Azolla is a small aquatic fern that harbors nitrogen-fixing cyanobacteria called Anabaena azollae within specialized leaf cavities. When used as a green manure for rice cultivation, Azolla can fix 40-60 kg of nitrogen per hectare and increase rice yields by 55% by supplementing 50-75% of the nitrogen needs of the crop. The cyanobacteria Anabaena azollae lives symbiotically within Azolla leaves and is able to fix atmospheric nitrogen through specialized cells called heterocysts, converting it into a form usable by plants. When incorporated into flooded rice fields before planting, the nitrogen fixed by Azolla promotes rice growth and reduces the need for chemical nitrogen fertilizers.

1. Azolla
Dr. S. PARTHASARATHY, M.Sc. (Agri)., Ph.D.
Assistant Professor (Plant Pathology)

2. INTRODUCTION
More than 50 % nitrogen can be supplemented when Azolla dual
cropped with rice.
Azolla (also known as mosquito fern, duckweed fern, or fairy moss)
is a genus in the family Salviniaceae is regarded as “Live Nitrogen
Manufacturing Factory” because, it harbors nitrogen fixing
Cyanobacteria.
The Azolla sporophyte (generally 0.5–7 cm in lenght but up to
40 cm in A. nilotica) consists of a
multibranched rhizome generating, on the ventral
surface, adventitious roots hanging down into the water to absorb
nutrients directly.
Azolla Cyanobacterial Symbioses

3. BLUE GREEN ALGAE
 Nitrogen fixing BGA – called - cyanobacteria
 Frequently abundant in soils fresh and salt waters
 Blue green colour of the cell – cyanophycin – absence of
organized nuculeus
 Rice ecosystem provides favorable for growth of BGA.
Some of them possess a peculiar structure known as 'heterocyst' and
all heterocystous forms can fix nitrogen from air.
Recently, some blue-green algae without heterocysts have also been
found to fix nitrogen under special conditions like low oxygen tension
Aulosira, Tolypothrix, Scytonema, Nostoc, Anabaena and
Plectonema

4. Filament of BGA

5. Role of heterocysts
• Free living Anabaena azollae only develops 5-10% of its cells into
heterocysts
• However, when living in conjunction with Azolla, Anabaena azollae
will increase its heterocyst production up to 25-30%
Role
• Heterocysts function as the sites for nitrogen fixation under aerobic
conditions
• Production of nitrogenase and other proteins for nitrogen fixation.
• Production of enzymes that eliminate oxygen from the cell.
• Rearrangement of the thylakoid, and the up-regulation of glycolytic
enzymes

6. Azolla-Anabaena symbiosis
Azolla with Anabaena
The Blue Green Algae - diazotrophic cyanobacteria Anabaena azollae
(Nostocaceae) present as a symbiont with this Azolla fern in the lower cavities
actually fixes atmospheric nitrogen

7. Anabaena azollae, although it can exist on its own, is usually found
within ovoid cavities inside the leaves of the water fern Azolla
The dorsal lobe is green and has nearly 2000-5000 immobilized cyanobacterial
symbiont (Anabaena azollae) within a central cavity.

8. Azolla dual cropping enhance rice yield by 55 % and supplement
nitrogen by 50-75 %
Azolla as a component of INM
Extensively used as biofertilizer and green manuring for rice cultivation.
Azolla can promote aerobic transformations such as methane oxidation
through enhanced aeration of the flood water in rice fields
Azolla has very fast growing character. It doubles its biomass in 3-5
days
Azolla fixes 40-60 kg N/ha in rice crop

9. Anabaena azollae is a small filamentous phototrophic
cyanobacteria generally seen as a multicellular organism with two
distinct, interdependent cell types.
1. The first is a small, circular, photoautotrophic “vegetative” cell
that performs oxygenic photosynthesis and is typically blue
green in color.
2. The second is a heterocyst; a larger, paler, more homogenous
cell produced by Anabaena to fix atmospheric nitrogen.

10. There are 7 living species of
• Azolla pinnata,
• Azolla caroliniana,
• Azolla rubra,
• Azolla filiculoides,
• Azolla nilotica,
• Azolla mexicana and
• Azolla microphylla.
Azolla pinnata is native to India but now many of these
species have been introduced.

11. Resting structure - Akinete
Under environmental stress, such as light limitation, cold temperatures, or
phosphate starvation, filamentous cyanobacteria such as Anabaena
azollae form specialized spores called akinetes
Anabaena filaments
Heterocyst
Akinete

12. Azolla can absorb nitrates from the water, it can also absorb ammonia
secreted by Anabaena within the leaf cavities.
• The ability of Azolla to fix N2 is about 1.1 kg N/ha/day.
• The doubling time varies between 2 and 10 days for most species and
maximum biomass ranged between 0.8 to 5.2 t dry matter/ha with an
average of 2.1 t/ha.
Biological Nitrogen Fixation

13. Application
• As green manure, Azolla is grown alone for two to three weeks in
flooded fields.
• Afterwards, water is drained out and Azolla fern is incorporated in
the field before transplanting of paddy. Otherwise, 4-5 q of fresh
Azolla is applied in standing water one week after planting of
paddy.
• Dry Azolla flakes can be used as poultry feed and green .
• Azolla is also a good feed for fish.
• It can be used as a bio-fertilizer, a mosquito repellent, in the
preparation of salads and above all as a bio-scavenger as it takes
away all heavy metals.

14. Advantages of Azolla
1. It can easily be produced in large quantity required as green manure in all the
seasons
2. It can fix atmospheric CO2 and nitrogen to form carbohydrates and ammonia
respectively and after decomposition it adds available nitrogen for crop
uptake and organic carbon content to the soil.
3. The oxygen released due to oxygenic photosynthesis, helps the respiration of
root system of the crops as well as other soil microorganisms.
4. It solubilises Zn, Fe and Mn and make them available to the rice.
5. Azolla suppresses tender weeds such as Chara and Nitella in a paddy field.
6. Azolla releases plant growth regulators and vitamins which enhance the
growth of the rice plant.
7. Azolla can be a substitute for chemical nitrogenous fertilizers to a certain
extent (25 kg/ha) and it increases the crop yield and quality.
8. It increases the utilisation efficiency of chemical fertilizers.
9. It reduces evaporation rate from the irrigated rice field.