The changes in growth, yield, and biologically active compounds of essential oil in Trachyspermum ammi L. upon rhizobacteria and seaweed applications

Using biostimulants to enhance plant growth and increase yield and secondary metabolites in medicinal and aromatic plants is an important strategy to achieve sustainable agriculture. The influence of two strains of nitrogen-fixing rhizobacteria (NFB) of Azotobacter chroococcum (NFB1) and Azospirillum lipoferum (NFB2), three levels of seaweed extract (SWE; 0 (SWE1), 250 (SWE1), and 500 mg/L (SWE2)) and their interactions have been investigated on Trachyspermum ammi L. (ajwain) growth, fruit yield, and essential oil constituents for two winter seasons. Growth traits (plant height, number of branches, and fresh and dry weights) and fruit traits (umbel number, 1 000-fruit weight, and fruit yield) were improved following NFB and/or SWE applications. Leaf pigments, total phenols, carbohydrates, free amino acids, and nutrient content were also enhanced. Ajwain plants that received NFB2 soil inoculation and foliarly sprayed with SWE1 observed the highest growth and yield values. Applying this treatment resulted in 27.6% and 32.7% higher fruit yield per plant for the first and second seasons, respectively, compared to the control. The results of GC-MS revealed that γ-terpinene, p-cymene, and thymol are the major components in ajwain essential oil. All applications used changed the percentages of the main components detected in ajwain essential oil. For instance, increasing SWE level caused a reduction in γ-terpinene with an increase in thymol content. The highest conservation rate from γ-terpinene to thymol was detected in NFB2 × SWE1-treated plants, with the highest thymol content and least γ-terpinene. Azospirillum lipoferum soil inoculation with SWE1 foliar application is recommended to enhance ajwain production, in terms of fruit yield and oil quality.