Kaolin application improved growth performances, essential oil percentage, and phenolic compound of Thymus vulgaris L. under drought stress.

Drought stress is one of the most significant climatic challenges, severely affecting the productivity of medicinal and aromatic plants. Kaolin, a mineral with highly effective reflective properties, has been identified as a useful tool for enhancing plant resilience to abiotic stresses. This study examined the effects of kaolin concentrations (0%, 2%, 4%, and 8%) on the growth, physiological, and phytochemical attributes of Thymus vulgaris L. under drought conditions (100%, 75%, and 50% of field capacity (FC)). The findings revealed that the best performance in growth parameters (plant diameter, height, leaf dimensions (length and width), fresh and dry weight, as well as chlorophyll concentration) was observed with the 4% kaolin foliar application under 100% field capacity conditions. The highest concentration of proline, malondialdehyhe, hydrogen peroxide and cell membrane damage was observed under drought stress (50% FC), However, foliar application of kaolin 8% led to reduced these levels compared to untreated plants grown under the same water deficit (50% FC). Foliar application of kaolin (8%) under water deficit (50% FC) also increased total phenolic concentration, total flavonoid concentration, antioxidant activity and essential oil percentage. Rosmarinic acid content in plants treated with kaolin (8%) under drought conditions (50% FC) increased 1.6 times compared to untreated plants. An investigation of changes in enzymatic and non-enzymatic systems showed that kaolin improves the antioxidant capacity of T. vulgaris under drought stress and increases its resistance. Notably, kaolin also facilitated faster restauration of physiological functions during stress recovery. In summary, kaolin foliar sprays alleviated the negative effects of water stress in T. vulgaris performance by modulating distinct physiological and biochemical responses. This suggests kaolin application can support the cultivation and production of T. vulgaris in low-yielding and arid lands.