AUTECOLOGY OF THE BIOCONTROL STRAIN PSEUDOMONAS-FLUORESCENS CHA0 IN THE RHIZOSPHERE AND INSIDE ROOTS AT LATER STAGES OF PLANT DEVELOPMENT

A spontaneous rifampicin-resistant mutant of the biocontrol agent Pseu domonas fluorescens CHAO was released as soil inoculant in large outdo or lysimeters and its ability to colonise the roots of winter wheat, s pring wheat (grown after Phacelia) and maize at the later stages of pl ant development was investigated by colony counts. The inoculant (i.e. CHAO-Rif) colonised the rhizosphere and the interior of the roots of both wheat varieties but CFUs at ripening were about 2 log (g root)(-1 ) or lower. In contrast, the roots of maize were colonised poorly by t he pseudomonad at flowering, but the latter was found at 3 or more log CFU (g root)(-1) on and inside the roots in late ripening stage. Furt hermore, CHAO-Rif was recovered at more than 5 log CFU (g root)(-1) fr om the interior of several maize root samples. Whereas most cells of C HAO-Rif in soil were small and did not respond to Kogure's viability t est, the pseudomonad was present as viable, unusually large (7 mm long ) rods inside maize roots. In a microcosm experiment performed with si milar sandy-loam soil, the CFUs of maize root-associated CHAO-Rif were higher where the shoots of the plant had been cut off, confirming tha t older and/or decaying maize roots represent a favourable niche for t he inoculant. Overall, the results indicate that Pseudomonas inoculant s have the potential to colonise the roots of certain crops (e.g. maiz e but not wheat for strain CHA0-Rif) at later stages of plant developm ent.