Rhizobacteria and 24-epibrassinolide cooperatively enhance tolerance, phenolic accumulation and antioxidant defense in salt-stressed Satureja khuzistanica

Abstract

Satureja khuzistanica is an endemic medicinal plant of Iran, valued for its essential oil and phenolic compounds, but its cultivation in arid and semi‑arid regions is often limited by salinity stress. This study aimed to evaluate the combined effects of rhizobacterial inoculation (Azotobacter chroococcum+Azospirillum lipoferum) and foliar application of 24‑epibrassinolide (EBL) at 0, 1, and 2 µM on morpho‑physiological traits, mineral uptake, and phytochemical properties of S. khuzistanica under salt stress (0, 3, 6, and 9 dS/m NaCl). The experiment was conducted as a completely randomized factorial design with three replications. The results showed that the triple interaction of salinity× bacteria× EBL significantly affected most measured traits, except calcium content. Salinity reduced growth parameters, chlorophyll content, protein, and mineral elements, while increasing proline and soluble sugars. Bacterial inoculation combined with 2 µM EBL under non‑saline conditions resulted in the highest values for stem length (27.7 cm), root length (18.8 cm), fresh weight (11.11 g), dry weight (3.95 g), total chlorophyll (1.61 mg/g), and protein content (178.1 mg/g). The highest total phenolic content in the extract (3.27%) was recorded under bacteria× 2 µM EBL× 6 dS/m, while the highest antioxidant activity in the essential oil (72.53%) and phenolic content (9.67%) were obtained under the same treatment. Principal component analysis (PCA) confirmed the separation of treatments based on salinity levels and revealed strong associations between growth‑related traits and bacteria × EBL treatments, supporting the synergistic effects of these treatments. Overall, the combined application of rhizobacteria and 2 µM EBL is recommended as an effective strategy to enhance savory productivity and phytochemical quality under saline conditions.