1 Introduction

Wheat is considered one of the most important crops in the winter season in Egypt. While rice is dominant among the summer crops, the local production of wheat is about 8.8 million tons obtained from 3.13 million feddan (FAO 2018) and it requests about 30% of caloric intake of the Egyptians dietary. Nearly half of the world’s population relies on rice as a staple food (Lu 2004). Undoubtedly, weeds are representing severe biotic stress toward these important economic crops. More specifically, weeds are responsible for reduction of 23% of wheat yields worldwide (Oerke 2006) also, caused a significant decline in rice output. Some weeds are also shown for their allelopathic or toxic effect on the wheat germination and seedling growth (Fragasso et al. 2013). Chenopodium album one of the most predominant weed in the wheat fields (Siddiqui and Bajwa 2001) which lead to significant reduction in grain yield up to 65% (Siddiqui 2005). Echinochloa crus-galli (L.) Beauv. is the most prevalent rice weed. One of the worst rice weeds is thought to be this one. Echinochloa spp. infestations in rice can result in extremely severe and variable yield losses depending on the cultivar and the length of the competition (Fischer et al. 1997). Accurate weed control is needed for wheat- and rice-based food security for every country of the world. The expense of cultivation is increased by using conventional weed management techniques that involve human weeding (Singh et al. 2016).

Synthetic pesticides are used in agriculture all over the world, but their widespread usage has led to pest resistance, and they can also have a negative impact on both the environment and human health. This has caused alternative development tactics for novel environmentally friendly biodegradable compounds, particularly in organic farming, to be given more thought (Jabran et al. 2015). Therefore, replacement of them with natural compounds would significantly advance environmental protection. Plant-derived natural compounds have advantages over synthetic chemicals such as fast biodegradability, low risk of subsequent pest/weed resistance, and relatively weak toxicity to non-target organisms (Chandler et al. 2011; Isman 2015; Pavela and Benelli 2016). A biological approach to weed control provides a fantastic alternative in organic systems. Allelochemicals form the basis of the biological process. The majority of allelochemicals, including O, alkaloids, tannins, and glycosides, are categorized as secondary metabolites in plants and are used as substitute weed-control methods (Kruse et al. 2000). Allelochemicals can simulate or inhibit the germination and growth of plants. Proverbially, plant-derived allelochemicals do not exert residual or toxic effects. Therefore, they are considered as the perfect alternates for synthetic herbicides (Cheng and Cheng 2015). According to Bonanomi et al (2006) and Macías et al. (2007), plant essential oils (EOs) and water extracts can be employed as a medium for the production of allelochemicals activity to inhibit the growth of other species. Allelochemicals have been recommended by a number of studies for usage in field applications as well as laboratory settings for the reduction of weeds (Farooq et al. 2011). According to Jamil et al (2009), using allelopathic water extract to control weeds is a significant and practical technique to take advantage of the allelopathic potential of agricultural plants. Due to their volatility, essential oils make effective fumigants. EOs are less hazardous to humans, animals, and the environment than synthetic products. Due to their unique composition and quick degradation in soil, EOs are regarded as environmentally safe (Grosso et al. 2010). EOs have been shown to produce allelopathic effects (Verdeguer et al. 2020). Terpenoids and allelochemicals in EOs can be utilized to stop the germination and growth of weeds (Nikolova and Berkov 2018).

The aims of this study were to characterize the chemical constituents of essential oils in two therapeutic plants, rosemary and thyme using GC–MS, and evaluate their water extracts and essential oils as allelopathic agents on seed germination and seedling growth of wheat and rice, as well as their associated weeds Chenopodium album and Echinochloa crus-galli.

2 Materials and Methods

2.1 Preparation of Plant Materials

The plant samples of rosemary and thyme were supplied from a local farm in Burg El-Arab (45 km west of Alexandria), Egypt. The leaves of the two species were carefully selected, rinsed under running water to remove dust and impurities from the surface, and then dried at room temperature for 5 days before being ground into powder using a Wiley mill (Model 4-GMI). Seeds of wheat and rice were purchased from the Agriculture Research Centre, El-Dokki, Giza, Egypt. While during the spring of 2022, seeds of Chenopodium album and Echinochloa crus-galli were gathered from some agricultural fields near Alexandria. The seeds were lightly washed on surface and kept in paper bags in the refrigerator until needed.

