Effect of N concentration and N source on root colonization by Pseudomonasfluorescens 2-79RLI

Wheat (Triticum aestivum L.) was grown in nutrient solution with low or hig h N supply (NH4NO3 as N source). To further evaluate the influence of N for m and its interaction with the nutrient solution pH, wheat plants were grow n with NH4+ or NO3- either in an conventional nutrient solution or in a nut rient solution in which the pH was maintained at pH 6.5 using a pH-stat sys tem. The nutrient solution was inoculated with Pseudomonas fluorescens 2-79 RLI, a genetically modified bacterium that contains lux genes activated by a ribosomal promoter. Cell numbers and physiological status of P. fluoresce ns 2-79RLI (length of the lag phase of bioluminescence) in the rhizosphere were determined at the root tip and in the lateral root zone. Nitrogen defi ciency decreased both plant growth and root colonization by P. fluorescens 2-79RLI at the root tip while it had no effect on root colonization in the lateral root zone. The physiological status of P. fluorescens 2-79RLI was n ot affected by nitrogen deficiency. Ammonium nutrition increased root colon ization by P. fluorescens 2-79RLI at the root tip and in the lateral root z one when the pH of the nutrient solution was allowed to change according to the N form provided. Under these conditions, the physiological status of P . fluorescens 2-79RLI was higher in the lateral root zone than at the root tip. In contrast, N source had no effect on root colonization or physiologi cal status of P. fluorescens 2-79RLI in the nutrient solution maintained at pH 6.5. It is concluded that the stimulation of root colonization by NH4in the nutrient solution, not maintained at a constant pH, may be due to in creased leakage of solutes into the rhizosphere as a result of impaired exu date retention by high H+ concentration in the rhizosphere or the apoplast.