Phytotoxic Effect of Caraway Essential Oil and Its Main Compounds against Germination of Spring Wheat, Wild Oat and Chamomile ^†

Abstract

Caraway oil is a promising botanical herbicide. This study tested the phytotoxic potential of caraway oil, carvone and d-limonene on germination and seedlings’ growth of spring wheat, wild oat and chamomile. As a result, an inhibiting effect of caraway oil and carvone on all the tested species was found. Contrary, d-limonene displayed selective toxicity against chamomile and wild oat, which should be investigated further.

1. Introduction

Increased interest in the use of biological crop-protection methods has led to research on using the allelopathic potential of various plant species to reduce agricultural pests [1,2]. Essential oils of plant origin show phytotoxic properties that can be used to produce so-called botanical herbicides, i.e., substances of plant origin for weed control [3]. Essential oils as herbicides could be leaf-sprayed [4,5,6] or soil-applied [7,8]. However, the most frequently analysed effect of the use of essential oils against weeds is their influence on seed germination and the growth of seedlings [9,10]. For comparison of the allelopathic potential of the essential oils, the effective dose of the oil, causing a 50% inhibition of seeds germination (so-called ED50 dose) [11], is being widely applied [9,12].

One of the promising oils for herbicidal application is caraway oil [4]. The yield of essential oil obtained from caraway fruits is in the range of 1–6%. The oil is rich in oxygenated monoterpenes, mainly carvone and d-limonene, constituting up to 95% of all oil compounds [13].

The biological action of essential oils is attributed to synergistic effects of their compounds, as was found, i.e., for clove oil [5,14]. Our study aimed to investigate the phytotoxic potential of water solutions of caraway oil and its main compounds: carvone and d-limonene, to the germination and initial growth of spring wheat (Triticum aestivum L.), wild oat (Avena fatua L.) and chamomile (Matricaria chamomilla L.).

2. Material and Methods

Caraway essential oil (EO) of Polish origin was purchased from the producer HerbaNordPol Sp. z o. o. (Gdansk, PL). The fractionation of the oil and analysis of its composition were carried out at the Institute of Natural Products and Cosmetics, Lodz University of Technology. Under reduced pressure, the essential oil was rectified with a fractional column (Vigerous column, 3 × 20 cm). Next, the collected fractions were analyzed by the GC-MS-FID method on a Trace GC Ultra gas chromatograph connected to a DSQ II mass spectrometer (Thermo Electron, Waltham, MA, USA). Simultaneous analysis of GC-FID and MS was performed thanks to the MS-FID splitter (SGE Analytical Science, Ringwood, Vic, Australia). The components of the EO were identified by comparing the experimental retention indexes (RI) of the analysed components with the indexes in the Mass Finder 4.0 library and the NIST MS Search database (2012) and by comparing the mass spectra of the tested compounds with the patterns contained in the computer library Wiley Registry 10th Edition/NIST (2012) Mass Spectral Library.

A Petri dish (11 cm diameter) experiment was carried out in glass dishes, in four series with three repetitions. The essential oil was applied as an oil in water (o/w) solution, with 5% acetone as a carrier. Five doses of caraway oil: 0.004; 0.007; 0.01; 0.02; 0.03 g per dish; five doses of carvone: 0.002; 0.004; 0.008; 0.012; 0.02 g per dish and five doses of d-limonene: 0.001; 0.002; 0.005; 0.007; 0.01 g per dish were used. The control treatment was composed of acetone and water only. The doses of EO components, carvone and d-limonene reflected their percentage content in the oil, 60% and 35%, respectively. The filter paper was soaked with 7 mL of an appropriate o/w solution of EO or its component. The seeds of spring wheat (T. aestivum L. cv. Tybalt) and weed seeds–wild oats (A. fatua L.) and chamomile (M. chamomilla L.) were used. Mature wild oats seeds were collected from a spring wheat field in southern Poland in 2016. Certified chamomile seeds were purchased (PlantiCo Sp. Z o. o., Krzeszowice, PL). Twenty seeds of the tested species were placed on two layers of sterile filter paper in each dish. The seeds were allowed to germinate for seven days in the dark at room temperature (±25 °C). The seedlings were counted seven days after, and the lengths [mm] of their roots and shoots were measured.

