Ameliorative effects of plant growth-promoting rhizobacteria on salinity tolerance of sweet basil (Ocimum basilicum L.)

Salinity in soil and irrigation water has negative effects on plant morphology, physiology, and biochemical processes. In this study, an experiment was conducted to investigate the effect of two plant growth-promoting rhizobacteria (PGPR) Bacillus subtilis and Pseudomonas fluorescens on sweet basil (Ocimum basilicum L.) growing under salinity stress: 0 (control, without NaCl), 50, and 100 mM NaCl. Increasing salinity significantly decreased plant height, root length, aboveground and root biomass, and photosynthetic pigment content. However, bacterial inoculation reduced the negative effects of salinity. Both antioxidant enzyme activity and antioxidant metabolites were affected by salinity and PGPR inoculation. The activity of catalase was the highest in P. fluorescens-inoculated plants at 100 mM NaCl (0.084 U g-1 FW). The highest activity of superoxide dismutase (SOD) and ascorbate peroxidase (APX) was observed in B. subtilis-inoculated plants at 100 mM NaCl (2.83 and 0.61 U g-1 FW, respectively), and the highest percentage of essential oils (EOs) was found in plants inoculated with B. subtilis under 50 mM NaCl (0.87%). The results of gas chromatography-mass spectrometry (GC-MS) showed the main constituents of the basil EOs in all treatments were oxygen-containing monoterpenes. The highest content of malondialdehyde (MDA) and hydrogen peroxide (H2O2) (as indicators of stress conditions) was related to non-inoculated plants at 100 mM NaCl (2.675 nmol g-1 FW and 0.8525 mu g g-1 FW, respectively). The highest level of nitrate reductase activity was observed under non-saline conditions and in B. subtilis-inoculated plants: 4.33 mu mol NO2 h-1 g-1 FW. Increasing salinity significantly decreased glutamine synthetase (GS) activity and total protein content in leaves. According to the main effect of inoculation, the highest glutamine synthetase activity and protein content was found in P. fluorescens-inoculated plants: 6.065 mu mol GHA mu g-1 protein h-1 and 31.77 mg g-1 FW. The highest concentration of proline was found at 100 mM NaCl along with P. fluorescens inoculation (6.27 mu g g-1 FW). These results indicate a positive effect of PGPR on plant salt stress and physiology.