Elucidating the Correlations Between Phytochemical Characteristics with Agronomic Performance in Lavandula Species under Drought Stress and Biostimulant

Evaluating the relationships between physiological/biochemical traits and yield-related characteristics in medicinal plants under stress conditions is crucial. This study aimed to assess these relationships in lavender genotypes (English and French) under varying drought stress levels (100%, 70±5%, and 40±5% field capacity, respectively) and the effects of different biostimulant applications (individual and combined applications of bacterial inoculants (PGPRs) and Arbuscular Mycorrhizal Fungi (AMFs)). A split-plot factorial test relied on a randomized complete block design during the 2022-2023 growing season. The chlorophyll content peaked at 40.59 µg/g Fresh Weight (F.W.) with dual fertilizer application in French lavender under optimal conditions. Proline content significantly increased under severe drought stress, reaching 823.47 µmol/g F.W. with PGPRs + AMFs application, while it was lowest in non-fertilized conditions. Antioxidant enzyme activities varied with drought stress and fertilization, generally showing higher activities under severe drought stress. Severe stress significantly reduced flower yield, with optimal yields achieved through dual fertilizer application in French lavender (1226.9 kg/ha). The use of PGPRs alone resulted in the highest averages for α-Pinene (3.01%), Camphor (6.36%), Terpinen (7.50%), and Linalyl acetate (24.59%). The combined use of PGPRs and AMFs resulted in the highest averages for Linalool (30.83%) and Caryophyllene oxide (10.22%). Essential oil yields varied: English lavender achieved the best results with standalone PGPRs under non-stress conditions, while combined PGPRs + AMFs were optimal under drought stress. For French lavender, AMFs alone were effective under moderate stress, and PGPRs + AMFs provided the best outcomes under non-stress conditions. Photosynthetic pigments, enzymes, and proline levels influenced flower yield and essential oil content.