Introduction: Drought is one of the most important abiotic stresses that affects plant growth, development and biochemical processes of plants such as the production of secondary metabolites and aromatic chemical compounds. Salvia subgenus Perovskia belongs to the Lamiaceae family and it has numerous therapeutic properties. In general, the anti-oxidative defense mechanisms are mainly classified into enzymatic (e.g., guaiacol peroxidase, and ascorbate peroxidase) and non-enzymatic, i.e., polyphenolic compounds, terpenoids, proline, which can coordinately detoxify excess reactive oxygen species in the cells. Indeed, water deficit stress can impose profound effects on quality and quantity of essential oils in aromatic plants.
Materials and Methods: In order to investigate the effects of limited irrigation on polyphenol contents, antioxidant enzyme activities and essential oil constituents of two populations of S. abrotanoides viz., Abyaneh and Mezdeh populations, as endemic species of Iran, a randomized complete block design with three replications was conducted at the Research Farm of Isfahan University of Technology, in two consecutive years 2020-2021. Water deficit levels included field capacity (as control), 70% of field capacity (mild stress), and 50% of field capacity (severe stress).
Results and Discussion: The results showed that total phenolic content, total flavonoids, antioxidant enzymes activities, proline content, and essential oil yield substantially increased with the severity of stress as compared with well-watered plants. Furthermore, the highest contents of total phenolics, total flavonoids, and antioxidant activities were obtained in Abyaneh population in comparison to the other ones at the second year of cultivation. In other words, Abyaneh has a greater capacity in regulating water deficit stress than the other population. The essential oil content ranged from 1.01 to 2.82% in Abyaneh population and 0.78 to 2.04% in Mezdeh population. The analysis of the essential oils by GC/MS revealed the presence of a large number of components represented mainly by oxygenated monoterpenes. Camphor concentration ranged from 18.28% (Abyaneh population and mild stress) to 29.26% (Mezdeh population and severe stress). The amount of berneol ranged from 1.18% (Mezdeh population and well-watered level) to 4.34% (Mezdeh population and mild stress), and the level of δ-3-carene ranged from 1.62% (Mezdeh population and severe stress) to 11.81% (Abyaneh population and mild stress). The highest and lowest amount of 1,8-cineole were found in Abyaneh population under mild stress level (25.79%) and Mezdeh population under well-watered level (15.95%), respectively. Among main essential oil compositions, Abyaneh population showed the highest amount of α-pinene under well-watered level (10.81%), while this population had the lowest value (5.04%) under severe stress level. Furthermore, the concentration of myrcene were obtained in the range of 0.59% to 9.90%, in which the highest value was observed for the sample collected from Abyaneh population under severe stress level, while the sample of this population under well-watered along with mild stress levels showed the lowest values. In both plant populations, the proportions of oxygenated monoterpenes were elevated with increasing stress severity while the sesquiterpenes (sesquiterpene hydrocarbons and oxygenated sesquiterpenes) decreased. In addition, the percentage of all prominent volatile constituents including δ-3-carene, camphor, 1,8-cineole, berneol, myrcene and α-pinene elevated at the levels of mild and severe stress, and it suggested that water stress conditions improved the quality of the studied essential oils.
Conclusion: Finally, it can be concluded that mild and severe drought stress significantly increased secondary metabolites, antioxidant activities, and the percentage of prominent volatile constituents, which provided an insight for improving the qualitative and quantitative status of bioactive components of S. abrotanoides with moisture-stress management practices.