Enhancing growth and quality of Handroanthus impetiginosus (Mart. ex DC.) Mattos (Bignoniaceae) seedlings by rhizobacteria inoculation

Background: Handroanthus impetiginosus (lavender trumpet tree) is valued for its construction, medicinal, and ecological uses. However, its slow initial growth and weak root system hinder seedling production. Rhizobacteria from the genera Bacillus and Azospirillum enhance plant growth and resilience. This study evaluated their effects on H. impetiginosus seedling development. Methods: The experiment was conducted in a greenhouse at the Experimental Nursery of Ornamental and Forestry Plants - College of Agricultural and Veterinary Sciences (UNESP/FCAV), Jaboticabal, Brazil, during the summer 2022/23. A completely randomised design included five treatments: Bacillus subtilis, B. megaterium, B. amyloliquefaciens, Azospirillum brasilense, and a control (no inoculation). Each treatment had four replicates of 20 plants. Seeds were sown in 280 cm³ tubes with a commercial substrate, composed of peat, vermiculite, roasted rice husk, calcined dolomitic limestone, NPK 14-16-18, and micronutrients. The rhizobacteria were inoculated at 30 and 60 days after sowing. Growth parameters (shoot height, stem diameter, root length, and biomass) were assessed at 107 days. Photosynthetic performance and microbiological colonisation were also evaluated. Data were analysed using ANOVA, Tukey’s test and Pearson correlation. Results: Azospirillum brasilense significantly enhanced growth, with the highest averages for shoot height (13.9 cm), stem diameter (1.72 mm), shoot dry mass (0.172 g), and total dry mass (0.686 g). It also improved seedling quality indices, including the Dickson Quality Index and shoot height-to-stem diameter ratio. Photosynthetic efficiency increased when inoculated with Azospirillum brasilense, as indicated by greater leaf area and chlorophyll content. Colony Forming Units (CFU) analysis showed higher bacterial colonisation in the substrate, roots, and aerial parts of A. brasilense-treated plants, with strong correlations between colonisation and plant growth. Conclusions: Azospirillum brasilense was the most effective rhizobacterium promoting H. impetiginosus seedling growth and quality. Its use could enhance reforestation and nursery production efficiency, accelerating seedling establishment. These findings highlight the potential of rhizobacteria to improve seedling vigour and adaptation in early growth stages.