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

Wheat (Triticum aestivum L.) was grown in nutrient solution with low or high N supply (NH₄NO₃ as N source). To further evaluate the influence of N form and its interaction with the nutrient solution pH, wheat plants were grown with NH ₄ ⁺ or NO ₃ ^- either in an conventional nutrient solution or in a nutrient solution in which the pH was maintained at pH 6.5 using a pH-stat system. The nutrient solution was inoculated with Pseudomonas fluorescens 2-79RLI, a genetically modified bacterium that contains lux genes activated by a ribosomal promoter. Cell numbers and physiological status of P. fluorescens 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 deficiency 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 not affected by nitrogen deficiency. Ammonium nutrition increased root colonization by P. fluorescens 2-79RLI at the root tip and in the lateral root zone 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 physiological 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 NH ₄ ⁺ in the nutrient solution, not maintained at a constant pH, may be due to increased leakage of solutes into the rhizosphere as a result of impaired exudate retention by high H⁺ concentration in the rhizosphere or the apoplast. This revised version was published online in June 2006 with corrections to the Cover Date.