Biochar and plant growth-promoting rhizobacteria enhance physio-biochemical traits, secondary metabolites, oil, and grain yield of rapeseed under salinity stress

Salinity stress poses a significant constraint on global productivity. This study was designed to investigate the combined effects of inoculation with Azotobacter chroococcum (Az) and soil amendment with biochar on the performance of rapeseed ( Brassica napus L.) under varying levels of NaCl-induced salinity. A factorial experiment evaluated key physio-biochemical responses, oxidative stress indicators, defense compound accumulation, and yield components. Salinity stress negatively impacted rapeseed growth, significantly reducing photosynthetic efficiency (Fv/Fm), chlorophyll content, relative water content (RWC), ATP levels, and ultimately grain and oil yields. Concurrently, salinity (120 mM and control treatments) increased indicators of oxidative damage — electrolyte leakage by 54%, malondialdehyde by 37% — and elevated levels of reactive oxygen species (O 2 •− at 97 µmol g −1 FW h −1 and H 2 ​O 2 ​ at 86 µmol g −1 FW). Additionally, it induced defense responses such as increased antioxidant enzyme activity and osmolyte accumulation. However, the co-application of Az and biochar significantly mitigated these adverse effects of salinity. Notably, the combined treatment, particularly Az inoculation with an elevated biochar application rate (20%), demonstrated synergistic effects. This combination significantly enhanced photosynthetic efficiency, improved plant water status, and augmented antioxidant defense systems as well as osmotic adjustment mechanisms (including soluble sugars, proline, etc.) relative to untreated plants under saline conditions. Under these conditions, grain and oil yields increased substantially by 16.8% and 12.6%, respectively, compared to untreated saline-stressed plants. These findings highlight the potential of combining Az inoculation with biochar application as a promising and sustainable strategy to improve salinity tolerance and enhance the productivity of rapeseed in salt-affected agricultural environments.