Synergism of Bradyrhizobium and Azospirillum baldaniorum improves growth and symbiotic performance in lima bean under salinity by positive modulations in leaf nitrogen compounds

Salinity causes serious agricultural losses, and inoculation with plant growth-promoting bacteria (PGPB) represents an alternative to mitigate this and other abiotic stresses. This study aimed to investigate the effectiveness of five different rhizobia strains (B. elkanii BR 2003, B. pachyrhizi BR 3262, B. yuanmingense BR 3267, B. paxllaeri BR 10398, and B. icense BR 10399) and one Azospirillum baldaniorum strain (Sp245), alone or in combination, in inducing salt stress tolerance in lima bean. Plants were grown from seeds and inoculated with different rhizobia strains, A. baldaniorum, or both (coinoculation) and exposed to 0 (control) and 75 mM NaCl. Uninoculated plants were used as absolute control. Salinity negatively affects the absolute growth rate, root length, plant biomass (shoot, root, and nodules), number of nodules, and photosynthetic pigments. However, plants coinoculated with rhizobia and A. baldaniorum showed the highest value for these parameters. Salt stress provokes negative alterations in nitrogen compounds (nitrate, free ammonia, free amino acids, proline, and soluble protein). Nevertheless, these responses were minimized in plants coinoculated with rhizobia and A. baldaniorum — mainly BR 2003 and A. baldaniorum or BR 10399 and A. baldaniorum. The highest total N values were registered in plants coinoculated with BR 3267 and A. baldaniorum or BR 10399 and A. baldaniorum. Under salt stress, coinoculated plants show a minor increase in sodium and the highest potassium content values, nitrogen derived from the atmosphere, and nitrogen fixation efficiency than plants inoculated with rhizobia. Our findings indicate that lima bean coinoculation with rhizobia and A. baldaniorum was beneficial under salinity and improved salt tolerance. Synergistic interactions between rhizobia and A. baldaniorum, mainly among BR 2003 and A. baldaniorum and BR 10399 and A. baldaniorum, attenuated the deleterious effects of salt stress on lima bean. Furthermore, nitrogen fixation in root nodules was improved in coinoculated plants. Our results may be useful to develop adequate combinations of inoculants with rhizobia and A. baldaniorum for improving the growth and symbiotic performance of lima bean and others legumes under stressful conditions.