Improvement of legume inoculants: heat and desiccation tolerant rhizobia

Heat and desiccation tolerances of 8 commercial strains of rhizobia and 46 isolates from varieties soils were determined separately by studying growth and survival in petri plates or in liquid medium incubated from 37 deg C to 45 deg C for heat stress and in soil at approximately 15 bars water potential at 28 deg C for desiccation stress. Majority of the rhizobia tolerated 37 deg C. The fast growers (Rhizobium sp.) were more tolerant of higher temperatures (up to 40 deg C) than the slow growers (Bradyrhizobium sp.). Very few strains grew at 42 deg C and 45 deg C. Rhizobial strains that tolerated higher temperatures (40-42 deg C) lost their nodulating capacity on mungbean. Commercial inoculum strains TAL 420 and BVr 9 for mungbean and 3G4b20 and BAh 23 for peanut grew well in liquid medium incubated at 37 deg C for 7 days but not strain M6 for mungbean.Among the three inoculum strains for mungbean (BVr9, TAL 420, and M6) strains BVr9 retained its symbiotic effectiveness when grown at 37 deg C. When exposed to 40 deg C for brief periods of time (1-4 hours), strain BVr9 also retained its ability to grow and nodulate mungbean. In a field experiment, strain BVr9 maintained significantly higher populations on the seed up to 30 hours after sowing without irrigation than strain M6. Strains BVr9 and TAL 420 tolerated temperatures as high as 39 deg C in the field. However, the strains had comparable symbiotic performance in terms of nodulation, nitrogen uptake and grain yield with the control indicating that there maybe present in the soil competitive indigenous rhizobia which may also process heat and/or desiccation tolerance. Peanut rhizobia strains (3G4b20, NC92, NC123 and SG) grow in liquid medium during brief periods or heat stress tolerated 40 deg C to 44 deg C until 4 hours of incubation and retained their capacity to infect and nodulate peanut. The screening for desiccation toleance indicated that the4 rhizobial isolates and known effective inoculum strains varied in their ability to withstand dry soil conditions. Mungbean commercial strain M6 was sensitive to desiccation stress because its initial population decreased almost 100-fold 2.5 hours after drought stress was imposed. Mungbean inoculum strains BVr9 and TAL 420 are considered desiccation tolerant because they were able to maintain greater 1% of their initial population up to 24 hours (BVr9) and 72 hours (TAL 420) after inoculation. The populations of peanut commercial strains BAh23 and No.G4b20 decreased 100-fold when subjected to desiccation for 2.5 hours after inoculation. Strain 3G4b20 is considered tolerant to desiccation at 24 hours incubation time because it was able to maintain 1% of its initial population. The other peanut rhizobial strains NC 92, NC 123 and SG did not tolerate the dry soil condition up to 2 days. The second strategy for the improvement of rhizobial inoculants in terms of tolerance to heat and desiccation is the development of inoculant forms, formulations or additives that will promote Rhizobium survival in dry and hot environments. Gum arabic, an exudate from acacia species that increases longevity or rhizobia in peat based inoculant was added to a liquid medium (Yeast extract, mineral salts and mannitol) in varying concentrations of 1 to 5%. All the bradyrhizobia strains tested except strain M6 grow well in the medium with 1 to 2% gum arabic. Population levels of 8 log no of cells/ml were maintained by the strains BVr9, 3G4b20 and SG up to 38 days of incubation. Liquid inoculant formulations using the same basal medium as above and amended with 1% gum arabic or 1% cassava starch supported the growth of rhizobial strains M6, BVr9, 3G4b20 and SG with 10 sup 8 to 10 sup 9 cells/ml population levels up to eight months of storage. The addition of 1% gum arabic extended the shelf life to one year. The survival of mungbean and peanut inoculum strains on seeds was not affected by the different inoculant formulations and age up to 6 months. In growth room experiments, liquid inoculant formulations and age did not alter the nodulating ability of the inoculum strains for mungbean and peanut compared to Nitroplus legume inoculant in powdered form. This was observed up to 32 weeks of storage of the different liquid inoculant formulations. The dry matter yields of mungbean and peanut inoculated with the different liquid formulations at all storage periods were comparable with those inoculated with the existing inoculant Nitroplus although greater values of dry matter yield were obtained with the liquid inoculant formulations than Nitroplus. In pot experiments no significant differences in the nodulation of mungbean were observed when inoculated with different liquid inoculant formulation of strains M6 and BVr9.