INFLUENCE OF 2 PLANT-SPECIES (FLAX AND TOMATO) ON THE DISTRIBUTION OFNITROGEN DISSIMILATIVE ABILITIES WITHIN FLUORESCENT PSEUDOMONAS SPP

The distribution of nitrogen-dissimilative abilities among 317 isolate s of fluorescent pseudomonads was studied. These strains were isolated from an uncultivated soil and from the rhizosphere, rhizoplane, and r oot tissue of two plant species (flax and tomato) cultivated on this s ame soil. The isolates were distributed into two species, Pseudomonas fluorescens (45.1%) and Pseudomonas putida (40.4%), plus an intermedia te type (14.5%). P. fluorescens was the species with the greatest prop ortion of isolates in the root compartments and the greatest proportio n of dissimilatory and denitrifying strains. According to their abilit y to dissimilate nitrogen, the isolates have been distributed into non dissimilatory and dissimilatory strains, nitrate reducers and true den itrifiers with or without N2O reductase. The proportion of dissimilato ry isolates was significantly enhanced in the compartments affected by flax and tomato roots (55% in uncultivated soil and 90 and 82% in the root tissue of flax and tomato, respectively). Among these strains, t he proportion of denitrifiers gradually and significantly increased in the root vicinity of tomato (44, 68, 75, and 94% in uncultivated soil , rhizosphere, rhizoplane, and root tissue, respectively) and was high er in the flax rhizoplane (66%) than in the uncultivated soil. A highe r proportion of N2O reducers was also found in the root compartments. This result was particularly clear for tomato. It is hypothesized that denitrification could be a selective advantage for the denitrifiers i n the root environment and that this process could contribute to modif y the specific composition of the bacterial communities in the rhizosp here.