Root and rhizosphere colonization of soybean [Glycine max (L.) Merr.] by plant-growth-promoting rhizobacteria at low root zone temperatures and under short-season conditions

Co-inoculation of plant-growth-promoting rhizobacteria (PGPR) with B. japonicum has been shown to increase soybean [Glycine max (L.) Merr.] nodulation, nitrogen fixation, growth and physiological activity at suboptimal root zone temperatures (RZTs). We studied the survival and growth of seven PGPR inoculated on soybean in a sterile rooting medium at three RZTs (2.5, 17.5 and 15 degrees C) on a growth bench. The survival of the two most promising strains (Serratia liquefaciens 2-68 and S. protea-maculans 1-102) was studied under field conditions in methyl bromide fumigated and non-fumigated soils. In general, population densities varied with temperature. PGPR strains generally colonized the rhizosphere and root surface effeciently at higher RZTs; however, S. proteamaculans 1-102 colonized best at a low RZT (15 degrees C). The population of PGPR applied to the rhizosphere either with or without addition of B. japonicum increased over time in fumigated soil as compared to non-fumigated soil, indicating that the PGPR survive and proliferate better under fumigated conditions. S. liquefaciens 2-68 had higher population densities both on the root and in the rhizosphere, demonstrating, their ability to colonize under short-season conditions. The possible interactions between the two cultivars (Maple Glen and AG Braver) and the PGPR were generally not significant, despite observations that growth and yield of AC Braver respond more strongly to PGPR inoculation.