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Write short notes on : 

(i) Batch culture and continuous culture. 

(ii) Salinity resistance in crops.

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Tissues and cells cultured in a liquid medium produce a suspension of single cells and cells clumps of few to many cells, are called suspension culture. Suspension cultures are of two types: 

(i) Batch culture: In a batch culture, the same medium and all the cells produced are retained in the culture vessel e.g., culture flasks (100-250 ml), fermenters (variable size) etc. The cell number of biomass, of a batch culture exhibits a typical sigmoid curve represented by lag phase (cell number or biomass remain unchanged), log phase (rapid increase in cell number) and finally stationary phase (cell member does not change). The lag-phase lasts about 3-4 cell generation. The stationary phase is forced on the culture.

• due to depletion of nutrients 

• and due to accumulation of wastes. 

They are maintained by subculturing. They are unsuitable for studies on cell growth and metabolism because of constant change in cell density and nutritional status. 

Continuous culture: Here the cell population is maintained in a steady state by regularly replacing a portion of used or spend medium. Such culture systems are either 

• closed or 

• open system. 

In closed system, cells are separated from the used medium, taken out for replacement and added back to the culture so that biomass keeps on increasing. In contrast, both cells and used medium are taken out from open continuous system and replaced by equal volumes of fresh medium. 

(ii) A plant which bears a foreign gene for desired function of other organism is called transgenic crop. Using biotechnological approaches stress/salinity tolerant plants can be produced. They secrete stress-related osmolytes such as sugars (fructans and trehalose) sugar alcohols (mannitol) amino acids (betaine, glycine and proline) and other proteins. For example, betaine is highly effective osmolyte that accumulates in some plants during, water stress or high salinity: Betaine is synthesised both in bacteria and plants. A transgenic tobacco was prepared by transferring E.coli bet A. gene through Ti-plasmid. The transgenic tobacco was 80% salt tolerant (i.e. 300 mM) than the normal tobacco.

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