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Slow Growth Cultures - In this method, shoot cultures are maintained in a state of slow growth usually imposed by low temperature, a growth retardant or elevated osmotic concentration of the medium. Other factors like culture vessel size, nutrient restriction and gas restriction also affect culture growth.

Low Temperature
 Lowering the surrounding temperature to 2-18°C usually retards the growth of cultures so that they require infrequent (every six weeks to up to 30 months) Subculturing. Generally, shoot tips, nodal segments and somatic embryos are stored, but cell culture can also be preserved. Temperate species can tolerate temperatures between 5 and 10°C.

However, tropical species generally show chilling damage at these temperatures: they show a satisfactory reduction in growth at 15-20°C.

Nutrient Restriction
Low temperature storage may not be satisfactory in some cases. In such cases, lowering the concentration of some nutrients and/or altering the sucrose concentration in the medium may be useful. Reduction of MS salts to 75, 50 or 25 percent of the full strength reduces growth of plantlets in many species.

However, generally individual nutrients are restricted or eliminated. For example, in case of papaya, substitution of 1% fructose for.2% sucrose significantly reduced the growth rate of single node explants.

Use of Growth Retardants
Some chemicals like paclobutrazole, daminozide, tri idobenzoic acid (TIBA), chlormequat (CCC) and growth regulators like abscisic acid (ABA) retard shoot growth and thereby, increase the shelf life of cultures under standard tissue culture conditions. TIBA inhibits polar transport of auxins, the growth regulator that promotes plant growth, while ABA is a known gibberellin antagonist.

High Osmoticum
Culturing of explants under high osmotic concentrations created by high levels of sucrose, mannitol or sorbitol prolongs the storage duration for up to 30 months.

Very high concentrations (100 and 171.2 g 1-1) of sucrose have been used for extended storage of cultures. It has been concluded that when cost and risk of culture loss are considered, storage at 25°C under standard culture conditions with subculture every 10-12 months is the most suitable protocol.

Lower O2 Concentration
Growth of tissue cultures declines when O2 partial pressure decreases below 50 mm Hg and viability of callus culture increases when stored under 2 or 4 ml of mineral oil at 22°C. It appears that hypoxic conditions partially replace low temperature for culture storage.

Culture Vessel
The type and size of culture vessel may markedly affect contamination risk and even culture survival. Storage of strawberry plantlets in vitro in 5-section polythene bags (having gelrite medium at 4°C) showed a higher survival than that in plastic boxes and culture tubes.

Restricted Illumination
Use of reduced light intensities or total darkness in combination with reduced temperature helps in retarding culture growth. Further, the quality of light during storage affects the quality of plantlets, white light being preferable to red and blue lights.
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