Effect of excessive temperatures on rhizobia growth nodulation and nitrogen fixing activity in symbiosis with siratro

Abstract

The heat tolerance of 17 strains of Rhizobium sp. was studied on agar plates, in liquid culture medium and in symbiotic associations with Macroptilium atropurpureum. To test this the plants were exposed during six hours per day and during incubation with C₂H₂, to temperatures ranging from 28 to 40° C. There were significant differences among strains in their tolerance to high temperatures, and five of them were able to grow well at 40°C. Rhizobium growth on agar plates or in liquid medium however did not correlate with symbiotic performance. One strain which showed fastest growth in liquid medium at 40°C showed almost linear decreasing nitrogenase activity with increasing temperatures from 28 to 40°C. Another strain which also showed good growth up to 40°C in culture medium showed unaffected nitrogenase up to this temperature. The temperature sensitive strains showed maximal nitrogenase activity at 32°C but all tested strains were able to nodulate and to fix some N₂ (reduce C₂H₂) at 40°C.