2.2 Preparation of Aqueous Extracts

By soaking 100 g of the air-dried donor plant materials (rosemary and thyme) in 1 L of distilled water at (20 ± 2°C) for 24 h with shaking, stock aqueous extract was made. To remove the fibre debris, the mixture was filtered through four layers of cheesecloth. Then, Whatman No. 1 filter paper was used, and the purified extract was then adjusted to pH 6.8 with NaOH 10%. From the stock solution, various concentrations (10, 20, 50, and 100%) as well as the control with distilled water were prepared.

2.3 Extraction of Essential Oils

The essential oils were extracted from rosemary and thyme by hydro-distillation through passing steam produced in one round-bottomed flask, into one more 3-L flask containing 1 kg of the pure grinded leaves of rosemary and thyme along with 1 L distilled water. The steam crosses the plant, charged with essential oil so the vapour mixture of water–oil passes to the condenser, where it is condensed. The process was carried out for 2 h to obtain maximum oil and was repeated until no more oil was left. After condensation, the condensate (water–oil) was collected in a round-bottomed flask. Subsequently, the oil was separated from water by decantation after every fraction. Finally, the essential oil was dried on anhydrous sodium sulphate and stored at 4°C before, GC–MS analysis.

2.4 GC–MS analysis of Essential Oil

Shimadzu-QP-2010 ULTRA gas chromatography/mass spectrometry was used to evaluate both essential oil samples. The oven temperature was programmed at 60°C for 3 min and then raised to 260°C with 5 rate and hold time 10.00 min. In the TRB-WAX column (30 m × 0.25 mm internal diameter and film thickness 0.25 m) at a flow rate of 1.9 mL/min, helium carrier gas was used, injection mode was split, and injection temperature was 200°C. The oven temperature was programmed at 60°C for (3 min), then raised to 260°C with 5 rate, and hold time 10.00 min, and the ionization mode was electronic impact mode (SEI). By comparing their mass spectra with the National Institute of Standards and Technology (NIST11s) database library and other available references, the components of the oil were identified.

2.5 Germination Bioassay

The research was carried out in the lab of Faculty of Science, Botany and Microbiology Department in 2022. The experiment was run as a factorial with three replications using a completely randomized statistical design. Sodium hypochlorite solution was used to sterilize seeds of wheat and rice, as well as Chenopodium album and Echinochloa crus-galli for about 2 min. The seeds were then repeatedly rinsed in distilled water. Under standard laboratory settings with day temperature range of 25–30°C and night temperature range of 20–25°C, a total of seeds from each plant were distributed in 9-cm diameter Petri-dishes on 2 discs of Whatman No.1 filter paper wetted with the solutions previously prepared of water extracts with varying concentrations (10, 20, 50, and 100%) and essential oil with various doses of 2, 4, 8, and 16 µL per petri dishes in three replicates to test the inhibitory effect of the two plants: rosemary and thyme. Control was prepared by adding distilled water. Each Petri dish’s number of germinations was counted every day.

2.6 Specific Analyses

Germination parameters such as germination percentage (GP%), the coefficient of velocity of germination (CVG), speed of germination (SG), germination energy (GE), germination index (GI), mean germination time (MGT), seedling shoot length (SL), seedling root length (RL), shoot–root ratio (SL/RL ratio), and seedling vigour index(SVI) were recorded by the following equations:

Germination percentage (GP%) = number of germinated seeds/total number of seeds × 100.

The Coefficient of Velocity of Germination (CVG) = N1 + N2 + … + Ni/100 × N1T1 + … + NiTi where N is the total number of seeds germinated every day and T is the number of days corresponding to N and i is the last day of germination (Jones and Sanders 1987).

Speed of germination (SG) = Ni/Di number/day (Khan and Ungar 1998).

As Ni is the number of seeds germinated on day I, Di is the number of days after sowing,

Germination energy (GE) = Number of germinated seeds at 4 days/Total number of seeds tested × 100.

Germination index (GI) = ∑ (Et / Dt) where Et is the number of emergence seedlings in t days and Dt is the number of corresponding germination days.

Mean germination time (MGT) = Σ fx/(Σ (f1* × 1) + ………).

Seedling shoot length (SL), seedling root length (RL), shoot–root ratio (SL/RL ratio): Five plant individuals per treatment were used for determination of shoot and root lengths using measuring tape. The shoot/root ratio was calculated for each treatment.

Seedling vigour index (SVI) = Seedling total length (cm) × germination percentage as described by Hossain et al. (2006)

2.7 Statistical Analysis

Results were given as mean ± SE of the mean. Using the CoHort Software Company’s COSTAT 2.00 statistical analysis programme, the data from the current investigation were subjected to the standard one-way analysis of variance (ANOVA) (Zar 1984). Using Duncan’s multiple range test, the differences between treatment means were separated at a 5% level of significance.

