Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 339

Seed is the preferred propagule for use in the propagation and cultivation of most agronomic species. This is because they are easy to handle before and during the production of the plant, and seeds of most species can be stored for many years or decades. However, a number of major food crops and horticultural species are vegetatively (asexually) propagated as a preferred method, because of biological reasons (e.g. self‐incompatibility) and the lack of uniformity in seed progeny. Micropropagation is the in vitro clonal propagation or reproduction of plants. It is used more commonly for commercial propagation of ornamentals and other high‐priced horticultural species than for field crop species. Micropropagation can utilize preexisting meristems (specific regions where cells are undifferentiated or have no specific assigned roles or function) or nonmeristematic tissue. The common methods of micropropagation may be divided into three categories: (i) axillary shoot production, (ii) adventitious shoot production, and (iii) somatic embryogenesis.

Micropropagation may be summarized in five general steps:

1 Selection of explantThe plant part (e.g. meristem, leaf, stem tissue, buds) to initiate tissue culture is called the explant. It must be in good physiological condition and disease‐free. Factors that affect the success of the explant include its location on the plant, age, or developmental phase. Explants that contain shoot primordia (e.g. meristems, node buds, shoot apices) are preferred. Also, explants from younger (juvenile) plants are more successfully used in micropropagation.

2 Initiation and aseptic culture establishmentThe explant is surface sterilized (e.g. with bleach, alcohol) before placing on the medium. Small amounts of plant growth regulators may be added to the medium for quick establishment of the explant.

3 Proliferation of axillary shootsAxillary shoot proliferation is induced by adding cytokinin to the shoot culture medium. Cytokinin to auxin ratio of about 50 : 1 produces shoot with minimum callus formation. New shoots may be subcultured at an interval of about four weeks.

4 RootingAddition of auxin to the medium induces root formation. Roots must be induced on the shoot to produce plantlets for transfer into the soil. It is possible to root the shoot directly in the soil.

5 Transfer to natural environmentBefore transferring into the field, seedlings are gradually moved from ideal lab conditions to more natural climate room or greenhouse conditions by reducing the relative humidity and increasing light intensity, a process called hardening off.