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.