Nitrogen Fixation of Legumes: Biology and Physiology

Legumes (Leguminosae or Fabaceae) are considered the second most cultivated crop, covering 14% of the total cultivated land worldwide. Legumes considered as the major source of important oils, micronutrients, vegetable proteins, fiber, and minerals for both livestock and human consumption. Besides, they provide nitrogen (N) to agroecosystem through their exclusive capability to fix atmospheric N in symbiotic relationship with soil rhizobia. The symbiotic association among legumes and rhizobial bacterium occurs in the root nodules of legume where they access to atmospheric N. Hence, it is of great benefit for us to understand the mechanism of symbiotic nitrogen fixation (SNF) and their effects on the neighboring environment. The excessive application of chemical fertilizers has resulted in an alarming disturbance of the N cycle, e.g., buildup of nitrates in soil and water and atmosphere contamination with nitrogen oxides. In order to maintain a sustainable agricultural system, substitutes for chemical fertilizers should be sought on an emergency basis. Biological nitrogen fixation (BNF), known as a microbiological process, is used by legumes that transform atmospheric N into a plant in its usable form, which can be this alternative. Mineral N shortage is a recurring character of arid and semi-arid soils. Consequently, it is understood that BNF is a sustainable and environmentally friendly substitution to chemical fertilizers. There exists convincing proof that some non-leguminous crops, in some cases, may benefit from association with diazotrophs. Considerably, a natural association between plants mainly gains the potential benefit from N fixation and bacteria, which is seldom manageable as, part of agricultural practices. Especially in drylands, these associations are hard to maintain and are very unreliable. Nonetheless, through recent advancements in induction of nodular structures on roots of several cereal crops, i.e., wheat and rice, the possibility of dependable symbiosis with free-living diazotrophs, e.g., azospirilla and rhizobia may ultimately be achieved. Nevertheless, in this chapter, we have described the biology and physiology of N fixation by legumes.