Enhancing essential oil production in rosemary (Rosmarinus officinalis L.) with salicylic acid and methyl jasmonate and its relationship to spectral indices

Rosemary (Rosmarinus officinalis L.) is a well-known versatile herb that is utilised frequently in culinary preparations due to its distinctive aroma (Sasikumar, 2012). Additionally, rosemary has numerous health benefits and is economically important mainly due to its essential oil that is rich in bioactive compounds, including eucalyptol, α-pinene, camphor, camphene, and β-pinene (Sasikumar, 2012; Tadtong et al., 2015) that underpin its various therapeutic and medicinal properties. The oil is utilised widely in the pharmaceutical, cosmetic, and food industries. Research has documented that rosemary essential oils have antioxidant, anti-inflammatory, and antimicrobial activities, as well as stress-relieving properties, making them valuable in traditional medicine and complementary therapies (Peter and Shylaja, 2012; Aziz et al., 2022; Eid et al., 2022). In addition, it has been shown that essential oils from rosemary can have significant inhibiting effects on orange postharvest decay (Bakhtiarizade and Souri, 2019) as well as improve the nutrient uptake in plant seedlings (Souri and Bakhtiarizade, 2019).

Due to many benefits of rosemary, there have been several efforts to cultivate this herb to maximise productivity and increase the quality of its essential oils. During rosemary cultivation, it requires careful attention to environmental conditions, including temperature (15–25°C), light intensity (5–6 hr · day⁻¹), and water availability (well-drain soil) (Peter and Shylaja, 2012; German et al., 2016). Rosemary is a temperate-climate plant that grows best at 15–25°C (Peter and Shylaja, 2012). In addition, excessive watering or rainfall could induce stress in rosemary due to waterlogging, which can lead to stunted growth and even death (German et al., 2016). In Thailand, rosemary has been reported to grow in the highlands, such as in the north under the Royal Project (Skilbeck et al., 2014). Phetchabun province located in the highlands of Thailand had mean maximum and minimum temperatures of 34.97°C and 15.86°C, respectively (Kaewkrom et al., 2007). In addition, the precipitation in Phetchabun province normally exceeds 1000 mm in the rainy season from June to October (Ono et al., 2014). Such an environment suggests that rosemary plants could be grown in Phetchabun province, Thailand; however, there may be some issues associated with other unfavourable factors that are challenging for rosemary cultivation.

The application of plant growth regulators (PGRs) is one of the effective strategies to enhance plant resilience to unfavourable environmental conditions and to increase the productivity of various plant species (Sabagh et al., 2022). Environmental stress changes the endogenous hormones, such as abscisic acid and salicylate, in the rosemary plant (Asensi-Fabado et al., 2013), suggesting that these plant hormones may control the stress response. Among the group of plant hormones, jasmonate (JA) and salicylate are PGRs that have been reported to mitigate the impact of unfavourable environmental factors on plants. JA has been reported to improve plant resistance to salinity stress and water stress (Hossain et al., 2021), as well as being associated with an increase in the production of essential oil as a defence mechanism against insects (Wari et al., 2022). Therefore, it could be appropriate to consider the application of JA that is present in volatile herbs (particularly in the Lamiaceae family to which rosemary belongs). Additional members of this family, such as sweet basil (Talebi et al., 2018), lemon balm (Pirbalouti et al., 2019), thyme (Alavi-Samani et al., 2015), and lettuce leaf basil (Złotek et al., 2016), have demonstrated various uses of JA during concentrations of 0.001–0.1 mM in enhancing plant responses and essential oil production. Salicylate has been identified as a signalling molecule facilitating plant self-protection mechanisms in response to environmental stress (Souri and Tohidloo, 2019; Pan et al., 2021) and pathogen attack (Pan et al., 2021), as well as increasing essential oil production in various volatile plants, including cumin (Rahimi et al., 2013), lemon balm (Pirbalouti et al., 2019), and fragrant trees such as silver birch and black alder (Blande et al., 2010) when foliar apply salicylate during concentrations of 0.002–1 mM.

Therefore, the application of jasmonate and salicylate could be a potential strategy to support rosemary cultivation in regions with unpredictable climates or limited resources, as is often experienced in Thailand. Consequently, this study aimed to enhance the efficacy of utilising jasmonate and salicylate in promoting growth and maximising the essential oil yield of rosemary, based on evaluating their optimal concentrations.

