Co-inoculation of Phosphate-Solubilizing Bacteria and Mycorrhizal Fungi: Effect on Seed Yield, Physiological Variables, and Fixed Oil and Essential Oil Productivity of Ajowan (Carum copticum L.) Under Water Deficit

Rhizosphere symbiotic microorganisms play a significant role in mitigating the effects of water deficit on plant production. This work explores the effects of co-inoculation of phosphate-solubilizing bacteria (PSB) and arbuscular mycorrhizal fungi (AMF) on seed yield, essential oils (EOs), and oil composition of Ajowan under water deficit conditions. We assessed the response of Ajowan plants to individual or combined inoculation with AMF (Rhizophagus intraradices) and PSB (Pseudomonas putida + Pantoea agglomerans) under several irrigation regimes (irrigation at 100% FC, 75% FC, and 50% FC, corresponding to normal irrigation, moderate water stress, and severe water stress, respectively). Normal irrigation with AMF + PSB co-inoculation produced the highest seed yield (870 kg ha⁻¹). Moderate water stress with AMF + PSB co-inoculation yielded the most essential oil yield (22.3 kg ha⁻¹). For inoculated and uninoculated plants, severe water stress decreased chlorophyll a by 44.80% relative to normal irrigation. Proline, soluble sugars, and carotenoid contents, and peroxidase, superoxide dismutase, and catalase activities increased with increasing water deficit, more so with AMF + PSB co-inoculation than individual inoculations. Water deficit also increased the essential oil content in Ajowan seeds. Co-inoculation with AMF and PSB increased the percentage of major EO components—thymol (50.94%) and γ-terpinene (33.18%)—and the main fatty acids—petroselinic (64.28–70.11%), oleic (16.05–21.9%), and palmitic acids (6.50–9.10%)—under moderate water stress, relative to uninoculated plants. Co-inoculation with AMF and PSB significantly improved EO quantity and quality in Ajowan seeds under moderate water stress.