3 Results

3.1 Essential Oil Composition

The GC–MS analysis of the pale-yellow oil isolated from Egyptian rosemary leaves has been shown 50 identified components representing 100% of the total oil (Table 1 and Fig. 1). The dominant constituents were eucalyptol (1,8-cineole) with 48.01% of the relative area, α-pinene (11.50%) and camphor (8.65%). Concerning thyme, GC–MS analysis of the extracted reddish-brown oil has been shown 65 identified components representing 100% of the total oil (Table 2 and Fig. 2). The dominant components were thymol with 14.79% of the relative area, o-cymene (8.97%), caryophyllene (8.91%), and gamma-terpinene (4.66%).

Table 1 Composition of Rosmarinus officinalis L. essential oil (the dominant constituents illustrated in bold)
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Fig. 1
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Gas chromatography–mass spectrometry (GC–MS) chromatogram of Rosmarinus officinalis L. essential oil

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Table 2 Composition of Thymus vulgaris L. essential oil (the dominant constituents illustrated in bold)
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Fig. 2
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Gas chromatography–mass spectrometry (GC–MS) Chromatogram of Thymus vulgaris L. essential oil  

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3.2 Allelopathic Effect

The potential allelopathic effects of rosemary and thyme of both aqueous extracts and essential oils in germination of wheat and its associated weed, Chenopodium album, were illustrated in Fig. 3 and Plate 1. The inhibitory activity was observed significantly with the increase of concentration of both aqueous extracts and essential oils of both plants specially thyme essential oil which prevent the germination of weed totally (Plate 1d). Data showed that there is notable increase in germination percentage of wheat for both aqueous extracts of the two plants in low concentrations (10 and 20%) compared to respective control, while it reduced at high concentrations. In general, the inhibition in germination percentage was higher for the essential oil of thyme than rosemary, and it was more pronounced at increasing dose of the respective oil.

Fig. 3
figure 3

Effect of different concentrations of aqueous extract and essential oils of Rosmarinus officinalis L. (a) and Thymus vulgaris L. (b) on germination percentage (%) of Triticum aestivum L. and Chenopodium album L. Columns with same letters did not differ between treatments using a Tukey’s test at 5% of probability, figures expressed as mean ± standard error

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Plate 1
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Effect of different concentrations of aqueous extract and essential oils of Rosmarinus officinalis L. (a, b) and Thymus vulgaris L. (c, d) on germination of Triticum aestivum L. and Chenopodium album L

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The potential allelopathic effects of different doses for both aqueous and essential oils of rosemary and thyme in different germination parameters of Triticum aestivum (wheat) and its associated weeds Chenopodium album were illustrated in Table 3 and 4, respectively. Results showed that doses of essential oils for rosemary were affected less compared to thyme which has negative effect on the coefficient of velocity of germination (CVG), speed of germination (SG), germination index (GI), and seedling vigour index (SVI).

Table 3 Effect of different concentrations of aqueous extract and essential oils of Rosmarinus officinalis L. on different germination parameters of Triticum aestivum L. and Chenopodium album L
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Table 4 Effect of different concentrations of aqueous extract and essential oils of Thymus vulgaris L. on different germination parameters of Triticum aestivum L. and Chenopodium album L
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The germination energy (GE) of wheat was reported to increase as the dose of aqueous extract for both thyme and rosemary increase; however, it decreased as the dose of the two essential oils increased. In comparison to rosemary essential oil, thyme essential oil had a stronger allelopathic effect on the tested weed. The inhibitory effects of the two different essential oil types and doses against Chenopodium album were also found to differ significantly, according to the results.

Mean germination time (MGT) was increased as the concentration of aqueous and essential oil increased; the highest increase of wheat (5.28%) was attained at 10% aqueous extract of rosemary treatment when compared with control. While the highest increase (78%) was recorded at 8 and 16 µL essential oil of rosemary, results showed that there was notable increase for MGT of Chenopodium album in response to the effect of the aqueous and essential oil of rosemary, 20% attained with the highest increase (4%) and 16 µL essential oil attained with the highest value (3.36%) compared with control.

Wheat seedling shoot length (SL) increased significantly when aqueous extract and essential oils of thyme and rosemary were used at lower dosages. The highest increase recorded at 20% aqueous extract and 2 µL essential oil on the other hand seedling root lengths (RL) in all treatment of both two aromatic plants this inhibition is obvious clearly in Chenopodium album, Shoot Root ratio (SL/RL ratio) in wheat increase at lower doses of aqueous extract of the two aromatic species.

