be considered a potential bioherbicide for controlling E. plana, suggesting the development of further studies on concentration and formulation of a bioproduct.
Conceptualization, B.M.D., V.D.-S.F., M.V.T. and G.L.Z.; methodology, B.M.D. and R.P.; formal analysis, B.M.D.; investigation, B.M.D. and R.P.; writing—original draft preparation, B.M.D. and G.L.Z.; writing—review and editing, B.M.D., V.D.-S.F., M.V.T. and G.L.Z.; visualization, G.L.Z.; supervision, G.L.Z.; project administration, G.L.Z.; funding acquisition, G.L.Z. All authors have read and agreed to the published version of the manuscript.
Data will be made available upon request.
The authors thank UFSM for the physical structure and equipment for experiments.
The authors declare no conflicts of interest.
Footnotes
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Enlarge this image.Figure 1. Effects of the extracts obtained by the IF, PLE, and UAE methods with the plants C. barbatus, L. leucocephala, and E. camaldulensis, and the solvents distilled water and ethanol on the germination of E. plana seeds in a paper roll test: (A) statistical comparisons for each method and plant; (B) statistical comparisons for each solvent and plant. Lowercase letters for plants and uppercase letters for methods (A) and lowercase letters for plants and uppercase letters for solvents (B) do not differ statistically from each other by Tukey’s test at the level of 5% error probability.
Enlarge this image.Figure 2. Effects of the extracts obtained by the IF, PLE, and UAE methods with the plants C. barbatus, L. leucocephala, and E. camaldulensis, and the solvents distilled water and ethanol on the GSI of E. plana in a paper roll test: (A) statistical comparisons for each method and plant; (B) statistical comparisons for each solvent and plant. Different letters for each graphic (A,B) indicate statistical differences according to Tukey’s test at a 5% probability of error.
Enlarge this image.Figure 3. Effects of the extracts obtained by the IF, PLE, and UAE methods with the plants C. barbatus and E. camaldulensis and the solvents distilled water and ethanol on the germination of E. plana in a substrate test; (A) statistical comparisons for each method and plant; (B) statistical comparisons for each plant and solvent. Lowercase letters for plants and uppercase letters for methods (A), and lowercase letters for plants and uppercase letters for solvents (B) do not differ statistically from each other by Tukey’s test at the level of 5% error probability.
Enlarge this image.Figure 4. Effects of the extracts obtained by the IF, PLE, and UAE methods with the plants C. barbatus and E. camaldulensis and the solvents distilled with water and ethanol on the shoot length of E. plana in test with substrate; means followed by the same letter; comparisons for each group of treatment (plant, method, and solvent) do not differ statistically from each other by Tukey’s test at the level of 5% error probability.
Enlarge this image.Figure 5. Effects of the extracts of C. barbatus and E. camaldulensis obtained with the solvents distilled water and ethanol on the root length of E. plana in a test with substrate; means followed by the same letter do not differ statistically from each other by Tukey’s test at 5% probability of error.
Motality percentage of E. plana in a post-emergence experiment with substrate submitted to the application of extracts from leaves of C. barbatus and E. camaldulensis obtained by IF, PLE, and UAE with water or ethanol.
| Plant | Motality (%) * | ||||||
|---|---|---|---|---|---|---|---|
| IF Water | IF Ethanol | PLE Water | PLE Ethanol | UAE Water | UAE Ethanol | Control | |
| C. barbatus | 3.33 ± 23.30 bB | 3.33 ± 23.30 bB | 40.00 ± 51.96 bAB | 66.67 ± 20.81 aA | 56.67 ± 32.14 aA | 13.33 ± 15.27 bB | 26.67 ± 23.09 AB |
| E. camaldulensis | 60.00 ± 34.64 aAB | 76.67 ± 18.10 aA | 100.00 ± 0.00 aA | 26.67 ± 15.27 aB | 63.33 ± 5.77 aAB | 100.00 ± 0.00 aA | 26.67 ± 23.09 B |
| Control | 26.67 ± 23.09 ab | 26.67 ± 23.09 b | 26.67 ± 23.09 b | 26.67 ± 23.09 a | 26.67 ± 23.09 a | 26.67 ± 23.09 b | |
* Means followed by the same letter, lowercase in the column and uppercase in the row, do not differ statistically from each other by Tukey’s test at the level of 5% error probability; IF: infusion; PLE: pressurized liquid extraction; UAE: ultrasound-assisted extraction.
