Growth promotion of winter wheat by fluorescent pseudomonads under field conditions

The ability of some pseudomonad strains to promote growth and yield of winter wheat was assessed at field sites during 1988 and 1989. The Kernen and Hagen sites were located on Dark Brown Chernozemic and Orthic Black Chernozemic soils, respectively. Plants were inoculated either by injecting a bacterial suspension into the rhizosphere of 250-day old plants in the spring (1988 study) or by soaking seeds in a bacterial suspension prior to fall planting (1989 study). Resistance to rifampicin and nalidixic acid (Rifr Nalr) was used to differentiate introduced pseudomonads from indigenous soil bacteria. Pseudomonad inoculants injected into a rhizosphere produced winter wheat grain yields 1.05-2.08 times the control, but these increases were not significant due to variability between field plot replicates. The Rifr Nalr inoculant P. cepacia MR85 resulted in the highest number of tillers and increased dry weight of seeds by 1.53- and 2.08-fold at the 1988 Hagen and Kernen sites, respectively. When uses as seed inoculants P. cepacia R85 and P. putida R105 stimulated (P < 0.05) grain yield by 11% at both the Kernen and Hagen sites. Likewise, two Rifr Nalr inoculants, P. cepacia MR85 and P. putida MR111 significantly (P < 0.10 and P < 0.05) increased grain yield at the Kernen site by 6 and 11%, respectively. Inoculant population counts were monitored once every 2 weeks in the wheat rhizosphere at both field sites throughout the growing season in both years, and reached levels of 10(4)-10(8) cfu g-1 of roots. Our results show that these pseudomonad inoculants colonized winter wheat roots and survived in the rhizosphere, even overwinter. Although the ability of these strains to promote winter wheat growth under field conditions varied from site-to-site during the 2 yr study, the consistent trend for some strains (e.g. R85 or MR85) to increase plant yields demonstrates the potential of these PGPR as field inoculants.