The height response of the rosemary was pronounced in response to 0.1 mM MeJA (Figure 1A), while canopy width responded to both MeJA and SA applications at different concentrations (Figure 1B). However, the common use of SA and MeJA is associated typically with enhancing plant defence against environmental and biotic stress and not primarily on promoting growth. Studies have reported that JA inhibited cell division and reduced the cell mitotic index (Mahfouz et al., 2014; Huang et al., 2017), which was consistent with the current results, where MeJA did not significantly increase the growth of rosemary but rather demonstrated an upward tendency at specific concentrations. Nevertheless, MeJA has been documented to enhance cell expansion by working cooperatively with cytokinin during etiolated cotyledon development (Pandita, 2022). Jasmonate has been reported to be associated with the expansion of potato tuber cells, attributed to an increase in osmotic pressure resulting from sucrose accumulation and alterations in cell wall architecture affecting wall extensibility (Koda, 1997). SA has been reported to interact with stem diameter and canopy parameters in fennel under water stress conditions (Ghilavizadeh et al., 2019). Furthermore, the application of 1.25 mM SA has been shown to increase the number of cells in the G1 state during cell division (Mahfouz et al., 2014), which was consistent with the current result, where 1 mM SA increased the canopy width of rosemary. SA has also been reported to improve rice shoot growth and the cell cycle by inhibiting the effect of abscisic acid (Meguro and Sato, 2014). However, in this study, the number of branches did not respond to either the MeJA or SA applications (Figure 1C), which was consistent with another study on coriander, where neither SA nor irrigation affected the number of branches · plant⁻¹ (Hesami et al., 2012). Similarly, MeJA applications at concentrations in the range 0.01–1 mM did not increase the number of branches in cumin (Rahimi et al., 2013). In addition, the slight promotion of rosemary growth following the SA or MeJA treatments in this study might have been influenced because SA and MeJA treatments may mitigate high-temperature stress (up to about 30°C during May–July 2023).

The application of MeJA and SA did not significantly impact photosynthetic pigments. However, SA consistently increased both the total chl and chl a contents at all applied concentrations, while the 0.1 mM MeJA treatment also produced a similar trend with a higher total chl than the control (Figures 2A and 2B). This was consistent with the results observed in cucumber plants, where SA increased the chlorophyll content under both saline stress and normal conditions (Yildirim et al., 2008). The use of jasmonate in pigeon peas produced a similar trend to our findings, with a considerable increase in the total chl content after treatment with 1 μM jasmonate (Poonam et al., 2013). However, numerous studies have mentioned that jasmonate is an inhibitor of chlorophyll and carotenoid synthesis and that it reduced photosynthetic pigments in various plant species (Moreira-Rodríguez et al., 2017). Our findings did align with aspects of these reports in that MeJA application at all concentrations resulted in a lower chl a/chl b ratio compared to SA applications at 0.01 mM and 0.1 mM and tended to be lower than the control (Figure 2D). The ratio of chl a-to-chl b reflects a plant’s ability to use light energy. An increased chl a-to-chl b ratio suggests better acclimation to high light intensity leading to increase the photosynthetic electron transport rate (Jin et al., 2016). This suggests that SA applications might enhance the photosynthetic performance of rosemary.

The leaf spectral indices (NDVI, NDRE, SRPI, NPCI, Ctr1, and SIPI) were selected to investigate various responses in this study. NDVI and NDRE are commonly used to assess plant health and serve as indicators to evaluate plant pigment content and other biochemical compounds inside plant leaves (Stamford et al., 2023). The same levels of NDVI and NDRE in all the treatments in this study indicated that all the rosemary plants were at the same plant physiological stress level. The other indices were chosen based on their use of leaf reflectance values involved in the wavelength range 400– 500 nm, which were in the same range as the essential oil absorption wavelengths recorded in many plants such as clove, thyme, and cinnamon in the laboratory study (Ouaddari et al., 2022). High absorption in plant essential oil in the range 400–500 nm, with low absorption after 600 nm, has been reported by Ouaddari et al. (2022), implying that plants with a high essential oil content should produce low leaf reflectance at wavelengths in the range 400–500 nm and high leaf reflectance at 600 nm because leaf reflectance values are the reverse of leaf absorption values (Ustin and Jacquemoud, 2020). Thus, in this study, of the spectral indices calculated from those specific wavelengths (SRPI, NPCI, Ctr1, and SIPI), only NPCI and Ctr1 increased after the SA or MeJA applications (Figure 3). NPCI and Ctr1 were calculated from the 430 nm and 420 nm wavelengths, respectively (Table 1) and are within the 400–500 nm range, which could be useful in estimating the essential oil in plants (Ouaddari et al., 2022). NPCI has been used to estimate nitrogen deficiency in plant leaves and disease defence (Sapate and Deshmukh, 2019). Tan et al. (2018) reported that high NPCI values were highly correlated with the maximum quantum yield of photosystem II (F_v/F_m), which is a well-known parameter used for plant stress detection (Murchie and Lawson, 2013). Another spectral index (Ctr1) has been reported to decrease after abiotic stress occurrence (Rosa et al., 2023; Samarina et al., 2024). Therefore, the tendency for increased values for NPCI and Ctr1 following the SA or MeJA treatments implied that SA and MeJA might reduce stress in rosemary plants during growth.

