Substrate Dependent Microbial Biosynthesis of Auxins and Their Effect On Growth and Yield of Mung Bean (Vigna radiata L.).

Rhizobial inoculants have successfully been used in different parts of the world for improving growth and yield of legume crops. Co-application of L-Tryptophan (L-TRP) with rhizobia may further improve its efficacy due to precursor -inoculum interactions resulting into auxins biosynthesis in the rhizosphere. Rhizobium phaseoli strains were isolated using dilution plate technique from the mung bean nodules. Auxin biosynthesis by these rhizobial isolates was determined in the absence and presence of L-TRP, a physiological precursor of auxins. Rhizobial isolates varied largely in auxins biosynthesis capabilities and A23, N12 and N42 being most prolific in auxin biosynthesis were further evaluated in the presence of different levels of L-TRP (10^-3, 10^-4 and 10^-5 M) for improving the growth and yield of mung bean (Vigna radiata L.) in a field trial conducted at Agronomy Section, Ayub Agricultural Research Institute, Faisalabad, Pakistan. Mung bean seeds were inoculated with peat-based inocula and sown following randomized complete block design with four replications. Fertilizers, NP were applied at 60-50-0 kg ha^-1 as urea and single super phosphate (SSP) in all plots. Results revealed that although rhizobial inoculation significantly improved the growth and yield of mung bean but when rhizobia applied in combination with 10^-5 M L-TRP, the effect was more pronounced than their separate application on growth parameters of mung bean. Results indicated that plant height, total plant biomass, number of pods plant^-1, number of grains pod^-1, dry matter, grain yield,   number of nodules plant^-1, nodular mass plant^-1 and 1000-grain weight significantly increased compared to un-inoculated and untreated control. Similarly, N and P concentration in grain also increased significantly by the combined application of 10^-5 M L-TRP and rhizobial inoculation. These results imply that supplementation of rhizobia with L-TRP could be a useful approach for improving growth, nodulation and yield of mung bean.

Key words: Rhizobium, inoculation, L-tryptophan, yield, nodulation, mung bean