Regulating shoot proliferation

The growth and proliferation of axillary shoots in shoot cultures is usually promoted by incorporating growth regulators (usually cytokinins) into the growth medium. Most often such a treatment effectively removes the dominance of apical meristems so that axillary shoots are produced, often in large numbers. These shoots are used as miniature cuttings for plant multiplication.

Removing the apex. In some plants, pinching out the main shoot axis is used as an alternative, or an adjunct, to the use of growth regulators for decreasing apical dominance. Pinching was found to be effective for some kinds of rose (Bressan et al., 1982) and for some apple cultivars (Yae et al., 1987). Pinching or 'tipping' is usually done when plant material is removed for subculturing, for example removing the apical bud at the first subculture increased the branching of Pistacia shoot cultures (Barghchi, 1986). An effective kind of shoot tipping occurs when shoots are cropped as microcuttings. Standardi (1982) and Shen and Mullins (1984) obtained effective shoot proliferation of kiwi and pear varieties by transferring the basal shoot clump that is left at this stage, to fresh medium for further proliferation. (Note however that this practice can increase the likelihood of obtaining deviant plants - see below). In just a few plants neither cytokinins nor pinching effectively remove apical dominance. Geneve et al. (1990) reported that seedling shoots of Gymnocladus dioicus produced 1-5 shoots, but only one grew to any appreciable length. If this shoot was removed, another took over.

Placing explants horizontally. In pear, pinching out the tips of shoots resulted in the growth of larger axillary shoots than in the controls, but the number of shoots was less. The most effective physical check to apical dominance was achieved by pinching the tips, and/or placing shoot explants horizontally on the medium (Lane, 1979; MacKay and Kitto, 1988). The treatment can be effective with many other woody plants: horizontal placement of shoot sections, consisting of 2-3 nodes, resulted in more axillary shoots being produced in cultures of Acer rubrum, Amelanchier spicata, Betula nigra, Forsythia intermedia and Malus domestica, than when explants were upright (McClelland and Smith, 1990). Favourable results have also been reported with lilac (Hildebrandt and Harney, 1983) and some apple cultivars (Yae et al., 1987).

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