In addition, this study investigated other spectral indices calculated using wavelengths related to the essential oil content as reported by Ouaddari et al. (2022). Specifically, the R400, R425, R450, R475, and R500 indices were calculated from leaf reflectance at 400, 425, 450, 475, and 500 nm, respectively, compared to leaf reflectance at 600 nm (Table 1). The results showed that the R425 and R450 indices significantly responded to the SA and MeJA treatments at specific concentrations (Figures 4B,C). This result was associated with the increase in the rosemary essential content after SA or MeJA treatments (Figure 5). Another study highlighted eucalyptol as the main volatile compound in rosemary essential oil, constituting over 82% (Tadtong et al., 2015). The eucalyptol was associated with the alteration of light absorption of the photosynthetic pigment of Chlorella vulgaris, especially at 413, 453, and 457 nm (Zhao et al., 2016). Consequently, the R425 and R450 indices indicated a significant response to the increase of the essential oil after SA or MeJA applications, which will be used for non-destructive estimation of the essential oil content in rosemary in the future.

The impact of SA application on the increase in the essential oil content recorded in this study has been documented in various plant species (El-Esawi et al., 2017; Pirbalouti et al., 2019; Sabagh et al., 2022; Ahmed and Shehata, 2023). However, the impact of SA and its concentration varied depending on the plant species. For example, with lemon balm, SA applications in the range 0.14–14 g · L⁻¹ considerably increased the essential oil content (Pirbalouti et al., 2019). Similarly, there was an increase in the essential oil content at 400 ppm SA with rosemary grown in sandy calcareous soil (Ahmed and Shehata, 2023). In cumin, SA applications at concentrations of 0.01–1 mM increased the essential oil content in the fruit (Rahimi et al., 2013), while under stress conditions, SA had a positive impact on the accumulation of essential oil (El-Esawi et al., 2017; Sabagh et al., 2022) due to its role as a key signalling molecule in plant defence mechanisms. SA stimulates the binding of proteins constituting the cell wall structure, enhancing wall strength (Napoleão et al., 2017). Additionally, SA inhibits mechanisms responding to plant injury, inhibits ion leakage through cell membranes, promotes rapid water and mineral absorption in the roots, reduces leaf transpiration by regulating stomatal aperture, and stimulates leaf senescence (Shaukata et al., 2022; Tyagi et al., 2022). The application of MeJA (0.01 mM) increased the essential oil content in this study (Figure 5), which could be attributed JA modulating gene expression (Kianersi et al., 2021), influencing glandular trichome development (Cappellari et al., 2019), and interacting with other signalling pathways (Yu et al., 2021) involved in essential oil biosynthesis.

There were significantly positive correlations between some spectral indices (R425, R450, NPCI, and Crt1) and the rosemary essential oil contents (Figure 6) with increased SA or MeJA application (Figures 3–5). Based on these results, leaf reflectance values in the range of 400–500 nm could be used for the estimation of the essential oil content in rosemary (Figure 7). In addition, this study showed that only chl a was positively correlated with total chl (Figure 6), which was supported by Böszörményi et al. (2020), who found that the level of chl a in rosemary leaves was about three times higher than for chl b. The relationship between an increase in chl b and some rosemary growth parameters, such as stem height and canopy width (Figure 6), might have been due to the increase in chl b synthesis enhancing light-harvesting proteins that resulted in increased plant growth (Biswal et al., 2012).

Figure 7.

The summary effects of 1 mM SA and 0.01 mM MeJA on growth and essential oil production in rosemary. The R425 and R450 indices showed a high correlation to rosemary essential oil. MeJA, methyl jasmonate; SA, salicylic acid.

The outcomes of this study suggested that SA and MeJA had the potential to enhance stem height, canopy width, and the essential oil yield in rosemary, particularly at specific concentrations (Figure 7). These findings should provide valuable insights that can be applied to optimise rosemary cultivation practices.

The application of 1 mM SA and 0.01 mM MeJA significantly enhanced growth, spectral indices (R425 and R450), and essential oil production in rosemary. The application of SA and MeJA had the potential to apply for rosemary cultivation in regions with similar climates to Phetchabun province, Thailand, which had the highest temperature up to 30°C.