Salt-tolerant plant growth-promoting bacteria enhanced salinity tolerance of salt-tolerant alfalfa (Medicago sativa L.) cultivars at high salinity

Alfalfa (Medicago sativa L.) plant growth decreases when cultivated under salinity or irrigated with salty water. Inoculation with plant growth-promoting bacteria (PGPB) is a method for mitigating the harmful effects of salinity on plants growth. To investigate salt-tolerant PGPB with salt-tolerant and salt-sensitive alfalfa cultivar interactions under salinity, some physiological and agronomical aspects were investigated. The inoculated plants of alfalfa cultivars with Hartmannibacter. diazotrophicus and Pseudomonas sp. bacteria were compared with non-inoculated plants. Plants were grown in growth room and irrigated with tap water until 6–7 weeks, and then, salinity stress imposed by irrigating with tap water (control), 10 dS m−1 and 20 dS m−1 NaCl. Salinity reduced relative water content (RWC), membrane stability index (MSI), K+, photosynthesis rate (Pn) and stomatal conductance (gs), leaf number, height, and dry weight, and increased sodium in all cultivars. Inoculation of cultivars with both PGPB mitigated the negative effects of salinity on plants growth by increasing the root length and weight, nodule number, chlorophyll pigments, RWC, MSI, Pn, and gs. Chlorophyll pigments, plant height and leaf number, Na+, K+/Na+, and nodule number improved more pronounced through inoculating with Pseudomonas sp., whereas K+, carotenoids, and RWC improved more pronounced through H. diazotrophicus under salinity. The results showed inoculation with two bacteria improved growth performance in salt-tolerant and salt-sensitive cultivars under 10 dS m−1, but at high salinity (20 dS m−1), inoculation was successful only in salt-tolerant alfalfa cultivars.