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 colonisati on of the wheat rhizosphere and establishment in bulk soil were invest igated by inoculating lux-marked Pseudomonas fluorescens onto seeds. T he coated seeds were placed in soil microcosms, maintained at a consta nt matric potential. The wheat seedlings were harvested every 7 days o ver a 28 days period. Survival and colonisation were determined over a 28 days period by performing dilution plate counts on seed, above-gro und material, ectorhizosphere, endorhizosphere and bulk soil samples. Selection of the marked inoculum was made on the basis or resistance t o kanamycin and rifampicin, with the ability to emit light. Soil bulk density and temperature had a significant effect on colonisation of th e rhizosphere, with greatest rates of colonisation occurring at the lo wer bulk density (0.82 g cm(-3)) and the highest temperature (22 degre es C). Colony forming units (CFU) were approximately 100-fold higher i n 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 o rders of magnitude greater at 8 degrees C than at 22 degrees C.