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Culture Medium for Meristem Explant - In general, MS medium has been found satisfactory for most plant species. But for some species a much lower salt concentration may be adequate or even necessary since the high salt concentration of MS medium may be deleterious or even toxic.
For example, for blueberry, 1/4 MS salts are the best, while full MS is often toxic. Agar gelled medium is the most widely used mainly for convenience. But in some species, use of liquid medium is either necessary, e.g., Cattleya, or beneficial, e.g., Cephalotus.
The GR requirement depends on the stage of culture process, viz.,

(i) culture initiation,
(ii) shoot multiplication,
(iii) rooting of shoots and
(iv) transfer of plantlets to pots/soil.
Culture initiation consists of surface sterilization of explants and establishing them in vitro. The main feature of this stage is detection and elimination/control of contamination; growth of explants mayor may not occur.
Generally, a GR-free basal medium is used. In cases of heavy contamination or entophytic contamination (bacteria/fungi present inside explant) a suitable antibiotic, e.g., trimethoprim, and/or fungicide, e.g., Bavistin, may be added to the culture medium.

After 2-3 weeks, the cultures are transferred to a shoot multiplication medium designed to promote axillary branching. This medium generally contains a cytokinin (usually 1-2 mg/I, but up to 30 mg/l has been used) either alone or in combination with an auxin (commonly 0.1 -1 mg/l) chiefly depending on the plant species.

BAP is the most commonly used cytokinin, but with some species, e.g., blueberry, garlic, rhododendrons, etc., 2-ip is much more effective. Among auxins, NAA, IBA and IAA are generally employed; 2, 4-D is not used as it promotes callusing.

Higher concentrations (>2 mg/l BAP) of cytokinin induce adventitious buds and retard shoot growth; the latter may necessitate a culture of shoots on GR-free/low cytokinin/GA3 medium for shoot elongation prior to rooting.

Therefore, a GR combination should be determined to obtain optimum shoot multiplication rates with the minimum risk of adventitious shoot buds and, if possible, without the need of shoot elongation step (to save time, labour and cost).
 

 

 
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