Plant Growth-promoting Rhizobacteria and Soybean [Glycine max(L.) Merr.] Growth and Physiology at Suboptimal Root Zone Temperatures

Application of plant growth-promoting rhizobacteria (PGPR) has been shown to increase legume growth and development under optimal temperature conditions, and specifically to increase nodulation and nitrogen fixation of soybean [Glycine max(L.) Merr.] over a range of root zone temperatures (RZTs). Nine rhizobacteria applied into soybean rooting media were tested for their ability to reduce the negative effects of low RZT on soybean growth and development by improving the physiological status of the plant. Three RZTs were tested: 25, 17.5, and 15 °C. At each temperature some PGPR strains increased plant growth and development, but the stimulatory strains varied with temperature. The strains that were most stimulatory at each temperatures were as follows: 15 °C—Serratia proteamaculans1–102; 17.5 °C—Aeromonas hydrophilaP73, and 25 °C—Serratia liquefaciens2–68. Because enhancement of plant physiological activities were detected before the onset of nitrogen fixation, these stimulatory effects can be attributed to direct stimulation of the plant by the PGPR rather than stimulation of plant growth via improvement of the nitrogen fixation symbiosis.