Introduction

Most plants exhibit allelopathic effects on seed germination, growth and development of other plants by releasing allelochemicals into the soil, either as exudates from living organs or by decomposition of plant residues (Narwal 1999; Singh et al. 2003; Noguchi and Ino 2005). Allelochemicals are present in almost all plants and their tissues such as leaves, stems, roots, flowers, seeds, bark, and buds (Weston and Duke 2003). Allelopathic plant interactions can have either a harmful or beneficial effect and are generally evaluated by testing some physiologic mechanisms that result in the inhibition/stimulation of seed germination, plant growth and development due to the presence of another plant (Suman et al. 2002; Weston and Duke 2003).

The genus Nepeta (family Lamiaceae) contains approximately 280 species, which are widely distributed in Central and Southern Europe, and South-West, Central and Southern Asia (Dabiri and Sefidkon 2003). Previous research on Nepeta species indicate that it is represented by 44 taxa, including 22 endemic to Turkey (Aytac and Yildiz 1996; Dirmenci 2005). The endemic and non-endemic species are commonly found in East Anatolia and the Taurus Mountains (Dirmenci 2005). Some Nepeta species have antioxidant properties (Dapkevicius et al. 1998) and significant effects on viruses, bacteria, fungi and acarus (Bourrel et al. 1993; Nostro et al. 2001; Çalmaşur et al. 2006). Through our numerous field observations, it was determined that N. meyeri has not permitted the germination of other wild plant species into its natural environment. Such inhibition zones around dominant plants species can provide an important ecological context for studies of allelopathy (Inderjit and Callaway 2003). Therefore, it has been hypothesized that N. meyeri has significant allelopathic potential.

In this study, aqueous extracts from roots and leaves of Nepeta meyeri were studied for their allelopathic effect on seed germination and early seedling growth of five economically important crop plants: barley, wheat, canola, sunflower and safflower.

Materials and methods

Preparation of aqueous extracts from roots and leaves of Nepeta meyeri

Nepeta meyeri plants during the flowering stage were collected from various zones between Erzurum and Kars provinces in East Anatolia, Turkey. The plant material was dried and was separated into roots and leaves. The dried samples were cut into 1-cm pieces and ground in a blender (Dapkevicius et al. 1998). A measure of 5 g of dry powder was stirred (125 rpm) in 100 ml of distilled water for 8 h at 20°C. The resulting solution was filtered through four layers of cheesecloth to remove debris, and centrifuged at 5,000×g for 30 min. The supernatant was then filtered through one layer of filter paper (Whatman no. 1). The filtered solutions (stock solution) were held for a short time at 4°C until bioassayed. The extracts used for bioassay were performed by diluting the stock solution (5% w/v) at concentrations including 0.125, 0.25, 0.5, 1.0, 2.5 and 5.0 g of the dried powders per 100 ml of distilled water (Yu et al. 2003).

Plant materials

Plant seeds of barley (Hordeum vulgare cv. Tokak), wheat (Triticum aestivum cv. Dogu-88), canola (Brassica napus cv. Oleifera), safflower (Carthamus tinctorius cv. Dincer) and sunflower (Helianthus annuus cv. AS-508) were obtained from the Agriculture Faculty of Atatürk University, Turkey. Before germination tests, the plant seeds were surface-sterilized with 10:1 water/bleach (commercial NaOCl) solution for 5 min and then washed six times with distilled water.

Seed germination and seedling growth

Germination tests were performed for each of the root and leaf extracts from N. meyeri as follows: According to seed size, 25 or 50 the sterilized seeds were evenly placed on two layers of filter paper in sterile Petri dishes (9 cm). Ten ml of the extract solutions at five concentrations of stock solution (0.125, 0.25, 0.5, 1.0, 2.5 and 5.0%) were added to each Petri dish, and distilled water was used as the control. The Petri dishes were placed in a growth chamber (25°C, 70% humidity and continuously dark). Treatments were arranged in a completely randomized design with four replications. Germination was determined by counting the number of germinated seeds at 24-h intervals over a 5-day period. After 5 days, root and shoot length, and dry matter weight of the plants was calculated. Final germination was expressed as percentage after statistical analyses were performed on the raw data. Data were analyzed using a one-way ANOVA test in SPSS 11.0, and means were compared using Duncan’s multiple range test at the 1 or 5% confidence level.

Results and discussion

Previous researches have studied the composition and content of the essential oils in Nepeta species including N. meyeri, because of the interesting biological activities of the Nepeta species and their use in traditional medicine. Most oils of Nepeta species contain nepetalactones as the main components, but different oil compositions have been detected in several Nepeta species (Baser et al. 1998; Sajjadi and Khatamsaz 2001; Sefidkon and Shaabani 2004; Esmaeili et al. 2006). In the present study, however, we were interested in the allelopathic potential of the aqueous extracts from Nepeta meyeri in Turkey. The effects of aqueous extracts (0.125, 0.25, 0.5, 1, 2.5 and 5%) from the roots and the leaves of N. meyeri were determined on seed germination and early seedling growth (root and shoot length, and dry matter weight) of several crop plants (barley, wheat, canola, safflower and sunflower). The root and leaf extracts from N. meyeri had an ameliorative effect on seed germination and early seedling growth of some of the crop plants studied while having an inhibitory effect on others.

