HIGH TEMPERATURE SCREENING OF RHIZOBIUM species CWP G34A MUTANTS FOR NITROGEN FIXATION ON COWPEA (Vigna unguiculata (L.) Walp)

Abstract

This project was carried out to select thermophlic mutant Rhizobium species CWP G34A strains compatible with cowpea (Vigna unguiculata). Many strains of the mutant of Rhizobium species CWP G34A were primarily screened for growth in culture medium and nitrogen fixation through nodulation at high temperature of 40oC to 60oC at 10oC interval with 28oC as the control temperature. The wild-type and mutants obtained from the Department of Microbiology, Federal University of Technology Akure were tested for high temperature tolerance by growing them individually in nutrient broth at 60oC for 24 to 48 hours. Optical density (OD) (670 nm) of each was read before and after incubation at 24 hr interval. The mutants were placed into 8 classes based on their responses to the high temperature as reflected in the percentage difference in OD values obtained before and after exposure at 60oC. The selected mutants were then inoculated into already prepared potted cowpea plants and allowed to grow until harvest. Some of the mutants of Rhizobium species CWP G34A produced functional nodules on the cowpea consistently on three different plant tests. At the different temperatures tested, 5 mutants (M571, M522, M853, M891, M637) and the wild-type produced nodules at 28oC, 7 mutants (M860, M59, M126, M459, M635, M330 and M912) produced nodules at 40oC, 12 mutants (M126, M637, M393, M799, M571, M59, M671, M330,M853, M891, M7 and M156) formed nodules at 50oC, while at 60oC five mutants (M459, M860, M522, M635 and M330) produced nodules. Most of the nodules produced were active as observed in the physiology of the plants, characterized by flowering and fruits formation. The M393 among all the mutants produced the highest number (23) of effective nodules but M330 was the most stable nodule former of all the mutants that nodulated the cowpea plants; it was able to nodulate cowpea at 3 different temperatures of 40oC, 50oC and 60oC. Therefore, the mutant M330 is a promising variant that can be used to inoculate cowpea in arid zones by farmers to grow cowpea throughout the year especially during the dry season