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 esti mated using the thymidine and leucine incorporation techniques. Bacter ia were initially released from rhizosphere soil using homogenisation and centrifugation before adding the labelled substrates. The incorpor ation rate for both thymidine and leucine was higher in the rhizospher e 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 slig htly higher for the thymidine than for the leucine incorporation rate. The total incorporation rate, the cell-specific incorporation rate an d bacterial numbers increased in the rhizosphere with time during a 2- week growth period of the seedlings, while all these variables were co nstant in the bulk soil. The R/S ratio of the percentage culturable ba cteria increased 4-fold during this 2-week period. When comparing inco rporation into rhizosphere bacterial communities from zones at differe nt distances from the root tip, the incorporation rate and the cell-sp ecific 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 on ly differentiate between bacterial activity in the rhizosphere and bul k soil, but were also sensitive enough to allow studies along the root . (C) 1998 Elsevier Science Ltd. Ail rights reserved.