Root plasticity and enhanced secondary metabolites biosynthesis in Zataria multiflora Boiss. under controlled drought stress

Zataria multiflora Boiss., a valuable medicinal aromatic plant native to the arid regions of Iran, faces significant challenges due to drought stress, which impacts both plant growth and secondary metabolism. This study evaluated the adaptive physiological, morphological, and phytochemical responses of this ecologically significant Lamiaceae species, including essential oil production, under irrigation levels of 100 %, 75 %, and 50 %. Drought significantly inhibited shoot growth parameters while concurrently enhancing root traits, particularly at 50 % field capacity, indicating a reallocation of resources. The observed 36.33 % reduction in growth rate, indicating physiological decline, was offset by enhanced antioxidant defenses, attributed to increased antioxidant enzyme activities that effectively alleviated oxidative stress. Gene expression analysis revealed the upregulation of McWRKY57 and ABAR6, confirming the activation of abscisic acid-dependent and transcriptional stress pathways. Drought conditions also triggered metabolic reprogramming, resulting in significantly elevated concentrations of key secondary metabolites, particularly carvacrol and thymol, which are recognized for their industrial and pharmacological significance. Under 50 % field capacity, carvacrol and thymol levels increased by 54.20 % and 48.13 %, respectively, compared to the control, accompanied by a substantial enhancement in essential oil yield, 28.74 % at 50 % FC and 32.7 % at 75 % FC. Principal component analysis revealed treatment-specific responses; plants at 100 % irrigation prioritized shoot growth, whereas those at 50 % exhibited a root-dominated architecture with increased secondary metabolites. The 75 % group displayed a balanced adaptation. Moderate to severe water stress enhances phytochemical production and stress resistance in Z. multiflora, indicating that controlled irrigation could effectively increase the production of bioactive compounds in arid agroecosystems.