Increased Root Exudation of ¹⁴C-Compounds by Sorghum Seedlings Inoculated with Nitrogen-Fixing Bacteria

Root exudation of Sorghum bicolor seedlings in relation to inoculation with nitrogen-fixing bacteria was studied by exposing the plants to ¹⁴CO₂ and measuring the ¹⁴C recovery in the root exudates. Sorghum seedlings were aseptically grown in nutrient solution for 2 weeks and inoculated with Azospirillum brasilense, Azotobacter vinelandii, and Klebsiella pneumoniae Nif(-). A week after the inoculation, the plants were exposed to 10 μCi ¹⁴CO₂ for 2 hours and harvested 24 hours after the exposure. The amounts of root exudates increased with increasing nitrogen originally added to nutrient solution. Inoculation with Azospirillum and Azotobacter increased root exudation of ¹⁴C-labelled compounds by 33% of total ¹⁴C recovered from shoots, roots, and nutrient solution was found in the root exudates. When sucrose was added to the nutrient solution as a carbon source, bacteria failed to stimulate root exudation.