Pre-sowing seed priming and seedling inoculation with bioelicitor improve growth and phytochemical constituents of medicinal plants. This study was aimed to investigate the role of seed priming with silicon nanoparticles (nSi, 0, 100 and 500 mg/L) and inoculation of seedling originated from primed seeds with rhizobacteria strains (Pseudomonas fluorescens and P. putida) on physiological and metabolic attributes of Melissa officinalis L. Foliar application of P. putida on plants raised from nSi-primed (at 500 mg/L) seeds showed the wider deposition of nSi on root surface compared with that of P. fluorescens, however, P. fluorescens application on plants grown from nSiprimed (at 100 mg/L) seeds showed more open stomata on leaf surface than non-inoculated controls. Individual treatments of seed priming and seedling inoculation significantly (P < 0.05) increased plant biomass indices, leaf relative water content, photosynthetic pigments values, total soluble protein and phenolic contents, essential oil yield and their all (except thymol) major constituents, neral, geranial, and geranyl acetate compared with the control. However, the maximum increase in measured traits was recorded in mixed treatment of seed priming and inoculation of plants raised from primed seeds with rhizobacteria. Furthermore, the employed treatment combinations enhanced the free radical scavenging activities of plant extracts compared with the individual treatment and untreated control samples. According to the multivariate analyses, the treatments applied in combinations (especially seed priming at 100 mg nSi/L and seedling inoculation with P. putida) scores were substantially farther from the other treatments mean. Overall, seed priming with nSi along with seedling inoculation with pseudomonas strains play vital role in the increase in both primary and secondary metabolites of lemon balm plants.
