Effect of salicylic acid on phenolic compounds, antioxidant and antihyperglycemic activity of Lamiaceae plants grown in a temperate climate

Background: The Lamiaceae family, one of the most important herbaceous and shrub plant families, includes a wide variety of plants with biological and medicinal uses. This study aimed to conduct a comparative analysis of phenolic compounds content and biological activity of extracts from eight species of Lamiaceae plants, cultivated in a temperate climate, and to study the effect of the foliar salicylic acid application on these parameters. Methods: Lamiaceae plants (Lavandula angustifolia, Salvia officinalis, Hyssopus officinalis, Agastache foeniculum, Thymus serpyllum, Mentha $\times{}$ piperita, Origanum vulgare, Monarda didyma) cultivated on field experimental sites. Plants were sprayed with salicylic acid at a concentration of 1 mM. The untreated with salicylic acid plants were used as control. Results: The highest contents of hydroxycinnamic acids and flavonoids in control plants were observed in the Agastache foeniculum (6.4 $\pm{}$ 0.6 mg g ${}^{-1}$ and 6.5 $\pm{}$ 0.4 mg g ${}^{-1}$ respectively). The highest content of phenolic compounds was found in Monarda didyma (13.8 $\pm{}$ 0.7 mg g ${}^{-1}$ ). Among the control plants, Agastache foeniculum, Hyssopus officinalis, and Mentha $\times{}$ piperita were characterized by the highest antioxidant activity. All the studied extracts had the ability to inhibit $\alpha{}$ -amylase and $\alpha{}$ -glucosidase. Significant positive correlations were revealed between the antioxidant activity and the contents of hydroxycinnamic acids, total phenolic compounds, and flavonoids. A high degree of correlation was found between the $\alpha{}$ -amylase inhibitory activity and the content of hydroxycinnamic acids (r = 0.72, p $\leq$ 0.05), as well as between the $\alpha{}$ -glucosidase inhibitory activity and the content of flavonoids (r = 0.83, p $\leq$ 0.05) and hydroxycinnamic acids (r = 0.81, p $\leq$ 0.05). The foliar treatment with salicylic acid led to an increase in the contents of hydroxycinnamic acids (in 6 species), flavonoids (in 2 species), total phenolic compounds (in 7 species), antioxidant activity (in 5 species), as well as in $\alpha{}$ -amylase (in 4 species) and $\alpha{}$ -glucosidase (in 5 species) inhibitory activity compared to the control plants. Lavandula angustifolia was the most susceptible to foliar treatment with salicylic acid. In this plant species, a significant increase in all studied biochemical parameters was noted. Conclusions: The results obtained on the stimulating effects of salicylic acid can be used to increase the nutritional and pharmacological value of plants of the Lamiaceae family cultivated in temperate climates.