Assessment of cumulative microbial respiration and their ameliorative role in sustaining maize growth under salt stress

Cumulative microbial respiration reflects microbial activities and their potential to support plant growth, where salt tolerant rhizobacteria can optimize their respiration, and ensure plant survival under salt stress. We evaluated cumulative microbial respiration of different salt tolerant rhizobacterial strains at different salinity levels, and checked their ability to sustain plant growth under natural saline conditions by using maize as test crop. Our results revealed that at the highest EC level (10 dS m−1), strain ‘SUA-14’ performed significantly better, and exhibited the greatest cumulative respiration (4.2 fold) followed by SHM-13 (3.8 fold), as compared to un-inoculated control. Moreover, results of the field trial indicated a similar trend, where significant improvements in shoot fresh weight (59%), root fresh weight (80%), shoot dry weight (56%), root dry weight (1.4 fold), leaf area (1.9 fold), straw yield (41%), cob diameter (33%), SPAD value (84%), yield (99%), relative water contents (91%), flavonoid (55%), 1000 grain weight (∼100%), soluble sugars (41%) and soluble proteins (45%) were observed due to inoculation of strain ‘SUA-14’ as compared to un-inoculated control. Similarly, substantial decline in leaf Na+ (34%), Na+/K+ ratio (69%), electrolyte leakage (8%), catalase (54%), peroxidase (73%), and H2O2 (50%) activities were observed after inoculation of ‘SUA-14’ with a concomitant increment in the leaf K+ contents (70%) under salinity stress than un-inoculated control. Hence, among all the tested rhizobacterial isolates, ‘SUA-14’ served as the most efficient strain in alleviating the detrimental impacts of salinity on maize growth and yield. The 16S rRNA sequencing identified it as Acinetobacter johnsonii.