The effects of organic and conventional fertilization on oregano (Origanum onites L.) yield and quality factors

Leaf samples were collected in a full flowering stage. Samples were dried at 40 °C in an oven for 2 days. Then, dry leaf samples of 20 g were suspended in 200 mL distilled water. Ground mass was subjected to hydrodistillation using Clevenger’s apparatus. After 3 h, the EOs were collected (Karık et al., 2018). Then, essential oil yield (EOY) was measured by using the following formula: EOY(kg⋅ha−1)=EO(%)×dryherbyield(kg⋅ha−1)

The EO composition of samples was analyssed by gas chromatography (Agilent 5975 C; Agilent Technologies, Santa Clara, CA, USA) coupled to a flame ionisation detector and mass spectrometry equipment (Agilent 5975 C) using a capillary column (HP Innowax Capillary; 60.0 m × 0.25 mm × 0.25 μm). EOs were diluted at 1:50 ratio with hexane. GC–MS/FID analysis was carried out at split mode of 50:1. Injection volume and temperature were adjusted as 1 μL and 250 °C, respectively. The relative percentage of components was calculated from GC-FID peak areas, and components were identified by Wiley 7n, Nist 05 and Flavor and Fragrance Natural, and Synthetic Compounds (ver.1.3) libraries.

Extraction of the samples was accomplished according to the method of Škerget et al. (2005) with some modifications. This extract was used: total phenolic and flavonoid contents with antioxidant activity using DPPH (2,2-diphenyl-1picrylhydrazyl). Total phenolic content (TPC) was analyzed by the Folin-Ciocalteu method (Škerget et al., 2005). Total flavonoid content (TFC) was determined by Chang et al. (2006). The antioxidant activity of the samples was analyzed by the DPPH assay according to the procedure of Maisuthisakul et al. (2007). The percent inhibition of the DPPH radical was calculated using the following equation: IP (%) = [(Ac – As)/Ac] × 100, where IP is the inhibition percentage and Ac and As are the absorbance values of the control and test sample, respectively. The extract concentration providing 50% inhibition [IC50 (milligrammes of dry weight (DW) of plant material per milligramme of DPPH)] was calculated by plotting the concentration versus IP (Dinçer et al., 2013).

O. onites were in the full flowering stage; fresh leaf samples were collected, washed and dried at 65 °C until the last two weighing values become constant, and then, they were made ready for analysis by milling at the grinding mill. The total concentration of phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), zinc (Zn), manganese (Mn) and copper (Cu) of the solution obtained by wet combustion of the plant samples in the nitric-perchloric acid mixture was determined by inductively coupled plasma optical emission spectrometry (ICP-OES) (Varian 720-ES; Agilent Technologies, Santa Clara, CA, USA). The total nitrogen (N) concentration of plant samples was determined based on the modified Kjeldahl method (Kacar and Inal, 2008).

The statistical analysis was made according to the principles set of Yurtsever (1984). All data were analysed using the JMP Statistical package programme developed by SAS (SAS Institute, Cary, NC, USA). Means were compared by analysis of variance (ANOVA), and the least significant difference (LSD) test at p ≤ 0.05 was significant.

The fresh herb yield of O. onites L. ranged from 20 800 kg · ha⁻¹ to 25 320 kg · ha⁻¹ in the 1st cultivation year. In the 2nd year of cultivation, the fresh herb yield varied from 24 060 kg · ha⁻¹ to 28 630 kg · ha⁻¹. In both years, the lowest fresh herb yields were obtained by the control, and the highest values were determined by chemical fertilizer (F) (Table 3). Fresh yield increased by 22% and 19% with chemical fertilization and dry yield increased by 24% and 15%, respectively depending on the year of cultivation. Fresh yield increased by 3.36%–11.44% and dry yield increased by 2.21%–7.50% as compared with the control in the 1st year with organic fertilization. FYM was the most effective organic fertilizer. Fresh and dry yields were increased by 5.61%–13.60% and 3.76%–14.26% with residual effects of organic fertilization. Edris et al. (2009) found that fertilization had a positive effect on the yield of oregano herbage, regardless of organic or chemical origin. Kutlu et al. (2019) determined that bacterial inoculations had a positive effect on the yield of O. onites L., but chemical fertilization with 31.4% increase in drug herb yield was the most effective application. Bajeli et al. (2016) reported that the highest yield of Japanese mint (Mentha arvensis) was recorded in the combined application of FYM, VC and poultry manure.

