THE EFFECT OF TEMPERATURE ON NODULATION AND NITROGEN-FIXATION BY 5 BRADYRHIZOBIUM-JAPONICUM STRAINS

To establish a highly effective symbiosis between a legume and an inoculated Bradyrhizobium, it is important to examine how environmental factors will affect the strain's nodule-forming ability in the presence of other competing bradyrhizobia. We examined nodulation, growth and N-2 fixation of soybean (Glycine max L. Merrill) at 15, 25 and 35 degrees C, as affected by single or dual strain inoculation with the following B. japonicum strains, designated El, E2, E3 (all effective strains), 12 (uncertain effectiveness) and Il (ineffective). Irrespective of the strain inoculated, nodulation, nitrogen derived from atmospheric N-2 fixation (Ndfa) and growth of soybean were lowest at 15 degrees C, with significant improvement at the higher temperatures and generally best at 25 degrees C. The highest temperature (35 degrees C) decreased N-2 fixation more than dry matter yield, and nodules occurred deeper in soil at 25 and 35 degrees C than at 15 degrees C. Significant strain effects on the symbiosis were observed at the lowest and highest temperatures. At the optimum temperature (25 degrees C), E1 E2, E3 and 12 were of equal effectiveness as assessed by %Ndfa in soybean. However, strains E3 and 12 which fixed substantial amounts of N-2 at both 25 and 35 degrees C were found to be ineffective at 15 degrees C. Thus, depending on the temperature regime, strains E3 and 12 could be classified as effective or ineffective. The nodulation ability of the ineffective strain I1 was greatest at 25 and 35 degrees C, therefore resulting in significant reductions in nodulation by the effective strains and in N-2 fixation by the dual strain inoculations at 25 and 35 degrees C, but not at 15 degrees C.