act of nature is self-regulation. As all living organisms act as tools for bio
geo chemical cycles, nitrogen cycle is highly regulated. Life on earth depends
on nitrogen cycle. Nitrogen occurs in atmosphere in the form of N2
(N≡N), two nitrogen atoms joined
together by strong triple covalent bonds. The process of converting atmospheric
nitrogen (N2) into ammonia is termed as nitrogen fixation. Nitrogen
fixation can occur by two methods: 1. Biological; 2. Non-Biological (Figure 12.5).
Preparation of Solution Culture to find out Mineral Deficiency
Take a glass jar or polythene bottle
and cover with black paper (to prevent algal growth and roots reacting with
Add nutrient solution.
Fix a plant with the help of split
Fix a tube for aeration.
Observe the growth by adding specific
Nitrogen fixation by chemical process in industry.
Natural electrical discharge during lightening
fixes atmospheric nitrogen.
bacterium like Rhizobium fixes
atmospheric nitrogen. Cyanobacteria found in Lichens, Anthoceros, Azolla and coralloid roots of Cycas also fix nitrogen. non-symbiotic (free living bacteria) like Clostridium also fix nitrogen.
Rhizobium bacterium is found in leguminous plants and fix atmospheric nitrogen.
This kind of symbiotic association is beneficial for both the bacterium and
plant. Root nodules are formed due to bacterial infection. Rhizobium enters into the host cell and proliferates, it remains separated from the host cytoplasm by a
membrane (Figure 12.6).
plants secretes phenolics which attracts Rhizobium.
2. Rhizobium reaches the rhizosphere and enters into the root hair, infects
the root hair and leads to curling of root hairs.
3. Infection thread grows inwards and separates the infected tissue from normal tissue.
A membrane bound bacterium is formed inside the
nodule and is called bacteroid.
Cytokinin from bacteria and auxin from host plant
promotes cell division and leads to nodule formation
Collect roots of legumes with root
Take cross section of the root
Observe under microscope. Discuss
your observations with your teacher.
Alnus and Casuarina contain
the bacterium Frankia. Psychotria contains the bacterium Klebsiella.
The following plants and prokaryotes are involved
in nitrogen fixation.
Anabaena and Nostoc
Azolla - Anabaena azollae
Cycas - Anabaena and Nostoc
living bacteria and fungi also fix atmospheric nitrogen.