Enhancing drought resistance and phytochemical composition of Trachyspermum ammi L. through mycorrhizal fungi and salicylic acid

• Co-application of AMF and SA improved drought tolerance in Trachyspermum ammi L. • AMF under 75% FC yielded the highest essential oil content (4.01%). • 300 ppm SA with AMF increased chlorophyll content by 7.57%. • Drought enhanced γ-terpinene and p -cymene levels in essential oil. • AMF and SA boosted fixed oil yield and proline under stress conditions. Drought stress adversely affects plant growth, productivity, and secondary metabolite biosynthesis. This study evaluated the effects of arbuscular mycorrhizal fungi (AMF; Glomus intraradices ), salicylic acid (SA), and irrigation regimes on growth, yield, and phytochemical traits of Trachyspermum ammi under greenhouse conditions. The experiment was conducted as a three-factor factorial based on a completely randomized design (CRD) with four irrigation levels (100%, 75%, 50%, and 25% field capacity, FC), two AMF treatments (inoculated and non-inoculated), and three SA concentrations (0, 150, and 300 ppm). AMF inoculation significantly increased proline content (5.16 vs. 4.22 µg g⁻¹) and chlorophyll content (92.59 vs. 79.14 µg g⁻¹) compared with non-inoculated plants. The highest chlorophyll content (130.99 µg g⁻¹) was observed under AMF inoculation at 50% FC, whereas the maximum proline content (5.49 µg g⁻¹) was recorded under the combined AMF and SA (300 ppm) treatment. Essential oil (EO) percentage was significantly affected by AMF and irrigation levels, with AMF increasing EO content by 19.6% relative to non-inoculated plants. The highest EO content (4.0%) was obtained under AMF inoculation at 75% FC, while the lowest value (2.6%) occurred under 25% FC without AMF. Changes in EO composition were dependent on irrigation level and treatment combinations, with thymol increasing under severe water deficit (25% FC), whereas γ-terpinene and p-cymene varied across irrigation regimes and SA treatments, indicating treatment-dependent modulation of monoterpene biosynthesis. Fixed oil content increased by 21.2% under AMF inoculation, reaching its maximum under the combined AMF and SA (300 ppm) treatment. Statistical analysis revealed significant two-way interactions (AMF × irrigation and SA × irrigation; p < 0.05), whereas the three-way interaction (AMF × SA × irrigation) was not significant, indicating that treatment effects were primarily additive rather than synergistic. Overall, AMF inoculation and SA application improved physiological performance and phytochemical traits of T. ammi under water deficit conditions, with more consistent responses observed under moderate irrigation (50% FC).