SURVIVAL AND EFFECTIVENESS STABILITY OF COWPEA RHIZOBIA AS AFFECTED BY SOIL TEMPERATURE AND MOISTURE

Vigna unguiculata (L.) Walp and Arachis hypogaea L. are economically important food legumes in the cowpea cross-inoculation group that are nodulated by Rhizobium. Benefits from inoculation depend on survival and effectiveness stability of rhizobia introduced into the soil. The objectives of this research were to evaluate the effect of soil temperature and moisture on rhizobial survival and the effectiveness of surviving rhizobia. To determine survival of rhizobia in soil, five strains of cowpea rhizobia were used. They were incubated under moisture conditions of air dry, moist (-0.33 bar), and saturated and temperature conditions of 20(DEGREES)C, 35(DEGREES)C, and 45(DEGREES)C for nonsterilized soil and 40(DEGREES)C in sterilized soil. Three strains of cowpea rhizobia were used to determine effectiveness stability of surviving rhizobia. Incubations were under dry and moist (-0.33 bar) soil conditions at 40(DEGREES)C in sterilized soil for 45 days and at 45(DEGREES)C in nonsterilized soil for 15 days. To evaluate effectiveness, eighty single colony isolates of each strain were obtained from nodules of plants inoculated with soil samples at time 0 and at 15 days of incubation in dry and moist nonsterilized soils. Forty isolates were obtained from pour plates of soil dilutions of sterilized soil at time 0 and 60 isolates were obtained from soil samples of dry and moist soil after 45 days. Effectiveness of isolates was measured as dry matter produced by siratro plants.;Cowpea rhizobia strains 201 and T-1 were relatively stable in effectiveness. Changes in effectiveness of strain TAL309 isolated from nodules were observed. Relatively ineffective strains were found in comparatively high frequency (7.5%) in the population. The variation was much reduced when isolates were obtained directly from soil. Strain 201 was not only relatively to hot, dry temperatures but was also relatively stable in effectiveness. Therefore, there is a potential for selection of strains that are resistant to high temperatures and have effectiveness stability for inoculation in tropical and subtropical regions.;The number of rhizobia determined by the most probable number plant infection method, as compared to plate counts under pure culture conditions, using cowpea, siratro or peanut underestimated the population by 10 to 100-fold but it was the only available method for use with nonsterilized soil. Survival of rhizobia, in nonsterilized soil, was affected by temperature and moisture. At 35(DEGREES)C, the populations of T-1, TAL309, and 3281 decreased approximately 100-fold for the dry soil and the saturated soil during 45 days of incubation. However, populations of strain 201 decreased by only 10-fold under the same environmental conditions. When strains 201 and TAL309 were incubated at 45(DEGREES)C the population decreased rapidly and could not be detected at 45 days. In sterilized moist soil, at 40(DEGREES)C the population of all strains increased during the first 2 weeks. For dry soil conditions the populations began decreasing within 5 days but in contrast to the nonsterilized soil, the populations did not decline under saturated soil conditions. Survival of effective and ineffective rhizobia was similar for most comparisons but ineffective TAL309 was more adversely affected by hot and dry conditions than effective TAL309.