Shoot length of E. plana plants in a post-emergence experiment with substrate submitted to the application of extracts from leaves of C. barbatus and E. camaldulensis obtained by IF, PLE, and UAE with water or ethanol.
| Plant | Shoot Length (cm) * | ||||||
|---|---|---|---|---|---|---|---|
| IF Water | IF Ethanol | PLE Water | PLE Ethanol | UAE Water | UAE Ethanol | Control | |
| C. barbatus | 2.64 ± 0.67 bA | 1.57 ± 1.36 aA | 2.75 ± 0.77 bA | 2.15 ± 0.40 aA | 2.85 ± 0.92 aA | 2.98 ± 0.21 bA | 2.43 ± 1.68 A |
| E. camaldulensis | 1.20 ± 1.04 abA | 1.92 ± 0.10 aB | 0.00 ± 0.00 aA | 2.04 ± 0.28 aB | 1.94 ± 0.53 aB | 0.00 ± 0.00 aA | 2.43 ± 1.68 B |
| Control | 2.43 ± 1.68 b | 2.43 ± 1.68 a | 2.43 ± 1.68 b | 2.43 ± 1.68 a | 2.43 ± 1.68 a | 2.43 ± 1.68 b | |
* Means followed by the same letter, lowercase in the column and uppercase in the row, do not differ statistically from each other by Tukey’s test at the level of 5% error probability; IF: infusion; PLE: pressurized liquid extraction; UAE: ultrasound-assisted extraction.
Root length of E. plana plants in a post-emergence experiment with substrate submitted to the application of extracts from leaves of C. barbatus and E. camaldulensis obtained by IF, PLE, and UAE with water or ethanol.
| Plant | Root Length (cm) * | ||||||
|---|---|---|---|---|---|---|---|
| IF Water | IF Ethanol | PLE Water | PLE Ethanol | UAE Water | UAE Ethanol | Control | |
| C. barbatus | 1.90 ± 0.24 bA | 1.03 ± 1.19 aA | 1.50 ± 0.17 bA | 1.68 ± 0.17 aA | 1.64 ± 0.55 aA | 1.78 ± 0.09 bA | 1.33 ± 0.20 A |
| E. camaldulensis | 0.86 ± 0.55 aB | 1.28 ± 0.86 aB | 0.00 ± 0.00 aA | 1.65 ± 0.90 aB | 1.12 ± 0.66 aB | 0.00 ± 0.00 aA | 1.33 ± 0.20 B |
| Control | 1.33 ± 0.20 ab | 1.33 ± 0.20 a | 1.33 ± 0.20 b | 1.33 ± 0.20 a | 1.33 ± 0.20 a | 1.33 ± 0.20 b | |
* Means followed by the same letter, lowercase in the column and uppercase in the row, do not differ statistically from each other by Tukey’s test at the level of 5% error probability; IF: infusion; PLE: pressurized liquid extraction; UAE: ultrasound-assisted extraction.
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Details
; Pfeifenberg, Renan 1
; Viviane Dal-Souto Frescura 2
; Tres, Marcus Vinícius 1
; Giovani Leone Zabot 1
1 Laboratory of Agroindustrial Processes Engineering (LAPE), Federal University of Santa Maria (UFSM), 3013, Taufik Germano Rd., Cachoeira do Sul 96503-205, RS, Brazil;
2 Natural and Exact Sciences Center, Federal University of Santa Maria (UFSM), Roraima Av., 1000, Santa Maria 97105-900, RS, Brazil