EFFECT OF SOIL BULK-DENSITY AND TEMPERATURE ON WHEAT RHIZOSPHERE COLONIZATION BY LUX-MARKED PSEUDOMONAS-FLUORESCENS

The effects of bulk density and temperature on the rates of colonisation of the wheat rhizosphere and establishment in bulk soil were investigated by inoculating lux-marked Pseudomonas fluorescens onto seeds. The coated seeds were placed in soil microcosms, maintained at a constant matric potential. The wheat seedlings were harvested every 7 days over a 28 days period. Survival and colonisation were determined over a 28 days period by performing dilution plate counts on seed, above-ground material, ectorhizosphere, endorhizosphere and bulk soil samples. Selection of the marked inoculum was made on the basis or resistance to kanamycin and rifampicin, with the ability to emit light. Soil bulk density and temperature had a significant effect on colonisation of the rhizosphere, with greatest rates of colonisation occurring at the lower bulk density (0.82 g cm(-3)) and the highest temperature (22 degrees C). Colony forming units (CFU) were approximately 100-fold higher in the ectorhizosphere than in the endorhizosphere. Colonisation of the above-ground material was affected by temperature and was unaffected by changes in bulk density. Populations of lux-marked cells were 2-3 orders of magnitude greater at 8 degrees C than at 22 degrees C.