Laboratory Prescreening of Bradyrhizobiumjaponicum for Acid pH Tolerance to Predict Their Survival in Acid' Soils

Abstract

Introduced Brwiyrhizobium japonicum strains are often unable to tolerate acid soil stress factors in a new environment. There is a need to improve the survival of B. japonicum in acid soils. This experiment was conducted to study the selectivity oF four agar media in screening acid tolerant strains. Sixteen strains of B. japonicum were stab inoculated onto four screening media at five pH levels 3.8, 4.2,4.5, 5.0, and 6.8 and the growth of each strain was observed and scored everyday. To test the usefulness of this screening method, B. japonicum strains were inoculated to two sterile acid soils which differ in chemical properties. The survival of each strain was determined at days 1, 8, 18, and 28 after inoculation. The results showed that the selectivity of each screening medium was different, where the more acid stress factors incorporated to that medium the more selective was that me. dium. The relationship between screening in acidic agar media and survival capability in sterile acid soils was quite well estab lished, where the strains that survived well or poorly in sterile soils were those that were identified as acid tolerant or acid sensitive strains in acidic agar media. Results from this experiment confirm that laboratory prescreening of B. japonicum for acid, Al and Mn tolerance using acidic agar media was successful in selecting strains which were tolerant in low pH soils, and those which were less so.