Effect of Indole Acetic Acid on the Synergistic Interactions of Bradyrhizobium and Glomus fasciculatum on Growth, Nodulation, and Nitrogen Fixation of Dalbergia sissoo Roxb

The legume Dalbergia sissoo Roxb is an important forest tree of the arid and semi-arid regions. The efficiency of Bradyrhizobium (Ds Rhz-9) and Glomus fasciculatum (Thaxter sensu Gerd.) Gerd. and Trappe to promote growth of D. sissoo and the effect of indole acetic acid (IAA) on the tripartite symbiotic association was studied in a pot experiment. Parameters like plant growth, nodulation, total phosphorus, total nitrogen content, nitrogenase activity of root nodules (acetylene reduction assay), and arbuscular mycorrhizal fungi (AMF) root colonization were analyzed. The dual inoculation of Bradyrhizobium along with G. fasciculatum showed synergistic increase in growth, nodulation, and nitrogen fixation of the seedlings. The maximum nitrogen (1.45%) and phosphorus (34.6%) content of dry weight was recorded in the dual inoculated plants when treated with IAA (1.39 × 10-3 mole/liter). The nitrogenase activity of the nodules in dual treatments showed an enhancement of 34.9%, which became further enhanced by 15.7% when treated with IAA. The presence of Bradyrhizobium in the rhizosphere was found to decrease AMF root colonization. In dual inoculated plants, the exogenous application of IAA (1.11 × 10-3 moles/liter) increased AMF root colonization by 60%. The tripartite synergistic interaction of Bradyrhizobium and G. fasciculatum with D. sissoo seems to be promising and can be utilized for its growth promotion in forest nurseries and reforestation of degraded areas. Exogenous application of IAA will be beneficial and recommended.