The production of plants from axillary buds or shoots has proved to be the most generally applicable and reliable method of true-to-type in vitro propagation. Two methods are commonly used:
• Shoot culture
• Single, or multiple, node culture.
Both depend on stimulating precocious axillary shoot growth by overcoming the dominance of shoot apical meristems.
3.1.1. Shoot (or shoot tip) culture
The term shoot culture is now preferred for cultures started from explants bearing an intact shoot meristem, whose purpose is shoot multiplication by the repeated formation of axillary branches. In this technique, newly formed shoots or shoot bases serve as explants for repeated proliferation; severed shoots
(or shoot clumps) are finally rooted to form plantlets which can be grown in vivo. This is the most widely used method of micropropagation.
Explant size. Shoot cultures are conventionally started from the apices of lateral or main shoots, up to 20 mm in length, dissected from actively-growing shoots or dormant buds. Larger explants are also sometimes used with advantage: they may consist of a larger part of the shoot apex or be stem segments bearing one or more lateral buds; sometimes shoots from other in vitro cultures are employed. When apical or lateral buds were used almost exclusively as explants, the name 'shoot tip culture' came to be widely used for cultures of this kind. As the use of larger explants has become more common, the term shoot culture has become more appropriate.
Large explants have advantages over smaller ones for initiating shoot cultures in that they:
• better survive the transfer to in vitro conditions
• more rapidly commence growth
• contain more axillary buds
However, the greater the size of the explant, the more difficult it may be to decontaminate from micro-organisms; in practice the size used will be the largest that can be gained in aseptic conditions. Shoot cultures are also frequently started directly from the shoots obtained from meristem tip cultures. Virus eradication then proceeds the shoot multiplication phase. Occasionally fragmented or macerated shoot tips are used (see elsewhere in this Chapter). Meristem tip or meristem cultures are used for virus and bacteria elimination. Meristem cultures are initiated from much smaller explants and a single plantlet is usually produced from each. This terminology is very often abused.
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