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BIOFERTILIZERS

Microorganisms (bacteria, cyanobacteria and fungi) used to enhance the availability of nutrients, like nitrogen and phosphorus for the crops are called biofertilizers. Chemical fertilizers are short in supply and the production of (nitrogenous) these fertilizers needs use of fuels. Unlike chemical fertilizers, biofertilizers do not give quick responses, have to compete for the native microorganisms and substitute for only a part of the chemical fertilizer. Still, biofertilizers are preferred as they are cheaper, ecofriendly and have long-term sustain ability. For optimum crop yields biofertilizers are used in combination with the chemical fertilizers. Some important microorganisms with practical or potential application as biofertilizer are:

(i) Rhizobium Sp.: They are G symbiotic N2 fixing bacteria forming pink coloured root nodules in leguminous plants, like gram, mung, pea, pigeonpea (arhar dal) etc. (Fig. 19.3). Inside the nodules, many bacteria change into non-dividing bacteroids which fix the atmospheric nitrogen into ammonia with the help of the enzyme nitrogenase. Nitrogenase is highly sensitive to O2 and the pink coloured leghaemoglobin binds with O2 to maintain a correct concentration of O2 to protect the nitrogenase. The estimated N2 fixed by Rhizobium ranges from 50-150 kg/ha/year and increases about 10-15% yield. The culture of Rhizobium is grown in flasks or fermenters and mixed with a currier material like peat. This Rhizobium biofertilizer is used for coating the seeds.

(ii)  Azotobacter: It is a free-living N2-fixing bacterium utilizing the organic matter present in the soil and is capable of fixing 15-30 kg/N/ha/year. Low O2 concentration in the cells is maintained by high rate of respiration. It is used for crops, like wheat, rice, and vegetables.

Fig. 19.3 Soybean root nodules

(iii)   Azospirillum: It is found in association with roots of many plants of grass family (associative symbiosis) and is used for crops like jowar, wheat, bajra, etc. and fix about 15-30 kg N/ha/year.

(iv) Cyanobacteria (blue-green algae): They are photosynthetic, prokaryotic organisms, many of which fix N2 a symbiotically, e.g., Nostoc, Anabaena, Aulosira, Tolypothrix, etc. (Fig. 19.4). Generally, nitrogen fixation occurs in specialized cells called heterocysts where there are mechanisms (e.g., impermeable cell wall to inhibit the diffusion of gases, absence of O2 evolving Photosystem II, high rate of respiration, etc.) to make the environment anaerobic in order to protect the enzyme nitrogenase from O2 damage. They are preferably used as biofertilizer, containing a mixture of 2-4 cyanobacterial cultures and a carrier material (generally soil), in tropical rice fields. Cyanobacterial biofertilizer not only contributes for 20-30 kg N/ha/year but also provides organic carbon, various growth promoting substances und improves the physical properties of the soil. After application for 3-4 successive years in the field, there is no need to apply this biofertilizer again as sufficient population builds up in the soil. They are also useful in reclamation (curing) of alkaline and saline soils. Cyanobacteria are grown in small tanks or ponds in the sunlight and after harvesting the biomass, it is dried in the sun and used to sprinkle the rice field as biofertilizer.

Cyanobacterial bloom

Nostoc filaments

Calothrix filaments

Anabaena filament

Fig. 19.4 Cyanobacterial bloom in the tank and some biofertilizer strains of cyanobacteria.

(v) Azolla: Azolla is a fern and the symbiotic cyanobacterium, Anabaena azollae, as observed under the microscope found in the cavity on the underside or its leaves. It is widely used as biofertilizer in rice fields of temperate countries, like Vietnam (Fig.19.5).

(vi)Phosphaste Solubilizing Bacteria (PSB): Some bacteria, e.g., Pseudomonas, Bacillus, Thiobacillus, etc. convert insoluble non-available inorganic phosphorus (rock phosphate) present in the soil into soluble form which is utilized by the crop plants. These bacteria also protect the plants from harmful microorganisms by producing siderophores which chelate the iron present in the root zone, thus, iron becomes unavailable for the growth of the harmful microorganisms.

Fig. 19.5 Cultivation of Azolla in cement tank.

(vii) Vesicular Arbuscular Mycorrhizal (VAM) Fungi: Certain fungi, e.g., Glomus, are found in association with plant roots and are called mycorrhizae. These fungi may be located at the root surface (ectomycorrhizae) or inside the roots (endomycorrhizac) and convert non-available phosphorus into the available form, produce growth promoting substances and also protect crop plants against soil pathogens. These VAM fungi are yet to be exploited on a commercial scale. In USA these are commercially applied in Citrus, where seedlings are inoculated with VAM.

QUESTIONS

 1. Discuss the applications of biotechnology in the field of agriculture industry.

2. Write short notes on the following:

i. Micropropagation in crop improvement
ii. Somaclonal variation
iii. Haploid plants in crop improvement
iv. Somatic hybrids in crop improvement
v. Artificial seeds

3.Briefly describe the various approaches to large scale culture for plant cells and discuss their advantages and limitations.

4.Define micropropagation. Briefly describe the various approaches for micropropagation and discuss their advantages and limitations.

5. Define someclonal variation. Briefly describe their isolation, characterization, molecular basis and applications.

6. Briefly describe the mode of production of haploid plants and their various applications for crop improvement.

7.Define somatic hybrids and their possible applications.