Association between N2-fixing bacteria and pearl millet plants: responses, mechanisms and persistence

Responses to inoculation with N₂-fixing bacteria were studied in relation to genotypic differences in pearl millet, effect of nitrogen levels, and FYM additions in India. In some experiments, inoculation increased mean grain yield up to 33% over the uninoculated control, whereas in the remaining 11 experiments there was no significant increase. Increased grain yields, > 10% over the uninoculated controls were observed in 46% of the experiments with Azospirillum lipoferum (18.7% average increase) and with Azotobacter chroococcum (13.6% average increase). Yield increases were nil or reduced in three experiments with Azos. lipoferum and four experiments with Aztb. chroococcum. In two experiments continued inoculation for two or three years resulted in increased grain, plant biomass yield, and N uptake. Interactions of bacterial cultures with cultivars or years were not observed. The counts of the inoculated strains increased two to three-fold when inoculation was continued for three years. Repeated inoculations increased the mean cumulative N uptake from season 1 to season 3 by 19 kg ha⁻¹. Repeated inoculations with Aztb. chroococcum and Azos. lipoferum increased mean grain yield of a succeeding crop by 14.4% and 9.8%, respectively, over the uninoculated control. Inoculation increased the efficiency of N-assimilation by pearl millet. Marginal increase in nitrogenase activity, associated with the inoculated plants was observed during later stages of plant growth. Increased leaf nitrate reductase activity (NRA) was observed after inoculation with these bacteria. The responses to inoculation are mainly attributable to increased plant N assimilation which could be the effect of growth promoting substances secreted by the bacteria; and thus the contribution from BNF may be small.