Nodulation and Nitrogen fixation

Nodulation and nitrogen fixation are the unique characteristics of Fabaceae sub family of Leguminaceae. Nodules in Mucuna bracteata is formed by Rhizobium bacteria called as Bradyrhizobium. The nomenculture in Bradyrhizobium up to species level is carried out due to their promiscuous nature  in  nodulating legumes. They are gram negative, slow growing and non acid producing nitrogen fixing bacteria.  There are two types of nodules in Mucuna bracteata. The nodules in young seedlings are spherical and pink in colour. Those formed from the adventitious roots roots are coralloid and black in colour. Both spherical pink nodules and black branched coralloid nodules fix atmospheric nitrogen. The pink colour is due to presence of Leghaemoglobin where as in black nodules oxy-haemoglobin is present. Bradyrhizobium form nodules in 12 days  after sowing the seeds. Nodules formed in fibrous roots and partially exposed to sun light are light green in colour. About 40 days after nodule formation they dry and fall down. A pot culture study using field soil in 30 cm diameter mud pots revealed  five rooted cuttings of Mucuna bracteata fix 1.57 g nitrogen in four months Kothandaraman et. al. 1987) Another study on nitrogen fixation of Mucuna bracteata seedling was carried measuring acetylene reduction activity .  40 days old seedlings produced .8 g  nodules and  836 n.mol ethylene  from acetylene ( Mathew . Considerable differences in nitrogen fixation by different strains of Bradyrhizobium was also established.