Salinity-Induced Modulation of Phenolic Compounds and Antioxidant Activity in Lavandula viridis and Thymus lotocephalus

Mediterranean Lamiaceae species are important sources of bioactive compounds with potential applications in food, pharmaceutical, and cosmetic industries. In the present study, controlled salinity was evaluated as a biotechnological elicitation strategy to enhance the accumulation of bioactive metabolites in two species, Lavandula viridis and Thymus lotocephalus, cultured in vitro. Morphological, biochemical and metabolic responses of shoots grown for seven weeks in culture medium containing four NaCl concentrations (0, 25, 50, or 75 mM) were assessed. Mild salinity (25 mM NaCl) stimulated shoot growth in both species, whereas higher salt concentrations progressively reduced growth. Salinity induced oxidative stress, as indicated by elevated malondialdehyde (MDA) levels at 25 mM NaCl in both species, while hydrogen peroxide (H2O2) exhibited contrasting patterns, decreasing with increasing salinity in L. viridis but peaking at 50 mM NaCl in T. lotocephalus. Salt stress caused reductions in photosynthetic pigments, phenolic compounds, antioxidant capacity (DPPH, FRAP, ABTS, and ORAC), and overall metabolic performance in L. viridis, indicating limited tolerance to salinity. Conversely, T. lotocephalus displayed greater metabolic plasticity, characterized by increased levels of phenolic compounds, particularly rosmarinic acid and its derivatives, and higher antioxidant activity of the extracts, as well as the accumulation of proline and soluble sugars. These findings demonstrate marked species-specific differences in salinity responses and identify controlled salinity as an effective elicitation strategy to sustainably produce high-value bioactive compounds in T. lotocephalus.