Evaluation of Plant Growth-Promoting Properties of Native Azotobacter Isolates and the Effect of their Inoculation on Growth of Forage Maize under Salinity Stress

Large areas of Iranian soils have different levels of salinity; so the crop production in those area is very limited. Azotobacter can stimulate plant growth by various mechanisms. Inoculation of plants with superior native isolates of Azotobacter that are compatible with saline areas may enhance plant growth under salinity stress. In this study, 20 isolates of Azotobacter isolated from Iranian soils were evaluated in terms of phosphate solubility, potassium releasing, auxin and siderophore production. Salinity tolerance of isolates was investigated in bacterial culture medium at electrical conductivity of 10, 20, 30, 40 and 50 dS.m-1. Six selected isolates at three levels of salinity were evaluated on maize Single Cross 704 in a factorial completely randomized block design with three replications in a greenhouse condition. Salinity stress was applied through irrigation water (0.36, 3 and 6 dS.m-1). The laboratory results revealed that Az63, Az69 and Az70 were the most effective isolates in terms of phosphate solubilization and potassium releasing and siderophore production. All isolates of Azotobacter grown very well in saline media with electrical conductivity of 10 dS.m-1 but some of them could grow in saline media with electrical conductivity of 20 dS.m-1. Az22 and Az66 isolates were more tolerant to salinity and they survive at electrical conductivity of 50 dS.m-1. The greenhouse results showed that dry weight of clusters increased 64 percent compared to the control when plant inoculated with Az69 isolate under normal irrigation water. Inoculation had no significant effect on growth indices when electrical conductivity of irrigation water was 3 or 6 dS.m-1. In this study, Az69 isolate was suggested for further studies.