Effects Of High Temperature On Survival, Growth And Effectiveness Of Locally Isolated Rhizobium Phaseoli
Twenty six locally isolated Rhizobium phaseoli stains and two re-isolated strains originally obtained from the NifTAL (Nitrogen Fixation for Tropical Agricultural Legumes) culture collection were authenticated. The cultural, morphological characteristics of the strains and their performance in plant test confirmed that they were Rhizobium. Six of the strains were chosen for further experiments. Selection was based on growth characteristics on YEM and YEMA media, putative maximum growth temperature, dry matter increase of inoculated plants and site of isolation. When tested for their ability to withstand high temperature, two of the six selected strains (17 and 29B) showed growth up to 40°C on YEM and 42°C on YEMA media. Strains 17 and 29B also showed superior ability to survive in moist clay soil at 38°C and 42° C. When strains were tested on low pH media at high temperature, strains 17 and 29B did not grow as well as several of the other strains. Test for survival of the strains on bean seeds at room temperature (25°C) showed a rapid decrease of cells per seed. However an average of 3000 rhizobial cells per seed remained after 24 hours. All the six strains nodulated three Phaseolus vulgaris cultivars (Mwezi Moja, Rose Coco and Canadian Wonder). Number of nodules per plant showed significant difference between strains used for inoculation in each of the three bean cultivars at P= 0.05. There was significant difference between nodule dry weight attained by the various strains in combination with Mwezi Moja and Canadian Wonder cultivars at P=0.05. No significant differences were recorded between strains with Rose Coco at the same probability level. Percentage nitrogen (shool tissue) in the three bean cultivars showed significant difference at P=0.05 in Mwezi Moja and P=0.07 in Rose Coco and Canadian Wonder between strains. However shoot dry weight did not show any significant difference between plants inoculated with the six strains in Rose Coco and Canadian Wonder. In Mwezi Moja there was significant difference at P=0.05 All the six strains showed better nitrogen fixation as deduced by improvement in dry mater yield with the ’'early*' maturing bean cultivar (Mwezi Moja), than the "late” maturing cvs (Canadian Wonder and Rose Coco). Strains 445, 107, 105 and 13A were superior to strains 17 and 29B in the three cultivars at nitrogen fixation. Total nitrogen content was highly correlated with percentage shoot nitrogen. However nodule dry weight had low r-values though significantly correlated with shoot nitrogen content at P=0.05.