Temperature and desiccation tolerance of cowpea rhizobia

Strains of Rhizobium capable of nodulating cowpeas (Vigna unguiculata) were tested for their ability to tolerate desiccation and high temperature in soil. Eight strains were from a hot, dry region of Niger, and 13 strains were from cooler, wetter regions of Nigeria. All of the 21 strains tested at 30 °C grew somewhat in a moist sandy soil from Niger. Of the 21 strains tested in moist soil incubated at 37 °C, only the 8 strains from Niger did not decline in numbers. The moist soil was then allowed to dry slowly. After a 2-week period, the decline in the logarithm of cell numbers per gram of soil ranged from 0.19 to 0.41 at 30 °C and from 0.28 to 0.66 at 37 °C for the 8 strains from Niger and from 0.48 to 2.42 at 30 °C and 0.87 to 2.85 at 37 °C for the 13 strains from Nigeria. These results suggest that cowpea rhizobia from hot, dry areas are more temperature and desiccation tolerant than strains from cooler, more humid regions and that temperature and desiccation tolerance may be related to geographical origin.