The effects of rosemary and thyme of both aqueous extracts and essential oils in germination of rice and its associated weed Echinochloa crus-galli were showed in Fig. 4 and Plate 2. The study recorded that there was significant increase in germination percentage of rice which treated with both aqueous extract and essential oils of rosemary, and the highest germination percentage attained in 20% aqueous extracts and 4 µL essential oils (100% GP), while the percentage of germination reduced significantly for Echinochloa crus-galli specially at high concentrations treatments. Results showed that essential oil dose of thyme had inhibition effect on seed GP of both rice and its associated weed Echinochloa crus-galli, but inhibition was more remarkable in weed.

Fig. 4
figure 5

Effect of different concentrations of aqueous extract and essential oils of Rosmarinus officinalis L. (a) and Thymus vulgaris L. (b) on germination percentage (%) of Oryza sativa L. and Echinochloa crus-galli L. Columns with same letters did not differ between treatments using a Tukey’s test at 5% of probability, figures expressed as mean ± standard error

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Plate 2
figure 6

Effect of different concentrations of aqueous extract and essential oils of Rosmarinus officinalis L. (a, b) and Thymus vulgaris L. (c, d) on germination of Oryza sativa L. and Echinochloa crus-galli L

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Data showed that thyme has more negative affect on different germination parameters of rice and its associated weeds Echinochloa crus-galli when compared to rosemary (Table 5 and 6). Our experiment result also reveals that inhibition was noted in Echinochloa crus-galli. On the other hand, low concentrations of aqueous extract and essential oils for rosemary enhance the growth of rice, and the highest enhancement was recorded at 20% aqueous extract and 4 µL essential oil in both germination percentage (GP%), the coefficient of velocity of germination (CVG), speed of germination (SG), germination energy (GE), germination index (GI), and seedling vigour index (SVI), while aqueous and essential oil not-significantly reduced mean germination time (MGT) of rice compared to respective control treatments while it was noted that it increase in Echinochloa crus-galli.

Table 5 Effect of different concentrations of aqueous extract and essential oils of Rosmarinus officinalis L. on different germination parameters of Oryza sativa L. and Echinochloa crus-galli L
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Table 6 Effect of different concentrations of aqueous extract and essential oils of Thymus vulgaris L. on different germination parameters of Oryza sativa L. and Echinochloa crus-galli L.
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The inhibition was noted in Echinochloa crus-galli in most germination parameters; these results showed that essential oil dose of thyme had inhibition effect on seed GP of both rice and weed, but inhibition effect of Echinochloa crus-galli was more remarkable.

It was reported that there was significant enhancement of seedling shoot length (SL) of rice at lower doses (20%) of rosemary and (10%) thyme aqueous extract, while seedling root lengths (RL) fluctuated in both two aromatic plants but data recorded notable reduction in Echinochloa crus-galli especially under effect of thyme, shoot–root ratio (SL/RL ratio) in rice increase at all doses of aqueous extract of the two aromatic species.

4 Discussion

Data of the GC–MS analysis of rosemary leaves showed that the dominant constituents were eucalyptol (1,8-cineole), α-pinene, and camphor, in accordance with most results obtained by different authors in several countries (Micić et al. 2021). However, there are usually considerable variations in the percentage of the major components of rosemary EO cultivated in different geographical origins. Saviuc et al. (2016) identified α-pinene being 19.01%, followed by the 1,8-cineole (eucalyptol) with 5.49% and camphor (5.71%). Giarratana et al. (2016) recognized α-pinene, camphor and eucalyptol with 23.98%, 22.62%, and 18.76%, respectively. Atti-Santos et al. (2005) obtained α-pinene (40.55 to 45.10%), 1,8-cineole (17.40 to 19.35%) and camphene (4.73 to 6.06%) as the major components of Brazilian rosemary essential oils. Alternatively, the dominant constituents reported by Tawfeeq et al. (2018) were camphor (23.04%), 1, 8-cineole (eucalyptol) (14.01%), and terpinen-4-ol (13.8%), respectively. Our data recorded the dominant components of thyme were thymol, o-cymene, caryophyllene, and gamma-terpinene. Many researchers reported the chemical composition of the thyme essential oils, which were found to have abundant thymol, carvacrol, p-cymene, γ-terpinene, and caryophyllene oxide (Zeynep et al. 2018; Behnaz et al. 2020; Zhou et al. 2021). Although there are variations quantitatively, the composition of rosemary and thyme leaf extracts was qualitatively identical to that obtained by previous authors.

