Shoot Multiplication -Clonal Propagation
This is the most important step with respect to the rate of propagation and genetic uniformity of the product. The most reliable and, therefore, the most popular method of shoot multiplication is forced proliferation of axillary shoots. For this, cultures are initiated from apical or nodal cuttings carrying one or more vegetative buds. In the presence of a cytokinin alone or in combination with a low concentration of an auxin, such as IAA or NAA, the pre-existing buds grow and produce 4-6 shoots (sometimes up to 30-40 shoots) within 3-4 weeks. By periodic removal of individual shoots and planting them on fresh medium of the original composition, the shoot multiplication cycle can be repeated almost indefinitely, and a stock of large number of shoots built up in a short period of time.
Treatments with PGRs as described above can also help in a rapid buildup of shoots by inducing adventitious buds by the explant directly or after callusing. Somatic embryogenesis, which generally occurs after callusing of the explant, is another method of micro propagation. Somatic embryogenesis is not only fast, but may also allow partial automation of micro propagation and the propagules so produced (somatic embryos) bear both, shoot and root meristems. However, adventitive differentiation of shoots or somatic embryos, especially from callus tissue, has the risk of genetic variability in the progeny. Such variation, that develops in tissue culture called "somaclonal variation" is not desirable for micro propagation but is being exploited as a novel source of useful variations for crop improvement.