ON THE USE OF ANTIBIOTICS TO REDUCE RHIZOPLANE MICROBIAL-POPULATIONS IN ROOT PHYSIOLOGY AND ECOLOGY INVESTIGATIONS

No straightforward method exists for separating the proportion of ion exchange and respiration due to rhizoplane microbial organisms from th at of root ion exchange and respiration. We examined several antibioti cs that might be used for the temporary elimination of rhizoplane bact eria from hydroponically grown wheat roots (Triticum aestivum cv. Veer y 10). Each antibiotic was tested for herbicidal activity and plate co unts were used to enumerate bacteria and evaluate antibiotic kinetics. Only -lactam antibiotics (penicillins and cephalosporins) did not red uce wheat growth rates. Aminoglycosides, the pyrimidine trimethoprim, colistin and rifampicin reduced growth rates substantially. Antibiotic s acted slowly, with maximum reductions in rhizoplane bacteria occurri ng after more than 48 h of exposure. Combinations of non-phytotoxic an tibiotics reduced platable rhizoplane bacteria by as much as 98%; howe ver, this was generally a reduction from about 10(9) to 10(6) colony f orming units per gram of dry root mass, so that many viable bacteria r emained on root surfaces. We present evidence which suggests that insu fficient bacterial biomass exists on root surfaces of nonstressed plan ts grown under well-aerated conditions to quantitatively interfere wit h root nitrogen absorption measurements.