The internal regulation of seed germination is influenced by a variety of external variables. According to Kucera et al. (2005) and Finkelstein (2010), one of these elements is the existence of specific chemical substances (phytohormones or organic acids) that have a significant impact on this process. Data from the current experiment revealed that each two studied aromatic plants had a different allelopathic effect on seed germination; a wide negative effect was shown on weed seeds rather than in crops. According to Fischer et al. (1988), cineol was extremely poisonous to Schizachyrium scoparium but not to Leptochloa dubia in this regard. As a result, a compound’s inhibitory action on one species of plant may not always persist on another. Also, a compound’s inhibitory action on one species of plant may not always persist on another. According to Dudai et al. (2004), monoterpenes like carvacrol have highly potent inhibitors of wheat seed germination.

Our data recorded wide negative effect on weed seeds rather than in crops. Allelochemicals can inhibit the growth of weeds through inhibition of photosynthesis, decline in chlorophyll content, disruption of the cell membrane, and inhibition of enzymatic activity (Ghanizadeh et al. 2014). Aqueous leaf extract of Rosmarinus officinalis was reported with potential phytotoxic activity (Appiah et al. 2018 and Rahimi et al. 2015).

This might be because the essential oils in the two aromatic plants have different composition. According to Kotan et al. (2010), a high concentration of oxygenated monoterpenes is linked to strong phytotoxic action of plant essential oils, and it may be the responsible for the inhibitory effects on weeds and crops (Gitsopoulus et al. 2013; Uremis et al. 2009). These reports agreed completely with our findings. Our data also showed that thyme essential oils had the strongest allelopathic impacts on the plants and related weeds that were under study. The inhibitory effect of thyme oil was greater than that of rosemary oil because the monoterpenes in thyme oil are more diverse with larger percentages.

The seeds that germinate on the first day are given the most weight in the GI, while seeds that germinate later are given less weight. The seeds that germinated on the tenth day would have the lowest weight. As a result, the GI places emphasis on both the rate of germination and its percentage. A higher GI rating indicates a higher germination percentage and rate (Bench et al. (1991)). Our findings indicated that dosages of rosemary’s essential oils were less impacted by weed germination index (GI) than were those of thyme.

In comparison to the corresponding control treatments, rosemary essential oil and aqueous extract did not significantly reduce the seedling shoot length (SL) of wheat or rice. However, the rate of SL reduction in weeds was somewhat higher, particularly with the increasing dose of both types of essential oils. Compared to the essential oil of rosemary, the essential oil of thyme had a stronger allelopathic impact. Elghobashy et al. (2023) recorded that aqueous extract of natural medicinal plants have allelochemical potentials and showed a remarkable reduction in the organs length in weed Chenopodium album. Previous studies reported that essential oil effect on various cereal crops, including bread wheat germination inhibition (Dudai et al. 1999; 2004), and bread wheat germination inhibition (Atak et al. 2016a, b).

As the dose of thyme essential oil was increased, mean germination time (MGT) delayed. However, in the case of rosemary, the MGT increased as the aqueous and essential oil concentrations did as well. Previous research by Atak et al. (2016a, b) demonstrated that different essential oil concentrations and cultivars have different effects on MGT in durum wheat cultivars.

Our experiment reveals that thyme and rosemary can be used as an environmentally friendly technique to control weeds and pests in field crops, but they must be applied selectively so as not to harm crops that are intended to grow (Azirak and Karaman 2008). Numerous plant extract chemicals have specialized weed growth inhibition properties but do not adversely affect crop health (El-Darier et al. 2014). The different sensitivity of the target enzymes or particular weed receptors that recognize and respond to the chemicals may help to explain this (Hosni et al. 2013). The herbicidal effect of the essential oil should be assessed for this purpose both at the field level using different cultivars and relevant hazardous weed species. Our earlier research revealed that rice and wheat were less impacted compared to weed species, indicating that a suitable amount of these aqueous and essential oils could be utilized as a bio-herbicide to manage weeds. Both Dudai et al. (1999) and Gitsopoulus et al. (2013) found that cereal crops respond differently to essential oils. In the current investigation, thyme essential oil showed a stronger phytotoxic effect on germination and seedling characteristics for both two crops and associated weeds than rosemary oil.

5 Conclusion

Generally, the essential oils of the two studied species rosemary and thyme indicated different allopathic activity on seeds of Chenopodium album and Echinochloa crus-galli. These essential oils were promising as alternatives to conventional synthetic herbicides and could be used in the development of bioherbicide products. Further, pot experiments and field studies could provide more useful information concerning the wheat and rice crops, weed species, and applied dose to achieve more reliable weed control.