The percentage of germinated seeds was analysed using dose–response nonlinear analysis (‘drc’) [11]. Three parameters were used to fit the log-logistic curve (Y) according to [15], where the lower limit is equal to zero:

Y = d/(1 + exp(b(log x − log e)));

(1)

where e is the ED50 value, d is the upper limit, b denotes the relative slope around e and x is the percentage germination or root/shoot length. The ED50 value was calculated in the ‘drc’ package and further used to compare the phytotoxic effect of the EOs against the tested plants [4]. All statistical analyses were performed with the software R, ver. 4.0.1 [16].

3. Results and Discussion

The first fraction, analysed in the Petri dish experiment as limonene, contained d-limonene and carvone in 91.1%:6.9%. The second fraction was 53.3% limonene and 45.4% carvone. The residue of the rectification process consisted of almost pure carvone: 99.8% and was analysed in the Petri dish experiment as carvone.

Germination of the tested species was affected to a different level by the tested caraway EO and its main compounds (Figure 1). Germination of spring wheat was mostly affected by caraway EO and carvone (ED50 0.006 and 0.008, respectively). Contrary, in the presence of limonene, all wheat seeds germinated, even at the highest dose of limonene (Figure 1a). The germination of wild oat was inhibited already by the lowest doses of caraway EO (ED50 0.001) and carvone (ED50 0.003). However, in the presence of limonene, the weed germinated well (Figure 1b). Chamomile was highly susceptible to the presence of caraway EO and carvone (ED50 0.001 g for both) and less susceptible to the presence of limonene (ED50 0.003) (Figure 1c). The germination pattern of the studied species in the presence of caraway EO and its main compounds points to a strong herbicidal effect of the EO and carvone. The results are concurrent with other authors who underline the inhibiting effect of caraway EO and carvone on germination of weeds and crops [9,17].

The growth of seedlings of the tested species was highly affected by the caraway EO (

Table 1. The response of spring wheat, wild oats and chamomile seedlings to different doses of caraway oil (mean value ± standard error).

Dose [g]	Shoot Length [mm]			Root Length [mm]
	Wheat	Wild Oat	Chamomile	Wheat	Wild Oat	Chamomile
0	57.8 ± 0.66	61.7 ± 2.64	11.3 ± 0.7	103 ± 7.01	52.9 ± 17.2	3.1 ± 0.3
0.004	9.22 ± 0.08	1.31 ± 0.35	0 ± 0	12.3 ± 0.25	1.67 ± 0.17	0 ± 0
0.007	2.49 ± 1.99	0 ± 0	0 ± 0	3.83 ± 2.61	0 ± 0	0 ± 0
0.01	0 ± 0	0 ± 0	0 ± 0	0 ± 0	0 ± 0	0 ± 0
0.02	0 ± 0	0 ± 0	0 ± 0	0 ± 0	0 ± 0	0 ± 0
0.03	0 ± 0	0 ± 0	0 ± 0	0 ± 0	0 ± 0	0 ± 0
ED50	0.002	0.002	0.001	0.001	0.002	0.001

), especially in chamomile. Only single seedlings of wild oat grew in the lowest doses of the EO, whereas growth of chamomile seedlings was completely inhibited even at the lowest oil dose. The very low values of ED50 also represent the susceptibility of the tested species to the caraway EO.

The seedlings’ growth was inhibited by carvone even more than by the caraway EO (

Table 2. The response of spring wheat, wild oats and chamomile seedlings to different doses of carvone (mean value ± standard error).