Effect of Nepeta meyeri extracts on seed germination

The root and leaf extracts of N. meyeri significantly (P < 0.01) inhibited seed germination of barley and sunflower, compared to respective controls (Table 1). The determined inhibitions were higher for leaf extracts than root extracts, and were more pronounced at increasing concentrations of the leaf extract. The seed germinations of barley and sunflower, for instance, were inhibited by 87 and 60%, respectively, by the application of leaf extract at 5% concentration, and were inhibited by 83 and 46%, respectively, by the application of root extract (again at 5% concentration). Oueslati (2003), who studied the allelopathic effects of wheat varieties also suggested that leaf extracts appeared to have a more pronounced phytotoxic effect than roots extracts. In contrast, the germination of canola and safflower seeds was increased by both leaf and root extracts of N. meyeri at low concentration while that of wheat seeds was not affected at even the highest extract concentration (Table 1). The results show that allelochemicals in the extracts of N. meyeri could have a beneficial, harmful or harmless effect on seed germination of the crops.

Table 1 The effect on final germination percentages of some crops seeds of aqueous extracts in different concentrations prepared from roots and leaves of Nepeta meyeri Benth
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Effect of Nepeta meyeri extracts on early seedling growth

The extracts from roots and leaves of N. meyeri significantly (P < 0.01) inhibited the root length of germinating barley and sunflower seedlings, compared to respective controls (Fig. 1). The inhibitions were relatively enhanced with the increasing amount of both extract concentrations. The leaf extracts proved to have more effective allelopathic effect than the roots extracts. On the contrary, the root lengths of wheat, canola and safflower seedlings were promoted by 0.125, 0.25, 0.5 and 1% concentrations of both extracts while showing a slight inhibition at 2.5 and 5% concentrations (Fig. 1). It can be concluded that the low concentrations of the extracts could stimulate the root elongation of wheat, canola and safflower, but the same concentrations had harmful effects on barley and sunflower root elongation. The results obtained from root elongation tests also support the findings from germination tests (Table 1, Fig. 1). Nojavan (1990), who studied the allelopathic effect of N. meyeri on wheat, also reported a similar result. In addition, the extracts did not affect the germination of wheat while increasing root elongation. Some allelochemicals can stimulate the growth of plants while having neither harmful nor beneficial effects on the seed germination (Macias et al. 1999).

Fig. 1
figure 1

Effect of root and shoot extracts of Nepeta meyeri on the root lengths of barley, wheat, canola, safflower and sunflower. LE leaf extract, RE root extract. Means followed by the same letter are not significantly different at the 1% level as determined by Duncan’s multiple range test

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The extracts from N. meyeri significantly inhibited shoot length and dry matter weight of barley seedlings but the same extracts generally stimulated shoot growth of wheat, canola, safflower and sunflower seedlings (Figs. 2, 3). It is interesting that the extracts had an inhibitory effect on germination and root elongation of sunflower while having an ameliorative effect on shoot elongation and dry weight (Table 1, Figs. 1, 2, 3). However, the shoot length and dry weight results obtained from the crops studied were comparable with the findings from germination to root length tests of the same crops. The results show that allelochemicals in the extracts of N. meyeri could have both beneficial and harmful effects on growth of the crops.

Fig. 2
figure 2

Effect of root and shoot extracts of Nepeta meyeri on the shoot lengths of barley, wheat, canola, safflower and sunflower. LE leaf extract, RE root extract. Means followed by the same letter are not significantly different at the 1% level as determined by Duncan’s multiple range test

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Fig. 3
figure 3

Effect of root and shoot extracts of Nepeta meyeri on the dry matter weights of barley, wheat, canola, safflower and sunflower. LE leaf extract, RE root extract. Means followed by the same letter are not significantly different at the 5% level as determined by Duncan’s multiple range test

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It was determined that the aqueous extracts from N. meyeri leaf and root parts exhibited variable allelopathic activity on seed germination and seedling growth of the five food crops. Allelopathic activity depended on both the concentration levels of the extracts and the parts (leaf or root) of N. meyeri from which the extract was derived. This might be ascribed to differential allelopathic strength provided by constitutional variability of allelochemicals in N. meyeri. Differential response of various allelopathic compounds and/or their concentrations has been reported in previous findings (Wu et al. 1998, 2000; Om et al. 2002). The maximum reduction or induction generally occurred with leaf extracts. The advantages registered by low concentrations of the extracts from N. meyeri could not be observed at higher concentration (2.5 and 5%). High concentrations (2.5 and 5%) often inhibited both seed germination and seedling growth, except in the case of germination of wheat seeds. Similar findings were reported in studies of the allelopathic activities of Parthenium hysterophorus (Tefera 2002) and Chenopodiaceae extracts (Jefferson and Pennacchio 2003).

Overall, the phytochemicals having allelopathic properties in extracts of Nepeta meyeri are both species-specific and plant part (root and leaf)-dependent. The effects of the extracts cannot be categorized as only a phytotoxic effect because its extracts have both stimulatory and inhibitory effects on the crops studied. N. meyeri must be considered as an allelopathic species, presenting both risk and advantage to seed germination and growth of crop plants. Further research is required in order to define in detail the allelopathic characteristics of N. meyeri.