ABSTRACT
Inoculation density has
a marked effect on nodulation and N fixation in soybean (Glycine max L.). Therefore,
we conducted this study to determine the optimal inoculation density of Bradyhizobium japonicum SAY3-7 (SAY3-7) and Streptomyces griseoflavus P4 (P4) for plant growth,
nodulation, and N fixation, and to investigate the effect of co-inoculation on selected
soybean cultivars, using the optimal inoculation density. Nitrogen fixation, in
terms of an acetylene reduction activity value, was measured using a flame ionization
gas chromatograph. In this study, low-density single inoculation with P4 (105 or 106 cells mL-1) was associated with the highest plant biomass,
compared with normal- and high-density single inoculation with P4 (107 or 108 cells mL-1). Moreover, low-density single or co-inoculations
with SAY3-7 and/or P4 produced the highest nodule biomass and highest nitrogenase
activity, compared with single or dual inoculation at other inoculation densities.
Therefore, we evaluated low-density co-inoculation with P4 and SAY3-7, at the rate
of 105 cells mL-1, on selected soybean cultivars. Low-density
co-inoculation increased the plant biomass, compared with un-inoculated plants.
The effects of single and co-inoculation on nodulation did not differ significantly
for any of the cultivars, except “Yezin-9” in the first experiment and “Shan Seine”
in the second experiment. Low-density inoculation with both bacteria increased N
fixation by 15% - 75% for seven of the cultivars in the first experiment and by
15% - 39% for three of the cultivars in the second experiment, compared with single
inoculation with SAY3-7. Based on the overall results, we concluded that low-density
co-inoculation with P4 and SAY3-7 gave improved plant growth and N fixation.