Dose [g]	Shoot Length [mm]			Root Length [mm]
	Wheat	Wild Oat	Chamomile	Wheat	Wild Oat	Chamomile
0	57.8 ± 0.66	61.7 ± 2.64	11.3 ± 0.7	103 ± 7.01	52.9 ± 17.2	3.1 ± 0.3
0.004	3.54 ± 0.76	0 ± 0	0 ± 0	5.81 ± 1.07	0.5 ± 0.5	0 ± 0
0.007	0 ± 0	0 ± 0	0 ± 0	0 ± 0	0 ± 0	0 ± 0
0.01	0 ± 0	0 ± 0	0 ± 0	0 ± 0	0 ± 0	0 ± 0
0.02	0 ± 0	0 ± 0	0 ± 0	0 ± 0	0 ± 0	0 ± 0
0.03	0 ± 0	0 ± 0	0 ± 0	0 ± 0	0 ± 0	0 ± 0
ED50	0.001	0.001	0.001	0.001	0.001	0.001

). Only a few seedlings of wheat grew in the presence of the lowest dose of that compound. The ED50 values of both shoot and root length were also very low.

Contrary to the two other compounds, the seedlings of all the tested species germinated in the presence of limonene; however, the growth of wheat and wild oat seedlings was strongly inhibited, even by the lowest dose of the EO and its main compounds (

Table 3. The response of spring wheat, wild oats and chamomile seedlings to different doses of limonene (mean value ± standard error). Statistical differences were indicated with different letters.

Dose [g]	Shoot Length [mm]			Root Length [mm]
	Wheat	Wild Oat	Chamomile	Wheat	Wild Oat	Chamomile
0	57.8 ± 0.66 a	61.7 ± 2.64 a	11.9 ± 0.45 a	103 ± 7.01 a	52.9 ± 17.2 a	3.60 ± 0.30 a
0.004	10.5 ± 0.37 b	7.50 ± 0.35 b	11.8 ± 0.35 a	14.6 ± 0.34 b	10.7 ± 0.58 b	2.81 ± 0.21 a
0.007	10.3 ± 0.12 b	7.64 ± 0.29 b	11.1 ± 0.44 a	14.0 ± 0.28 b	12.4 ± 0.37 b	2.80 ± 0.28 a
0.01	10.0 ± 0.41 b	7.71 ± 0.33 b	8.73 ± 0.22 a	15.1 ± 0.09 b	12.5 ± 0.13 b	2.32 ± 0.23 a
0.02	9.99 ± 0.13 b	8.26 ± 0.20 b	10.6 ± 0.48 a	14.7 ± 0.68 b	10.4 ± 0.77 b	2.39 ± 0.21 a
0.03	9.45 ± 0.25 b	7.11 ± 0.36 b	8.85 ± 1.01 a	14.2 ± 0.18 b	11.4 ± 0.56 b	1.92 ± 0.43 b
ED50	0.001	0.001	0.07	0.001	0.001	0.02

). On the contrary, chamomile seedlings’ growth in the presence of a majority of d-limonene doses was similar to the control, except that the elongation of roots was significantly inhibited at the highest dose. The ED50 values expressed the differences in susceptibilities between grasses and chamomile to d-limonene.

The herbicidal effect of caraway EO and carvone on the seedlings of wheat, wild oat and chamomile was strong in laboratory conditions. On the contrary, the effect of d-limonene was weaker, as seedlings were able to grow but were strongly inhibited. Interestingly, the inhibiting effect of d-limonene on elongation of shoots/roots was more visible in monocotyledonous wheat and wild oat than dicotyledonous chamomile. Studies by other authors show that weed and crop species display different susceptibilities to various essential oils and compounds [9,18,19], which is a good selective effect for the future development of botanical herbicides.

4. Conclusions

In the laboratory conditions and with the doses applied, caraway EO and carvone inhibit germination and seedlings’ growth of spring wheat, wild oat and chamomile. On the contrary, d-limonene does not affect the germination of wheat and wild oat but significantly inhibits the growth of their seedlings. D-limonene inhibits germination of chamomile in a dose–response manner, but the growth of seedlings is similar to that of control. In summary, d-limonene should be studied further for its potential selective properties.