Legume is a diverse plants, which has a number of species that reaches about 20,000, including grain, agroforestry species and pasture. Due to such large diversity, legume holds one of the three leading places of largest families of higher plants (Graham and Vance, 2003). Grain legume supplies a considerable about a third of dietary protein nitrogen, which is extremely important for human health being one of the major sources of nutrition. In addition, legume contains processed vegetable oil, another important ingredient of human nutrition. Moreover, legume also contains important minerals that generate secondary compounds contributing to the reduction of the blood cholesterol and blood glucose (Gepts et al.,2005).
Specialists divide legume family into three subfamilies: Caesalpinieae, Papilionoideae, and Mimosoideae (Gepts et al.,2005).
2.1.2. Legume plant, rhizobial nodulation and symbiotic nitrogen fixation
Nitrogen (N2) is highly stable as it is triple bonded (NN). This stability contributes to the low reaction of molecular nitrogen in the atmosphere under normal conditions. There are This by two different methods in which the nitrogen fixation may take place. These methods are abiological and biological.
In case of abiological nitrogen fixation, the nitrogen is reduced to ammonia without
involving any living cell, by industrial and natural. For example, in the Haber’s process, synthetic ammonia is produced by passing a mixture of nitrogen and hydrogen through a bed of catalyst (iron oxides) at a very high temperature and pressure.
In case of natural method, the nitrogen is reduced to ammonia by lightning storms and nitrogen. Under the impact of lightening in the atmosphere, nitrogen can combine with oxygen to form oxides of nitrogen (Drapcho et al., 1982).
These oxides of nitrogen may be hydrated and trickle down to earth as combined
nitrite and nitrate.
Biological nitrogen fixation implies the reduction of molecular nitrogen to ammonia by means of a living cell such as bacteria, Rhizobium, in the presence of anenzyme nitrogenase (Kondorosi, et al., 1977).
The specific characteristic of legume is the ability of up taking the N from the atmosphere, manure or residual fertilizer through the use of some bacteria such as Rhizobium. The aforementioned bacteria is capable to live in nodules on roots of leguminous plants. This bacteria is able to accumulate large amounts of N. The enzyme nitrogenase contains protein and binds N2 at its binding site. After that N2 acts upon by hydrogen (from the reduced coenzymes) and reduces steadily, step by step.
At the first step, N2 produces dimide (N2H2) then hydrazime (N2H4) and finally ammonia
(2NH3). NH3 is not liberated by the nitrogen fixers. NH3 is toxic to the cells. Consequently, these
fixers combine NH3 with organic acids in the cell and form amino acids. Specialists suggest the following equation for nitrogen fixation (Pool and Hill, 1996):
N2+ 16ATP + 8H+ + 8e- →2NH3 16ADP + 16Pi + H2