PGPR Improves Barley Performance Under Saline Irrigation: Agronomic, Biochemical, and Transcriptional Evidence from a Two-Season Field Study

Saline irrigation is a major constraint to crop production in newly reclaimed desert lands, even when pre-sowing soil salinity is low. This two-season field study evaluated whether plant growth-promoting rhizobacteria could improve barley performance under saline irrigation water with an electrical conductivity of 11.8 dS m−1 in the El Moghra region, Egypt. The barley cultivar Giza 2000 was grown under five inoculation treatments: an uninoculated saline-irrigated control; a single inoculation with Azospirillum lipoferum; and combined inoculations with A. lipoferum and Bacillus coagulans, Bacillus circulans, or Enterobacter cloacae. Because freshwater was unavailable at the experimental site, treatment effects were evaluated relative to the saline-irrigated control. Across both growing seasons, single inoculation with A. lipoferum produced the most consistent improvements in growth, yield formation, nutrient accumulation, soil biological activity, and seed nutritional quality. The combined treatment of A. lipoferum and B. circulans was generally the second-most effective. Bacterial inoculation also improved adjustment to physiological stress, as indicated by greater proline accumulation, lower antioxidant enzyme activities, and enhanced expression of stress-related genes associated with proline biosynthesis and secondary metabolism. Overall, the results indicate that A. lipoferum applied alone was more effective than the tested combinations of bacteria under saline irrigation. These findings provide field-based evidence that inoculant performance depends on strain composition and that single-strain inoculation can be a promising strategy for improving barley production in reclaimed sandy soils irrigated with saline water.