ESTABLISHMENT OF PLANT-GROWTH PROMOTING BACTERIA IN THE RHIZOSPHERE OF SUBSEQUENT PLANTS AFTER HARVEST OF THE INOCULATED PRECROPS

Rifampicin-resistant mutants of two plant-growth-promoting rhizosphere bacteria, Pseudomonas fluorescens strain PsIA12 and the associative, nonsymbiotic Rhizobium leguminosarum by. trifolii strain R39, were inoculated with different crops in field experiments. Soil from these sites was collected after harvest. Non-inoculated maize, pea, lupine and two weeds (Amaranthus retroflexus, Echinochloa crus-galli) were subsequently grown in this soil in the greenhouse and tested for colonization with Pseudomonas strain PsIA12 and Rhizobium strain R39. The Rhizobium strain was able to re-establish in the rhizosphere of the plants without nodulation in between, even after dry storage of the soil up to 12 months. It could even colonize a second subsequent plant generation. Highest cell numbers were detected in legumes (mean value log 5.0 cfu . g root(-1)), followed by maize (log 4.3 cfu . g root(-1)) and the weeds (< log 4.2 cfu . g root(-1)). Rhizosphere colonization was modified by the inoculated precrop. The Pseudomonas strain was not able to re-establish cell numbers higher log 3.0 cfu . g root(-1) in the rhizosphere.