Nitrogen in Plants

• Nitrogen is a universally occurring element in all living beings.
• Apart from water and mineral salts, protein is the next major substance in a plant cell, making up about 10-12% of the cell
• Proteins, the building blocks of protoplasm, are made of nitrogenous substances called amino acids.
• Amino acids are synthesized when inorganic nitrogen from the environment is converted into organic nitrogen within plants.
• Plant cells can convert inorganic nitrogen into organic nitrogen.
• Nitrogen is in chlorophylls, cytochromes, alkaloids, vitamins, and nucleic acids.
• Nitrogen plays a role in metabolism, growth, reproduction, and heredity.

Sources of Nitrogen to Plants

• Plants can't use atmospheric nitrogen, which makes up about 78% of Earth's atmosphere
• Some bacteria, blue-green algae, and leguminous plants with root nodules can fix atmospheric nitrogen.
• Nitrate is the main form of nitrogen absorbed by plants from the soil.
• Many soil microorganisms use amino acids (organic nitrogen). Higher plants can uptake it as well.
• Insectivorous plants obtain nitrogen by catching and digesting small insects.

Conversion of Nitrate into Ammonia

• Nitrogen in nitrate (NO3) is in an oxidized state.
• Nitrogen in ammonia is in a reduced form.
• The conversion of nitrate to ammonia is a reductive process

Biological Nitrogen Fixation

• Nitrogen fixation converts molecular nitrogen (N2) in the atmosphere into inorganic nitrogenous compounds like nitrates or ammonia.
• The conversion of atmospheric nitrogen into ammonia is called biological nitrogen fixation when it occurs through living organisms.
• Natural processes fix an estimated 190 x 10^12 grams of N2 per year.
• Biological nitrogen fixation accounts for about 90% of the total.
• Non-biological nitrogen fixation (lightning (8%) and UV radiation (2%)) adds about 10%.
• The Haber-Bosch process produces ammonia industrially using high temperatures (200°C) and pressures (200 atmospheres).
• The Haber-Bosch process accounts for 80 x 10^12 grams of N2 converted to ammonia yearly.
• Biological nitrogen fixation happens at ambient temperatures and one atmosphere, carried out by bacteria, cyanobacteria, and symbiotic bacteria.

Nitrogen Fixing Organisms

• Only certain prokaryotic microorganisms can fix molecular nitrogen (N2).
• Free-living bacteria, cyanobacteria, and prokaryotic microorganisms can fix atmospheric nitrogen in symbiotic associations with legumes.

Free Living Nitrogen Fixers

Autotrophic

• Aerobic cyanobacteria contain heterocysts and can fix atmospheric nitrogen e.g. Nostoc and Anabaena
• Gloeotheca, a non-heterocystous blue-green algae, can fix nitrogen.
• Anaerobic species such as Chromatium and Rhodospirillum can fix nitrogen.

Heterotrophic

• Aerobic like Azotobacter, Azospirillum, Derxia and Beijerinckia can fix nitrogen
• Facultative bacteria like Bacillus and Klebsiella are capable of fixing nitrogen
• Anaerobic bacteria like Clostridium and Methanococcus are capable of fixing nitrogen

Symbiotic Nitrogen Fixers

Root Nodules of Leguminous Plants

• Rhizobia bacteria associated with legume root nodules can fix atmospheric nitrogen.
• Neither the bacterium nor the legume can fix nitrogen alone.
• Rhizobia are aerobic bacteria within the genera Rhizobium, Azorhizobium, Bradyrhizobium, Photorhizobium, and Sinorhizobium.
• Different rhizobia species infect specific leguminous host plants; Rhizobium leguminosarum is a common species.
• Rhizobia live freely in the soil before infecting host plants.

Root Nodules of Parasponia

• The rhizobial symbiont of Parasponia (Trema, family Ulmaceae) is Bradyrhizobium spp.

Actinorhizal Plants

• Woody dicot angiosperms' root nodules fix atmospheric nitrogen.
• The bacterial symbiont in angiosperms is Frankia (Actinomycetes).

Other Symbiotic Associations

• Gunnera and Azolla form symbiotic associations with Nostoc and Anabaena.
• Blue-green algae and fungi also fix atmospheric nitrogen.