Leguminous and non-leguminous plants are reported to belong to the flowering plant family in the plant kingdom. Leguminous plants have the property of fixing atmospheric nitrogen due to the presence of Rhizobium sp. in their roots, unlike non-leguminous plants. The nitrogenase complex present in these diazotrophs helps in the conversion of nitrogen to ammonia. However, some non-leguminous plants have been reported to fix atmospheric nitrogen. Examples include those which form symbiotic associations with Frankia sp. and Azolla sp, like alder trees and shrubs (Alnus sp.), pea plants and sweet-fern (Comptonia peregrina). Nitrogen is a limiting factor in plant growth and development. Plants are reported to absorb the nitrogen available in the soil by their roots in the form of ammonia. The ammonia is made available to the plants by the nitrogen-fixing bacteria with the aid of the nitrogenase complex.
Leguminous plants are the group of angiosperms that enrich the soil with nitrogen-rich compounds by the process of biological nitrogen fixation.
Certain prokaryotes have the capability to fix atmospheric nitrogen through a process known as biological nitrogen fixation (BNF) and convert the atmospheric N2 to NH3, which is absorbable by plants. The bacteria are called diazotrophs, and they are reported to have nitrogenase, the enzyme complex that catalyses the whole process. Beans and peanuts are leguminous plant examples that can fix nitrogen. These plants are agriculturally beneficial because they enrich the soil with important nutrients that are essential for the growth of the crops. Leguminous plants can also improve the quality of the soil and can prevent soil erosion. Leguminous crops always produce pods as their fruit. The seeds of the plants are enclosed in these pods. The leguminous plants have tap roots that penetrate deep into the soil. The leguminous plants having symbiotic relations with the diazotrophs receive an ample amount of nitrogen, reducing the requirement for chemical fertilisers.
Leguminous plants serve as the staple food in our diet. Beans, cowpeas, chickpeas, kidney beans, alfalfa and groundnuts are the most common legumes. The leaves of the plant are stipulated and compounded. The fruits are the pods.
Non-Leguminous plants belong to different classes, except Fabaceae. These plants do not bear pods, and they do not have Rhizobium sp. associated with the roots. These plants do not have the capacity to replenish the soil with nitrogen. Instead, they deplete the soil nitrogen.
Examples of non-leguminous plants include Tal or Asian palm B.flabellifer Sorisha or black mustard B.nigra. Other non-leguminous plant examples include Rose, mango, ficus, etc.
The process of biological nitrogen fixation involves the fixation of atmospheric nitrogen to ammonia with the help of the nitrogenase complex present in diazotrophs or the nitrogen-fixing bacteria. These bacteria pass the fixed atmospheric nitrogen into a plant host. Leguminous plants have Rhizobium sp. association in their roots that aid in this process. The non-leguminous plants are the exception, and they do not have Rhizobium sp. in their roots. Examples of nitrogen fixation in non-leguminous plants include an association between sugarcane and endophytic bacteria Gluconacetobacter diazotrophicus. Association of tropical grasses such as Digitaria sp. and Paspalum sp. with Azospirillum sp, and an association of coffee and maize with Burkholderia sp.
Parasponia sp. is the only exception; it belongs to a non-legume lineage to a rhizobial symbiosis system that is less advanced than the leguminous plants. In the non-leguminous plant Gunnera, the nitrogen-fixing bacteria Cyanobacterium sp. and Nostoc sp. are reported to enter the mucilage-secreting glands located in the stem and axis of the leaves. These form membrane-bound vesicles in the cytoplasm that is similar to rhizobium–legume symbiosis, but in this case, nodule formation does not occur.
Process of Nitrogen Fixation by Nitrogen-fixing Bacteria
Leguminous plants help in the enhancement of nitrogen-rich compounds in soil that is required for the growth and development of plants by the process of biological nitrogen fixation. The list of leguminous plants is given below.
The leguminous plants are popular for their nitrogen-fixing capacity; however, some non-leguminous plants are also reported to fix atmospheric nitrogen by maintaining symbiotic relation with Frankia sp.
Legumes can be annual, biennial, or perennial; these plants are reported to bear pods or seeds.
Leguminous plants are important in agriculture. They help in the enrichment of soil quality by enhancing the number of natural fertilisers (ammonium) in soil by the bacteria found in the roots.
The leaves of leguminous plants are pinnate, compound and stipulated. Whereas; leaves of non-leguminous plants can be simple or compound; stipulated or unstipulated.
Leguminous plants form symbiotic relations with Rhizobia sp. On the other hand, non-leguminous plants form symbiotic relations with actinomycetes.
Leguminous plants are used as a staple food in the human diet; they can be used in crop rotation as these plants can fix atmospheric nitrogen. On the other hand, non-leguminous plants can be used for animal grazing and can also be used in the human diet.
1. What are Nodules, and what state their role in Nitrogen Fixation?
Nodules are small outgrowths that comprise leghemoglobin that is reported to protect the nitrogenase complex from oxygen. The majority of the nitrogen fixation is performed by the nitrogen-fixing bacteria. Rhizobium sp. has a symbiotic relationship with the roots of leguminous plants such as alfalfa and sweet peas and helps in nitrogen fixation.
2. What is non-symbiotic nitrogen fixation?
The free-living microbes in the soil that can fix nitrogen are known as non-symbiotic nitrogen-fixing bacteria. They are not associated symbiotically with plants. Examples include Azotobacter sp.
3. What are the two types of nitrogen fixation?
The two types of nitrogen fixation include
Physical Nitrogen Fixation, which occurs under the influence of lightning and thunder. The N2 and O2 present in the air react to form nitric oxide (NO). The nitric oxides then combine with oxygen to form nitrogen peroxide (NO2). NO2 then combines with rainwater to form nitrous acid and nitric acid. These acids, when they fall on the soil during the rains along with the rainwater, react with the alkaline radicals to form water-soluble nitrates and nitrites.
Biological Nitrogen Fixation is the conversion of atmospheric nitrogen into ammonia that is utilised by plants. Nitrogen fixation is assisted by nitrogen-fixing bacteria, also called diazotrophs. They perform the process with the help of a nitrogenase complex.