The developmental stage and arbuscular mycorrhizal symbiosis influence the essential oil yield, chemical profile, and biological activities in Thymus pallidus, T. satureioides, and Lavandula dentata

Aromatic and medicinal plants (AMP) have gained a strong position on the global market due to their various biological activities and richness in secondary metabolites. However, more than 23 % of AMP species are threatened with extinction due to environmental and anthropogenic factors (drought, overharvesting). Several studies have highlighted the positive effect of arbuscular mycorrhizal fungi (AMF) on AMP development in these harsh environments. However, mycorrhiza establishment and functionality depend on the plant's developmental stage. This study aimed to evaluate the impact of the mycorrhizal potential of the rhizospheric soil and the plant developmental stage on the essential oil (EO) yield and biological activity in three AMP species: Thymus satureioides Batt, T. pallidus Coss, and Lavandula dentata L. Our results showed that the highest mycorrhizal intensity was recorded at flowering in the three AMP, while the highest number of propagules was recorded at senescence in T. pallidus and T. satureioides, but at flowering in L. dentata. The EO yield was highest at senescence in thyme (10-15 mu L/g dry matter) and flowering in lavender (14.3 mu L/g DM). Additionally, the developmental stage substantially impacted the chemical profile and the abundance of terpene compounds regardless of the AMP species. Some compounds were more impacted, such as borneol (0 < % < 27.7), thymol (9 < % < 18), and terpinene-4-ol (2.2 < % < 17.2) in T. satureioides, terpinene-4-ol (0.4 < % < 45.8), thymol (0 < % < 32), gamma-terpinene (0 < % < 18), and borneol (1.4 < % < 25.6) in T. pallidus, and camphor (28 < % < 74.4) in L. dentata. These chemical profile variations significantly impacted EO's biological properties, such as antibacterial, antifungal, and herbicidal activities. These activities were more effective at flowering and senescence than at the vegetative stage. In conclusion, harvesting during flowering or senescence stages is important to obtain a high EO yield with interesting properties. Due to the high abundance of AMF spores, these stages would be more convenient for identifying and multiplying the appropriate AMF strain for each AMP.