Bacterial activity along a young barley root measured by the thymidine and leucine incorporation techniques

The bacterial activity in the rhizosphere of barley seedlings was estimated using the thymidine and leucine incorporation techniques. Bacteria were initially released from rhizosphere soil using homogenisation and centrifugation before adding the labelled substrates. The incorporation rate for both thymidine and leucine was higher in the rhizosphere than in the bulk soil. This was due both to higher bacterial numbers and higher cell-specific incorporation rates for rhizosphere bacteria compared with bulk soil. The rhizosphere-to-soil ratio (R/S) was slightly higher for the thymidine than for the leucine incorporation rate. The total incorporation rate, the cell-specific incorporation rate and bacterial numbers increased in the rhizosphere with time during a 2-week growth period of the seedlings, while all these variables were constant in the bulk soil. The R/S ratio of the percentage culturable bacteria increased 4-fold during this 2-week period. When comparing incorporation into rhizosphere bacterial communities from zones at different distances from the root tip, the incorporation rate and the cell-specific incorporation rate of both thymidine and leucine increased with distance from the root tip, irrespective of the seedling age (4 to 8 d). The thymidine and leucine incorporation techniques thus did not only differentiate between bacterial activity in the rhizosphere and bulk soil, but were also sensitive enough to allow studies along the root.