Morphological and physiological responses of summer savory (Satureja hortensis L.) to azotobacter, mycorrhiza, and salicylic acid under drought stress conditions in Rey Region

This study aimed to investigate the effects of biological fertilizers and salicylic acid under water deficit stress conditions on the growth and yield of Satureja hortensis L. during two consecutive years (2018–2019 and 2019–2020). This experiment was performed as a split factorial plot arranged in a randomized block design with three replications. The main factor was irrigation at three levels (full irrigation, mild, and severe water deficit stress). The sub-factors included biological nitrogen fertilizer (non-inoculated, inoculated with Azotobacter, inoculated with Mycorrhiza, and inoculated with Azotobacter + Mycorrhiza) and salicylic acid (no spray and spray with a concentration of 0.6 mM). Although, water deficit stress decreased the total dry weight but, inoculated plants had higher total dry weight rather than control under water deficit conditions. The results showed that the inoculation of Azotobacter and Mycorrhiza decreased the content of total chlorophyll and carotenoids and increased leaf area and thymol content. Moreover, carvacrol content and essential oil harvest index had the highest value under a full irrigation regime, and inoculation treatment decreased these values. In contrast, mild water deficit conditions increased ɣ-Terpine and phenol content, but severe water deficit stress decreased these traits. The highest total chlorophyll content (3.46 mg g FW⁻¹) carotenoids content (0.22 mg g FW⁻¹), leaf area (649.88 cm), total dry weight (2540.9 kg ha⁻¹), essential oil content (1.7%), essential oil yield (43.1 kg ha⁻¹), thymol (3.58%), carvacrol content (8.47%) and essential oil harvest index (1.7%) were achieved in full irrigation and salicylic application. Based on our results, Azotobacter and Mycorrhiza can improve the morphological traits of summer savory plants, but in the case of photosynthesis pigments, they were not helpful. On the other hand, mild water deficit stress increased the thymol, carvacrol, ɣ-Terpine and phenol content, which means that summer savory plants by improving secondary metabolites, could ameliorate the water deficit damage. The current study proved beneficial in improving the physiological and morphological traits of summer savory, suggesting that the application of Azotobacter + Mycorrhiza or salicylic acid application can be a suitable solution to increase plant tolerance under water deficit conditions.