Abstract

Pigeonpea is an economically important kharif grain legume crop that nodulates poorly in the northern part of India. The role of siderophore production in nitrogen fixation and iron uptake in pigeonpea (Cajanus cajan) was assessed. 25 Tn5 siderophore producing mutants of pigeonpea-Rhizobium strain PP-18 were selected using Chrome Azurol S (CAS) agar plate as well as CAS assay solution. Finally, the efficacy in pigeon pea host was assessed under controlled conditions. PP-18 LSP-15 and PP-18 LSP-17 mutants did not produce detectable quantity of hydroxamate type of siderophore, while nine mutants over produced siderophores (HSP) as compared to wild type. The rhizobial mutants produced from 0.68 to 8.05 µg of hydroxamate N mg-1 protein, whereas wild type produced 2.21 µg of hydroxamate N mg^-1 protein. On overall mean basis, low siderophore producing group of mutants produced less nodule biomass, root, shoot dry weight and shoot weight ratio as compared to moderate siderophore producing and siderophore over-producing mutants. When pigeonpea plants were inoculated with HSP, they achieved the highest nitrogen and iron contents; 16.2 mg plant^-1 and 1408 ppm respectively. Hydroxamate type of siderophore production was highly correlated with N₂ fixing efficiency and iron contents of pigeonpea plants.

Key words: Rhizobium sp. (Cajanus), pigeonpea, nitrogen fixation, iron uptake, siderophore, nodulation.