Abstract
Commercial peat inoculants containing rhizobia of the cowpea miscellany (about 108 cell/g) were stored at 25, 35, 45 and 55°C to simulate extreme tropical storage conditions. At intervals of three weeks for 21 weeks, numbers of rhizobia were determined by the plant infection most probable number (MPN), plate MPN and plate colony enumeration techniques. Survival after 21 weeks at 25°C was good as measured by all counting methods. At 35°C, plate counts remained unchanged throughout the period, but plant infection counts declined slowly. Plate count declined slowly at 45°C, whereas plant infection counts decreased rapidly. After six weeks of storage at 55°C, there were no detectable viable rhizobia as determined by plant infection counts but plate counts showed approximately 104 rhizobia per gramme even after 15 weeks when sampling was discontinued at this temperature. At all sampling times for all temperatures, counts from the two plating techniques were very similar, but values obtained with the plant infection method were generally lower. This difference increased with storage time and was accentuated at the higher temperatures. This finding was attributed to a decline in cell vigour during storage such that the surviving cells, while able to grow when transferred to agar media, were incapable of surviving in plant nutrient solution and causing nodule development.
