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Nitrogen Fixation by Free Living Blue Green Algae General Aspects Heterocysts Algal Nitrogenase Ammonia Assimilation

Importance in Rice Cultivation

	Importance in Rice Cultivation In India and South East Asia, rice is the most important staple food and its nitrogen nutrition is associated with nitrogen-fixing blue-green algae. The soil conditions in rice fields provide a congenial environment for the growth of nitrogen-fixing blue-green algae. According to some earlier estimates, fixation of nitrogen by algae in rice fields amounts to approximately 49 kg/ha under normal conditional which can be doubled if optimum amounts of phosphate and molybdenum are available in the soil. Some of the nitrogen fixing algae isolated from rice fields belong to the genera Aulosira, Anabaena, Anabaenopsis, Calothrix, Campylonema, Cylindrospermum, Fischerella, Hapalosiphon, Microchaete, Nostoc, Westiella, Westiellopsis and Tolypothrix. Besides fixing atmospheric nitrogen blue-green algae synthesize and excrete several vitamins and growth substances (Vitamin B₁₂, auxins and ascorbic acid) which contribute towards better growth of rice plants
A survey conducted in rice growing areas in India has revealed that only about 33 per cent of the 2213 rice field soil samples harboured nitrogen fixing BGA species. Obviously management practices that are location specific may have influenced BGA occurrence. The situation is also true with regard to other Asian rice growing regions. Predators of BGA such as chytridious fungi, myxobacteria, cyanophages, copepods, snails and mosquito larvae graze algal growth. Phosphatic fertilizers stimulate the growth of BGA and similarly organic matter encourages the development of algal growth in general. Algal growth by itself improves the organic matter status of soils.
The flooded rice plant ecosystem is extremely complex, physically, chemically as well as microbiologically. One of the effects of flooding in uncropped rice field is a fall in O₂ content. However, in rice cropped soil, due to aeremyma in the rice plant, O₂ is capable of moving from the leaf blade to the root cortex. This results in the oxidation of soil around the actively growing root system. Flooding or soil results in ammonium accumulation and nitrate instability. Ammoniacal nitrogen, the dominant form of mineral nitrogen in low-land rice soil is liable to fixation by clay, loss by volatilization, nitrification, denitrification, leaching, run off and seepage. About 60-80% of N absorbed by crops (40-50 kg N/ha/Yr) can be attributed to the native nitrogen pool. Approximately 60% of the rice yields (2-4 t/ha) can be obtained without the application of N fertilizer.
The soil N does not show decreasing trends by crops (40-50 kg N/ha/Yr) can be attributed to the native nitrogen pool. Approximately 60% of the rice yields (2-4 t/ha) can be obtai9ned without the application of N fertilizer. The soil N does not show decreasing trends by rice plantings and harvest of grains indicating the existence of biological mechanisms to renew the depleted N form the soil N poll. Legumes, Azolla and N-fixing bacteria and blue-green algae take part in biological fixation of N. The fixed N is mostly mineralised to NH₄⁺ which is the key process of N nutrition in waterlogged soil is subjected to environmental stresses. Growth Response to Blue-Green Algal Inoculation to Rice Plants Grown in Pots
	Field trials conducted in different parts of India have shown significant increases in grain yields of many rice varieties by inoculation of rice fields with blue-green yields of many rice varieties by inoculation of rice fields with blue-green algae. The high yielding varieties of rice such as I.R. series demand heavy doses of nitrogenous fertilizers up to 120 kg N/ha. It is now well known that any form of combined nitrogen is detrimental to the process of nitrogen fixation by microorganisms. The observed increased yields of rice due to algal inoculation, even under heavy doses of nitrogenous fertilizers, could be attributed to the combined effect of biologically fixed nitrogen and the growth substances secreted by blue-green algae. Depending upon the requirement, algal biofertilizer can be produced by individual farmers and also on commercial basis, using one of the four methods, viz. (i) Trough method, (ii) Pit method, (iii) Field scale production, and (iv) Nursery-cum-algal biofertilizer production.

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Genetics Importance in Rice Cultivation The Through Method of Algal Production

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