Growth rate of rhizosphere bacteria as measured by the thymidine method

Abstract

Bacterial growth rates in soil in different proximity to roots was determined by adding ³H-thymidine and measuring incorporation of the compound into DNA. The method is presented in short together with preliminary results. The soil-plant system under study was separated into root and soil fractions and labeled immediately. Samples equivalent to 50–100 mg dry weight (DW) of root segments with adhering soil or soil without roots, was taken to microtubes and labeled as a slurry with ³H-thymidine (200 µl, 200 nM, 30 Ci mmole^-1) for 15 min. Soil samples added formalin (10% final cone.) before labeling were included as blanks. Extraction was performed with 0.5 M NaOH (1 h, 60°C) followed by centrifugation (10,000 xg, 10 min). Supernat-ants were diluted 10 times with distilled water, acidified to pH 2 at 0°C and filtered through 0.45 µm cellulose nitrate filters. The amount of activity collected on the filter was measured by liquid scintillation.