THE ECOLOGICAL SIGNIFICANCE OF PH AND MOISTURE ON THE SURVIVAL OF SELECTED RHIZOBIUM PHASEOLI STRAINS

The effects of pH and moisture on selected Rhizobium phaseoli strains were studied. Studies on acid broth showed that the R. phaseoli strains have differential sensitivity to pH. Some strains have inherent ability to grow at pH < 5.0 but the majority of them grew at a pH between 5.0 and 7.2. An acid-tolerant selection (S-442) was identified from the parent strain of 442 (P-442) during strain selection studies in acid media. This acid-tolerant selection combines acid-tolerance with improved aluminum and manganese tolerance.;Plate count and FA studies showed that there was no significant differential effect of moisture on the strains studied. Survival was excellent under both sterile and non-sterile conditions. Short-term flooding of soils did not affect the survival and symbiotic dinitrogen-fixing capacity of the bacteria. In fact, this treatment appeared to have stimulated the activity of the Rhizobium strains, as indicated by the increase in nodulation and yield. However, excess moisture stress severely reduced dinitrogen fixation.;Differences exist between R. phaseoli strains in their ability to withstand desiccation. In sterile soil, the order or resistance was: P-442 = S-442 > C-05 > QA 1062 > CIAT 255. A similar order was obtained in non-sterile soil, with P-442 > CIAT 255. The decline in population occurred during periods of severe desiccation, i.e. at moisture tensions greater than 15 bars.;Data obtained from these studies lead to the conclusion that nodulation failures associated with stress conditions like pH, high aluminum, and moisture stress in the tropics may be more an effect on nodule initiation and the symbiosis, or on the host plant, than on the survival of the specific Rhizobium.;Survival studies of P-442 and S-442 were done in Santo Angelo (Haplorthox) and Bom Jesus (Haplumbrept) using the fluorescent antibody (FA) technique. Acid and/or aluminum tolerance was demonstrated by S-442 in the Bom Jesus but not in the Santo Angelo soil, probably due to the high (5.0-5.0 me) and low (0.8 me) exchangeable aluminum contents of the soils, respectively. In the Bom Jesus soil, survival of both strains at population greater than 10('5) cells/g of soil after 30 days suggests that survival of R. phaseoli would not limit nodule initiation or dinitrogen fixation.