Screening of Rhizosphere-Associated Bacteria from Rhododendron arboreum for Plant Growth-Promoting Characteristics in the Northwestern Himalayas

Abstract

The rhizosphere of native Himalayan plants represents a potentially valuable reservoir of microorganisms adapted to distinctive edaphic and climatic conditions. The present study was undertaken to isolate culturable bacteria from the rhizosphere of Rhododendron arboreum and evaluate their plant growth-promoting rhizobacterial (PGPR) characteristics. Thirteen rhizospheric soil samples were collected from different locations in Kangra district, Himachal Pradesh, India. A total of 56 morphologically distinct bacterial isolates were recovered and screened for nitrogen-fixing ability, phosphate solubilization, siderophore production, hydrogen cyanide (HCN) production and indole-3-acetic acid (IAA) production. Among the isolates, 18 (32.14%) exhibited phosphate-solubilizing activity, five (8.93%) showed nitrogen-fixing ability, while seven isolates each (12.50%) were positive for siderophore, HCN and IAA production. Six promising phosphate-solubilizing and six IAA-producing isolates were subsequently selected for quantitative evaluation. Considerable isolate-dependent variation was observed in both traits. Phosphate solubilization ranged from 307.86 to 879.57 µg mL⁻¹ during different incubation periods, with isolate Dha1(1) recording the highest overall mean (798.99 µg mL⁻¹), followed by D3(10) (701.85 µg mL⁻¹). Quantitative IAA production ranged from 18.72 to 68.37 µg mL⁻¹, with Dha1(1) exhibiting the highest mean production (64.13 µg mL⁻¹). The occurrence of isolates possessing multiple plant growth-promoting characteristics demonstrates the functional diversity of culturable bacteria associated with the R. arboreum rhizosphere. The findings identify the northwestern Himalayan R. arboreum rhizosphere as a promising source of indigenous PGPR for further characterization and evaluation as microbial inoculants for sustainable agriculture.