Essential oils under stress: How drought and salinity shape the physiological and biochemical profile of Thymus ×citriodorus

Abstract

In the face of escalating environmental pressures, gaining insights into how medicinal plants adapt to adverse abiotic stresses is crucial for optimizing their growth and bioactive compound production. This study investigates how drought (40 % soil water capacity), salinity (120 mM NaCl), and their combination influence the growth, physiology, and biochemical responses of Thymus × citriodorus. Morphological analyses revealed reductions in shoot and root parameters across all stress treatments, with the most pronounced declines observed under combined stress. Chlorophyll, carotenoids and relative water content, were reduced under stressed treatments but mainly under salt and combined stresses. In response to these stresses, proline, soluble sugars, and total polyphenols increased significantly, reflecting enhanced osmotic adjustment and protective antioxidant mechanisms, while elevated hydrogen peroxide levels across all stress treatments, signalled intensified oxidative stress. Interestingly, antioxidant capacity significantly decreased under stress conditions, suggesting that the oxidative load exceeded the plant’s antioxidative defences. Essential oil content peaked in unstressed plants (0.45 mL/100 g DW), while GC-MS analysis identified 17 volatile compounds, such as geraniol, geranial, and neral that exhibited substantial fluctuations, indicating stress induced adjustments in secondary metabolism tied to their defensive roles. While the plant exhibits a degree of tolerance to drought, salinity alone or in combination with drought severely compromises overall productivity. These findings provide comprehensive insights into how T. × citriodorus modulates growth and biochemical pathways under abiotic stress, offering practical implications for improving the resilience and agricultural potential of medicinal plants in regions prone to abiotic stress.