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

Co-inoculation of plant-growth-promoting rhizobacteria (PGPR) with B. japon icum has been shown to increase soybean [Glycine max (L.) Merr.] nodulation , nitrogen fixation, growth and physiological activity at suboptimal root z one temperatures (RZTs). We studied the survival and growth of seven PGPR i noculated on soybean in a sterile rooting medium at three RZTs (2.5, 17.5 a nd 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 studi ed under field conditions in methyl bromide fumigated and non-fumigated soi ls. 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 degr ees C). The population of PGPR applied to the rhizosphere either with or wi thout addition of B. japonicum increased over time in fumigated soil as com pared to non-fumigated soil, indicating that the PGPR survive and prolifera te better under fumigated conditions. S. liquefaciens 2-68 had higher popul ation densities both on the root and in the rhizosphere, demonstrating, the ir ability to colonize under short-season conditions. The possible interact ions between the two cultivars (Maple Glen and AG Braver) and the PGPR were generally not significant, despite observations that growth and yield of A C Braver respond more strongly to PGPR inoculation.