Unfortunately not all the shoots arising in shoot cultures may originate from axillary buds. Frequently, adventitious shoots also arise, either directly from cultured shoot material, or indirectly from callus at the base of the subcultured shoot mass. For example, Nasir and Miles (1981) observed that in subcultures of an apple rootstock, some new shoots arose from callus at the base of the shoot clump; both adventitious and axillary shoots were produced in Hosta cultures (Papachatzi et al., 1981); and shoot proliferation from some kinds of potato shoot tips was exclusively from organogenic callus (Roca et al., 1978).
The precise origin of shoots can sometimes only be determined from a careful anatomical examination. Hussey (1983) has termed cultures providing both adventitious and axillary shoots, 'mixed cultures'. Adventitious shoots, particularly those arising indirectly from callus, are not desirable. For reasons described in Volume 2, shoots of axillary origin will normally be genetically identical to the parent plant, whereas there is a probability that those regenerated from callus may differ in one or more characters. Genetically deviant plants may not occur with high frequency from newly initiated callus, but could begin to appear in significant number if shoot masses incorporating basal callus are simply chopped up to provide explants for subculture. The use of a strict protocol, using only axillary shoots, may present problems with some plants where the rate of shoot multiplication is comparatively slow. This has led to attempts by some workers to use a more relaxed regime and accept a proportion of adventitious shoots (e.g. with Kalanchoe blossfeldiana - Schwaiger and Horn, 1988). The usual consequence is a degree of variation amongst ramets which may, or may not, be acceptable. The formation of callus and the subsequent development of adventitious shoots can often be controlled by modifying the growth regulators in the medium.
Fragmentation of a meristem tip, or its culture in a certain way, can lead to the formation of multiple adventitious shoots which can be used for plant propagation. These modifications of conventional shoot culture are described on in Chapter 10.
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