Isolation of Temperature and Acid Tolerant Strains of Rhizobium sp. on Pulse Crops

The present experiments were carried out during 2004 - 2007 at Dept. of Agrl. Microbiology, Tamil Nadu Agricultural University, Coimbatore. The UV mutants of five Rhizobium isolates were screened according to high survival percentage calculated after 5 min exposure. Further it was subjected to high temperature (35, 40, 45 and 50C) exposure and acid pH (5.5 and 6.0) and its survivability was estimated. The best performing nitrogen fixing Rhizobium strains viz. , CO 5, COG 15, TNAU 14, COS 1 and CRR 6 were selected for the development of temperature and acid tolerance study. In order to know the variation, these strains were subjected to UV irradiation treatment for 5 60 minutes which resulted a total of 51 mutants. Among them, five mutants (MB1, MG1, MS1, MC1 and MGn1) were selected and subjected to temperature (35 to 50C) and pH (5.5 and 6.0) stress and 40 isolates were obtained and its survivability of CRR 6 Rhizobium strain was found to be increased initially (0.40 % for 5 min) and then decreased gradually (0.16 % at 60 minutes) which was followed by other strain of Rhizobium. The population (8.21 to 8.36 log 10cfu ml-1) of UV mutant MB1 was increased to significant level for 24 72 h of incubation at 35C and pH 5.5. When the temperature increased from 35 to 45C, there was a gradual decline in the population level (8.21 to 7.72 log 10cfu ml-1) at pH 5.5. At 50C, the population was increased (7.30 log 10cfu ml-1 to 7.60 log 10cfu ml-1) upto 48 h of incubation beyond which the multiplication of rhizobia reduced further.