The effects of the applications on the EO content and yield of the oregano are shown in Figures 1 and 2. The EO content ranged from 3.61% to 5.42% in the first year of cultivation, the lowest EO was obtained with the control and the highest value was determined by CM. The applications increased the EO content in a range from 18.8% to 50.1%. In the second year of cultivation, EO content varied from 4.08% to 4.89% (p < 0.01). Fertilization increased EO content by 2.94%–19.85% as compared with the control. The residual effects of organic fertilizers continued, and FYM had the highest EO content. EOs are terpenoids, and biosynthesis of their components (isoprenoids) requires ATP and NADPH. Providing plant roots with the necessary nutrients results in an increase in the content of EOs in the plant because nitrogen and phosphorus are required for the synthesis of ATP and NADPH (Esmaielpour et al., 2017).

Figure 1.

Figure 2.

EOY ranged from 37.5 L · ha⁻¹ to 59.1 L · ha⁻¹ in the 1st year and 41.2 L · ha⁻¹ to 52.6 L · ha⁻¹ in the 2nd year. The EOY of oregano increased by 31.5%–57% with the direct effect of organic fertilizer, and the highest EOY was obtained by CM application. The residual effect of organic fertilizers increased EOY by 7.0%–27.6%, and the highest EOY was derived from VC and FYM. Bhaskar et al. (2001) noticed that the oil content of Pelargonium graveolens was increased with FYM (30 t · ha⁻¹) in the long period. Nikou et al. (2019) found that the application of 7 t · ha⁻¹ VC increased the content of oregano EO by 27%. Rajeswara Rao (2001) reported that application of 15 t · ha⁻¹ FYM increased palmarosa EOY by 10.3% as compared with control. Chemical fertilizer application increased EOY by 47.73% in the first year and 21.60% in the second year as compared with control. Kutlu et al. (2019) found that the content of EO of O. onites increased by 32.7% with chemical fertilization as compared with control. The increase in the EO content and yield by organic fertilizers equivalent or higher than those of chemical fertilizers indicate that organic fertilization provides more balanced nutrition of O. onites L. Khalid and Hussein (2012) reported that organic fertilizers accelerate metabolic reactions by increasing the activity of enzymes involved in EO synthesis. In this study, although leaf samples were collected in full bloom in both years of the study, both climatic differences between years and changing soil conditions due to decomposition of organic matter resulted in a change in the amount of EO. Alizadeh et al. (2010) reported that macronutrients, micronutrients and even heavy metals that occur with the decomposition of organic fertilizers promoted the biosynthesis of the plant’s EO.

Twenty-two constituents were identified in the EO of O. onites L. (Table 4). Carvacrol was the main constituent of EO, and its content was increased with direct effect of organic fertilizer by 0.58%–12.87% and 0.37%–11.46% by the residual effect of organic fertilizers. In the direct and residual effects, the increase ratio in carvacrol content was highest with FYM and lowest with CM. Carvacrol content of EO increased with chemical fertilization by 23.3% in the first year and 7.4% in the second year as compared with the control. The carvacrol content of plants grown with chemical fertilizers was highest in the first year, while the results obtained with chemical fertilization in the second year were close to the residual effects of organic fertilizers. Zarrabi et al. (2017) reported that the content of citronellol in EO of Melissa officinalis was increased with organic fertilizer, and the most effective application was 30% VC. Kocabaş et al. (2010) found that chicken and sheep manure give the highest content of 1.8 cineole in EO of Salvia officinalis Mill.

Thymol was the second main constituent in the EO of O. onites. Among the organic fertilizers, the lowest thymol content was determined by the direct and residual effect of FYM (1st year: 6.77%, 2nd year: 11.91%), while the highest values were determined by the direct and residual effect of CM (1st year: 19.95%, 2nd year: 16.51%). When the results of the 2 years were evaluated together, it was found that more stable levels of carvacrol and thymol could be obtained by applying SMC (Table 4). Matlok et al. (2020) reported that organic fertilizer increased the carvacrol and thymol content of EO by affecting the plant metabolism of oregano (O. vulgare L.). Edris et al. (2009) found that the thymol content of Thymus vulgaris L. increased by 61% when 50 t · ha⁻¹compost was compared with chemical fertilizers.

