The influence of high temperatures on the growth and survival of Rhizobium spp. in peat inoculants during preparation, storage, and distribution

Gamma-irradiated peat was used to prepare inoculants for 10 different species of tropical legumes. These inoculants were sent to cooperators in 14 cities in 13 tropical countries. Each cooperator received a package containing a maximum recording thermometer, plating-media components, special instruction sheets, and inoculants. Control experiments were pursued in the laboratory by exposing the various inoculants to 28, 37, and 46 °C. Temperatures reached in the inoculants during their transportation varied from 26 (Mexico) to 45 °C (Kenya). Arrival time of the inoculants ranged from 6 days (Hissar, India) to 54 days (St. Augustine, West Indies). Although a total loss of viability was reported for the chick-pea inoculant (Saudia Arabia) and a severe decrease in two others (bean and lentil inoculants in Kenya and Saudi Arabia, respectively), over 90% of the inoculants received had viable counts in excess of 1 × 10 8 cells per gram of moist peat. Laboratory data indicated that 28 °C was optimal for multiplication to maximal numbers in excess of 1 × 10 10 cells per gram of moist peat. Six inoculants studied for long-term storage showed excellent quality for 24 weeks at 28 °C. At 37 °C the cell multiplication was comparable with that at 28 °C with 8 of the 10 strains only during the 1st week. For all strains, 46 °C was lethal. Large reductions in viable counts were observed during inoculant preparation when broth cultures were added to peat.