Physiological and Photochemical Screening of Leguminous and Non-leguminous Plants Interacting with Nitrogen-Fixing Microbe Pseudomonas fluorescens (NAFP-19) Modulating Stress Tolerance in a Sub-optimal Condition

Predictable climate change has made salt stress the primary worldwide stressor that reduces global crop yield. An assessment was conducted on the role of Pseudomonas fluorescens strain (NAFP-19) on plant growth, light harvesting efficiency (specifically photosystem II functionality), antioxidant ability, and their subsequent effect on salt tolerance of leguminous and non-leguminous crop plants. Seeds were immersed in bacterial isolates @ of 10⁷ CFU mL⁻¹ whereas unprimed seeds were only soaked in distilled water for a set period of 16 h, afterward the seeds were dried in a laminar flow. Four weeks old plants were subjected to salt stress @ 75 and 150 mM NaCl. The plants were kept in an open field where humidity levels were 65–70 ± 5%, the temperature was within the range of 26–32 °C ± 2 °C, and the duration of light varied between 13.5 and 10.5 h. The assessment of subjected plants shows, in comparison to unprimed controls, P. fluorescens strain (NAFP-19) priming increases shoot (64.7 and 45.6% in corn and 41.3 and 77.1 in pea) and root lengths (23.3 and 30.5% in corn and 36 and 61.3% in pea) of plants grown under salt stress environment. Bacterial priming did substantial improvements in relative water content, osmotic potential, chlorophyll contents and stomatal conductance in non-legume (corn) than legume (pea) plants under mild and moderate stress. Stress markers like MDA and H₂O₂ concentration were significantly reduced by bio-priming with P. fluorescens. Furthermore, under mild and moderate stress, bacterial priming in non-legume plants demonstrated greater ascorbate peroxidase activities and sodium ions accumulation as compared to legume plants. Under salt stress conditions, bacterial priming did a considerable decrease in energy dissipation (NPQ) and a marked improvement in the performance index (PI) and maximal quantum yield of primary photochemistry (Fv/Fm) in non-legume. It is suggested that the P. fluorescens strain (NAFP-19) sustains photosystem II functionality to mitigate salt stress due to better antioxidant capacity in non-legume as compared to legume plants.