Biological significance of nickel on the nitrogen fixing ability of cowpea Bradyrhizobium

Abstract

Effect of nickel (Ni) at various concentrations on the growth and nitrogen (N) fixing ability of two strains of cowpea Bradyrhizobium RA 5 nodulating Pigeon pea (Cajanus cajan var. T 21) and MO 5 nodulating Green gram (Vigna radiata Var. PS16) were examined. At 1 μg#lbmL^‐1, Ni stimulated the growth of both bacteria when cultured in yeast extract mannitol (YM) and minimal medium (MM) whose sole source of N was glutamate. Decreased growth was noted when glutamate in the minimal medium was replaced by inorganic N, such as nitrate or ammonium. Higher concentrations of Ni further decreased bacterial growth. Similarly, Ni at 1 μg#lbmL^‐1 increased nodule number, nitrogenase activity, and plant dry weight in both the legumes studied when inoculated with their respective bacterial strains. Higher concentrations of Ni (>10 μg#lbmL^‐1) inhibited the nodulation, nitrogenase activity, and plant growth. It was concluded that Ni at 1 μg‐mL^‐1 had a beneficial effect with enhanced bacterial population, plant growth, and N fixation. Therefore, Ni should be used as an additional micronutrient to increase the productivity of pulse crops that are dependent on biologically fixed N.