Surviving the Extremes: The Synergistic Impact of Drought and Salinity on Thymus capitatus Growth, Physiology, and Biochemistry

Thymus capitatus, a Mediterranean medicinal plant, exhibits complex physiological and biochemical responses to environmental stress. This study investigated the effects of drought (40% SWC), salinity (70% SWC + 90 mM NaCl), and their combination (40% SWC + 90 mM NaCl) on the morphological, physiological, and biochemical traits of T. capitatus over 39 days. All stress treatments reduced shoot and root biomass, relative water content, chlorophyll, and carotenoids, with combined stress causing the most severe declines. Proline and soluble sugars accumulated, indicating osmotic adjustment. Total polyphenol content remained stable under single stress but increased under combined stress (123.28 mg GAE/g DW), suggesting an enhanced defense response. Hydrogen peroxide levels surged, particularly under combined stress (7.76 µmol H2O2/g FW), reflecting oxidative stress. Essential oil yield declined from 3.22 mL/100 g DW under control conditions to −30% and −34% under drought and combined stress, respectively, while carvacrol content increased (+4.71%) under combined stress, indicating a stress-induced metabolic shift. Antioxidant capacity significantly declined under salt and combined stress, likely due to oxidative stress overwhelming the plant’s defense mechanisms. These findings highlight Thymus capitata’s resilience, with combined stress having the most detrimental impact, followed by drought, while salinity had a more moderate effect. Despite these challenges, the plant retained key bioactive compounds, reinforcing its potential for stress prone environments.