Population and diversity of pigeonpea rhizobia from the Indo-Gangetic plains of India

Pigeonpea (Cajanus cajan) is an important pulse crop in India with an average yield of around 800 kg/ha. Rhizobia inoculation is known to improve the yield and quality of this pulse crop. Pigeonpea is known to be nodulated by both fast- and slow-growing rhizobia. India is believed to be a centre of origin for pigeonpea, however, the diversity of rhizobia nodulating this pulse crop has not been studied comprehensively. The population size and diversity of rhizobia nodulating pigeonpea were estimated in 35 sites across the Indo-Gangetic plains of India. The population of pigeonpea rhizobia was low in the study area with only four samples having a population of more than 10(4) cells/g dry soil. This suggests the need for inoculation with efficient and competitive strains of specific rhizobia to achieve optimum yield. Diversity analysis of the rhizobia isolated from pigeonpea nodules revealed that this host is nodulated by bacteria belonging to four genera : Bradyrhizobium, Rhizobium, Neorhizobium and Ensifer. Our study is the first report on pigeonpea being nodulated by Neorhizobium. Host range evaluation of a subset of pigeonpea rhizobia on soybean, mung bean, cowpea and urad bean has shown that none could nodulate soybean whereas cowpea and mung bean were nodulated with equal frequency by the tested strains.