Symbiotic effectiveness of Mucina bracteata bradyrhizobial isolates in acid conditions

Bradyrhizobia are symbiotic bacteria partners that form N2 fixing nodules with tropical legumes. Biological N2 fixation is a renewable source of nitrogen and forms an integral part of a sustainable agro-ecosystem. Mucuna bracteata originates from North-Eastern States of India and is rapidly recognized as a new and superior legume cover crop in oil palm plantations in Malaysia with favourable growth characteristics. However, there is a lack of information on the indigenous bradyrhizobial stain that could further enhance N2 fixation and growth of M. bracteata in tropical soils. Two experiments were conducted to isolate bradyrhizobia from root nodules of M. bracteata and to evaluate their symbiotic effectiveness on the host legume. In Expt. 1, root nodule bacteria isolated from M. bracteata grown on five different soil series (Briah (Typic Endoaquepts; ph-H20 4.6), Jawa (Sulfic Endoaquepts; pH 3.5), Penor (Terric Sulfisaprists; pH 4.9), Sabrang (Sulfic Endoaquepts; pH 3.6) and Selangor (Fluvaquentic Endoaquepts; pH 4.0)) from Jendarata ESate, Teluk Intan, Perak were identified (UPMR58, UPMR59, UPMR60, UPMR61 and UPMR62). The laboratory studies conclusively showed that all isolates were rod-shaped, Gram-negative slow-growers taking approximately a week to form very fine colonies on yeast extract mannitol agar and tolerant to lower pH (pH 4.5). Expt. 2 was conducted using modified Leonard jar assembly with N-free solution in four replicates and harvested after 49 days of growth. The observations involved leaf chlorophyll content, fresh weight of plant tops, roots and nodules and leaf nitrogen concentrations. Results showed that all isolates were able to nodulate M. bracteata and produced higher chlorophyll content of young leaves. However, plants inoculated with UPRM61 gave the highest nodule fresh weight, leaf N concentration and total plant N content while UPRM62 gave the lowest effect. The study indicated that bradyrhizobial isolates UPRM58, UPRM59, UPRM60 and UPRM61 could be used as effective inoculants for M. bracteata in acidic tropical soils.