Single and Multi-Strain PGPR Enhance Salinity Tolerance in Canola (Brassica napus L.) Seedlings

Salinity is a major abiotic stress limiting canola (Brassica napus L.) productivity, especially in arid regions. This study aimed to evaluate the efficiency of plant growth-promoting rhizobacteria (PGPR) in enhancing early seedling tolerance to graded sodium chloride (NaCl) stress and to compare single- and multi-strain inoculations. A greenhouse experiment was conducted under NaCl concentrations ranging from 0 to 200 mM using three treatments: uninoculated control, single strain (Bacillus subtilis, SB), and a triple-strain consortium (Bacillus megaterium + Pseudomonas fluorescens + B. subtilis, TB). Growth, physiological, and biochemical traits were analysed, including relative water content (RWC), electrolyte leakage (EC), photosynthetic pigments, phenolic compounds, oxidative stress markers, and antioxidant enzyme activities such as superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), and ascorbate peroxidase (APX). Salinity significantly reduced relative water content (RWC) from 60.10% to 48.63% and increased electrolyte leakage (EC) from 62.79% to 77.95%. TB inoculation mitigated these effects by maintaining higher RWC (53.57%) and lower EC (66.40%) than the control (71.93%). TB-treated plants showed higher chlorophyll (0.84 mg g⁻¹ FW), phenolics (10.34 mg GAE g⁻¹ DW), flavonoids (4.10 mg QE g⁻¹ DW), antioxidant capacity (85.98%), and proline (122.36 mg g⁻¹ FW) compared to controls. Oxidative damage decreased, with hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) reduced to 14.01 µmol g⁻¹ FW and 60.22 nmol g⁻¹ FW, respectively. Enzymatic responses differed: SB enhanced SOD and CAT under moderate salinity, while TB increased POD and APX activities under mild stress. PGPR inoculation, particularly multi-strain consortia, alleviates salt-induced oxidative stress and improves osmotic adjustment and antioxidant defense in canola seedlings, offering an eco-friendly strategy for sustainable production in saline soils.