Document Type : Research Article
Authors
1 Tea Research Center, Horticultural Science Research Institute, AREEO, Lahijan, Iran
2 Greenhouse and Controlled Environments Research Center (GCER), Horticultural Science Research Institute (HSRI), AREEO, Karaj, Iran
Abstract
Introduction
Hydroponic systems have emerged as an advanced method for cultivating medicinal plants, offering precise control over environmental and nutritional factors to enhance the production of secondary metabolites. Chemical elicitors, such as salicylic acid (SA) and methyl jasmonate (MeJA), play a key role in stimulating plant defense responses and increasing the synthesis of valuable phytochemical compounds. These compounds alleviate biotic and abiotic stress signals, triggering plant defense mechanisms and stimulating the biosynthesis of valuable secondary metabolites, including phenolics, flavonoids, and essential oils (Mehdizadeh and Moghaddam, 2023). Salvia officinalis L. (sage), a perennial aromatic plant from the Lamiaceae family, is known for its medicinal properties and rich content of bioactive compounds, including essential oils and phenolic derivatives. Recent studies have demonstrated that exogenous application of SA and MeJA can significantly influence plant growth and secondary metabolism. However, their effects are highly concentration-dependent and species-specific, necessitating further research to determine optimal dosages for different medicinal plants (Mohi-Ud-Din et al., 2025). While some studies have explored the individual effects of these elicitors, their synergistic interactions in hydroponically grown sage remain understudied (Jeyasri et al., 2023). This study investigates the synergistic effects of SA and MeJA foliar application on the growth, phytochemical composition, and antioxidant activity of sage in a hydroponic system.
Materials and Methods
This experiment was conducted as a factorial design based on a completely randomized design with three replications. Treatments included the foliar application of SA (0, 0.5, and 1 mM) and MeJA (0, 0.25, and 0.5 mM). Sage seeds were germinated under controlled conditions, and seedlings were transferred to a hydroponic system with a perlite-cocopeat (3:1, v/v) substrate. Plants were grown under natural light, with temperatures maintained at 20–30°C and relative humidity at 70–80%. Nutrient solutions were provided using Hoagland’s formula (EC 1.6 dS/m). Foliar treatments were applied at three growth stages: initial three-leaf, six-leaf, and flowering onset. Ten days after the final treatment, biomass, photosynthetic pigments, antioxidant enzyme activities (POX, APX, SOD), phenylalanine ammonia-lyase (PAL) activity, phenolic and flavonoid content, essential oil percentage, and antioxidant capacity (DPPH assay) were evaluated.
Results and Discussion
The experimental results demonstrated that the foliar application of salicylic acid and methyl jasmonate significantly influenced the growth, physiological traits, and phytochemical composition of hydroponically cultivated sage. The combined treatment of 0.5 mM SA and 0.25 mM MeJA produced the highest fresh (49.00 g) and dry (21.48 g) biomass, indicating a synergistic effect on plant growth. This enhancement can be attributed to the role of SA in promoting cell division and elongation, while MeJA at low concentrations stimulates metabolic activity without inducing excessive stress. However, higher MeJA concentrations (0.5 mM) exhibited inhibitory effects, likely due to the overactivation of defense mechanisms that divert energy away from growth-related processes. Photosynthetic pigments, including chlorophyll and carotenoids, were significantly influenced by the elicitor treatments. The combination of 1 mM SA and 0.25 mM MeJA resulted in the highest carotenoid content (2.34 mg g⁻¹ FW), suggesting that moderate elicitation enhances photosynthetic efficiency. Conversely, excessive MeJA (0.5 mM) led to a reduction in chlorophyll content, possibly due to accelerated senescence or oxidative stress. The application of SA and MeJA also induced a marked increase in antioxidant enzyme activities, including POX, SOD, and APX. Among the highest activities, POX was maximized in the combined treatment of 0.5 mM SA and 0.25 mM MeJA (20.49 U mg⁻¹ protein), while the highest APX (6.96 U mg⁻¹ protein) and SOD (1.95 U mg⁻¹ protein) activities were recorded in plants treated with 0.25 mM MeJA alone. These enzymes are critical for scavenging reactive oxygen species (ROS), which accumulate under elicitor-induced stress. The upregulation of these defense mechanisms not only protects cellular integrity but also stimulates the biosynthesis of secondary metabolites. A key finding was the significant increase in phenylalanine ammonia-lyase activity, a rate-limiting enzyme in the phenylpropanoid pathway. The highest PAL activity was observed in plants treated with 1 mM SA alone or in combination with 0.25 mM MeJA, correlating with elevated levels of total phenolics and flavonoids. Phenolic compounds, such as rosmarinic acid, are major bioactive constituents in sage, contributing to its antioxidant and antimicrobial properties. The increase in flavonoid content, particularly under the combination of 0.5–1 mM SA and 0.25 mM MeJA, further supports the role of these elicitors in activating secondary metabolic pathways. One of the most notable outcomes was the 39% increase in essential oil content under the 0.5 mM SA + 0.25 mM MeJA treatment. Essential oils in sage are synthesized via the terpenoid pathway, which appears to be strongly induced by these elicitors. The enhanced essential oil yield was accompanied by a significant improvement in antioxidant capacity, as evidenced by the DPPH radical scavenging assay. This suggests that the elicited plants not only produced higher quantities of bioactive compounds but also exhibited stronger free-radical quenching activity, which is valuable for medicinal and nutraceutical applications. These findings align with previous studies indicating that SA and MeJA act synergistically at optimal concentrations to enhance secondary metabolite production while maintaining plant growth. However, the negative effects of high MeJA levels (0.5 mM) underscore the importance of dosage optimization, as excessive elicitation can lead to phytotoxicity and metabolic imbalance.
Conclusion
This study demonstrated that the combined application of salicylic acid and methyl jasmonate in a hydroponic growth medium serves as a powerful tool for regulating growth and enhancing the production of secondary metabolites in sage plants (Salvia officinalis). A key finding was the identification of an optimal treatment combination (0.5-1 mM salicylic acid coupled with 0.25 mM methyl jasmonate), which synergistically increased biomass, essential oil content, and antioxidant activity. In contrast, higher concentrations of methyl jasmonate alone were found to inhibit growth. The implications of this research are significant, as it shows that by precisely modulating elicitor concentrations, the metabolic balance between growth and defense compound synthesis can be strategically managed to meet agricultural and pharmaceutical objectives.
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