The Ɣ-terpinene content of the EO varied from between 7.28% and 10.54% in the first year and 3.90% and 4.91% in the second year. The p-cymene content varied between 4.65% and 7.97% in the first year and 4.19% and 5.21% in the second year. Accordingly, the increased levels of carvacrol in association with the low levels of thymol and p-cymene probably reflect the close biosynthetic relation between these compounds in oregano. The enhancement of carvacrol biosynthesis with fertilization occurs by activating the enzymatic system responsible for p-cymene conversion to carvacrol (Karamanos and Sotiropoulou, 2013). In the study, organic and chemical fertilizers had a positive effect on the content and components of oregano EO. It is very important that the residual effects of organic fertilizers give similar and more positive results than chemical fertilizers. This condition is thought to be due to the influence of plant nutrients, enzymes, phenolic compounds and hormones that result from the degradation of organic fertilizers on the metabolic activity of plants (Kimera et al., 2021). Tabrizi et al. (2011) reported that plants synthesise compounds that originally contain carbon (monosaccharides, polysaccharides, secondary metabolites, vitamins and volatile oils) by using slow-release organic fertilizers.

The effects of the applications on the TPC of the oregano were not significant (Figure 3). The TPC of oregano varied between 13.5 mg GAE · g⁻¹ and 16.7 mg GAE · g⁻¹ in the 1st year and 12.4 and 14.1 mg GAE · g⁻¹ in the 2nd year. In the first year of cultivation, the lowest TFC of the oregano (4.8 mg · g⁻¹) was determined by the control and the highest value (7.6 mg · g⁻¹) by the VC (Figure 4). Compared with the TFC control, it increased by 25%–58% with organic fertilizers and by 48% with chemical fertilizers. The lowest TFC value (5.1 mg · g⁻¹) was found in the control, and the highest value (10.5 mg · g⁻¹) was determined by the residual effect of CM and VC in the 2nd year (p < 0.001). In the second year of cultivation, TFC increased by 14%–106% with the residual effect of organic fertilizers and by 19.6% with chemical fertilizer as compared with the control. VC may play an important role in the use of organic production systems to improve flavonoid biosynthesis in plants (Baktiari et al., 2020). Baktiari et al. (2020) reported that TFC of Satureja macrantha was greater in second-year plants treated with NPK + VC (15.2 mg CE · g⁻¹) and VC (14.8 mg CE · g⁻¹) than in other experimental plants. Assis et al. (2020) found that flavonoid content of M. officinalis L increased with the arbuscular mycorrhizal fungi and organic fertilizers. Kazimierczak et al. (2014) reported that the TFC of sage, rosemary and lemon balm plants increased by 27.5%, 12.2% and 35%, respectively, with organic fertilizers as compared with chemical fertilizers.

Figure 3.

Figure 4.

The DPPH radical scavenging activity of the plant was expressed by the IC₅₀ value, which was defined as the concentration that inhibited the free radical by 50%. IC₅₀ values varied between 12.7 mg DPPH · mL⁻¹ and 20.4 mg DPPH · mL⁻¹ in the first year and between 9.5 mg DPPH · mL⁻¹ and 12.5 mg DPPH · mL⁻¹ in the second year (Figure 5). VC stands out among the materials because a low IC₅₀ value indicates high antioxidant activity due to high radical scavenging (Delgado et al., 2010). Yang et al. (2000) reported that organically grown cabbage, spinach and green pepper generally had higher levels of antioxidant activity.

Figure 5.

The nitrogen (N), phosphorus (P), potassium (K) and calcium (Ca) concentrations of oregano varied from 1.85% to 1.97%, 0.18% to 0.22%, 1.92% to 2.13% and 1.21% to 1.34%, respectively, in 1st year. In the 2nd year, N, P, K and Ca concentrations of oregano varied from 1.96% to 2.11%, 0.20% to 0.23%, 2.12% to 2.29% and 1.23% to 1.38%, respectively (Table 5). The lowest values were determined by the control, and the highest concentrations were obtained by VC. Khomami (2011) reported that the highest N, P, K, Ca and Mg concentrations in Dieffenbachia were obtained from 100% VC. Results showed that oregano macronutrient concentrations grown with organic and chemical fertilizers were very similar. The residual effect of organic fertilizers on plant nutrition status was continued. Tepecik et al. (2014) reported that the nutrition status of Basil grown with organic and chemical fertilizers was similar.

The effects of applications on the microelement concentrations are given in Table 6. Fe concentration of oregano was changed between 256.25 mg · kg⁻¹ and 280.27 mg · kg⁻¹ in the 1st year. In the 2nd year, the lowest Fe (287.77 mg · kg⁻¹) was taken from control, and the highest value (343.67 mg · kg⁻¹) was obtained from FYM residual effect. Fe concentration of plant increased by 1.6%–19.4% with residual effects of organic fertilization as compared with the control. All of the applications caused an increase in Zn concentration. SMC gave the highest Zn with 21.00 mg · kg⁻¹ and 19.76 mg · kg⁻¹ in both years. Mn and Cu concentrations of oregano were increased by applications. Kocabaş et al. (2007) reported that the nutrient concentration of sage increased with organic manure applications.