The role of exopolysaccharide-producing-bacteria (Rhizobium sp.) in sunflower rhizospheric-soil aggregation: effect of inoculation on soil structuration and plant nutrition

The bacterial strain Y AS34, selected for exopolysaccharide (EPS) production, was isolated from the rhizoplane of sunflower (Helianthus annuus) in a Dystric Cambisol. From cartography and sequencing of the 16S rDNA gene it was concluded that strain Y AS34 belongs to the genus Rhizobium. The gelatinous EPS produced by this strain was different from succinoglycan usually produced by bacteria belonging to the genus Rhizobium. In the presence of 1 % (w/w) of the EPS purified from strain Y AS34, the water holding capacity of soil from St-Florent-sur-Cher was enhanced by 50%. Inoculation of sunflower seeds and soil with strain Y AS34 led to a significant increase of root-adhering soil mass per root biomass (+80%), under both, water stress and normal water supply conditions. Concomitantly with this bacterial effect on the mass of root-adhering soil, an increase of soil macro-porosity (pore diameter between 5 and 50 mm) was observed. In addition to these physical changes of root-adhering soil, nitrogen uptake by sunflower was significantly increased after inoculation with Rhizobium sp. Y AS34 in the presence of 1 N-nitrates. Strain Y AS34 seems to modify the soil structure around the root system in a way that the water supply of sunflower and its nitrogen uptake is increased counteracting the effect of water deficit on sunflower growth. (author)