Combined effects of plant growth-promoting rhizobacteria and genistein on nitrogen fixation in soybean at suboptimal root zone temperatures

Application of plant growth-promoting rhizobacteria (PGPR) or the plant to bacteria signal molecule genistein has been shown to increase nodulation and nitrogen (N) fixation by soybean [Glycine max (L.) Merr.] over a range of root zone temperatures (RZTs) and, specifically, off-sets at least some of the ill-effects of low RZTs. Two sets of controlled-environment experiments, one on a growth bench and the other in a greenhouse, were conducted to examine the combined ability of both PGPR and genistein to reduce the negative effects of low RZT on soybean nodulation and N fixation. Each of two the PGPR strains, Serratia proteamaculans 1-102 and Serratia liquefaciens 2-68 were coinoculated with Bradyrhizobium japonicum USDA 110 preincubated with 17.5 (somewhat inhibitory), and 15 degrees C (very inhibitory). At RZTs of 25 and 17.5 degrees C PGPR strains and genistein in combination increased the number of nodules and the amount of Nn fixed. The most stimulatory effect was observed at 17.5 degrees C for the combination: S. proteamaculans 1-102 plus B. japonicum USDA 110 pre-incubated in 15 mu M genistein under greenhouse conditions. For most treatment combinations the stimulatory effects of PGPR and genistein were additive at RZTs of 17.5 and 25 degrees C. Surprisingly, the combination of these two factors resulted in antagonism at the very inhibitory RZT of 15 degrees C. The results suggest that the negative effects of certain low RZTs could be more effectively off-set by combined treatments of PGPR plus geneistin preincubation of rhizobial cultures than by their individual treatment.