Читать книгу Biotechnology of Fruit and Nut Crops - Группа авторов - Страница 175

Pineapple is a vegetatively propagated plant, and commercial production is based on a monoculture which requires large quantities of clonally propagated materials. Traditional methods of pineapple propagation usually produce up to ten plants/year using crowns, slips and suckers from a single plant. Sectioning of these components, including the stem, can deliver up to 100 plants. Chlorfurenol can be used to enhance slip production as much as 30-fold. However, micropropagation is unequalled in terms of the number of plants that can be produced from a single axillary bud and, practically speaking, tens of thousands of plantlets can be produced from a single crown in a year through axillary bud proliferation. Multiplication rates are even higher if cell cultures are utilized and plants are regenerated by somatic embryogenesis or organogenesis.

The use of micropropagation for multiplying new hybrids from breeding programmes allows screening of larger populations at any number of sites with respect to fruit characteristics and agronomic performance and can shorten the interval needed to select those showing promise as a parent or for commercial release. Micropropagation also accelerates the release of new cultivars, e.g. in Latin America and Africa ‘Smooth Cayenne’ has been replaced by ‘MD-2’ and tens of millions of plants were released over several years (Hamill et al., 2009). In a pineapple farming system, micropropagation is normally used for the establishment of multiplication blocks, which then provide conventional planting material for larger production blocks. This is because of concerns that genetic off-types (somaclonal variants) will be produced in large numbers.

The earliest report of pineapple micropropagation (Aghion and Beauchesne, 1960) and many of the early studies (Mapes, 1973; Mathews et al., 1976; Drew, 1980) were concerned with culture establishment, multiple shoot formation and plant regeneration. Subsequent studies have concentrated on ways to optimize proliferation and recovery of plants, or to lower production costs (Be and Debergh, 2006).

The most commonly used explant for initiating cultures is the axillary bud dissected from crown leaves. In this way the fruit, as well as the whole plant, can be assessed for trueness-to-type before cultures are initiated. Fitchet (1990) suggested that the crowns should first be desiccated for a short period to break bud dormancy. Although contamination of the explants can occur, owing to the closely packed whorl of leaves in the crown trapping water and airborne particles, sufficient buds can usually be obtained to ensure successful establishment following surface disinfestation.

Murashige and Skoog (1962) semisolid medium (MS), supplemented with 20–40 g/l sucrose and a cytokinin, usually 2.2–22.2 μM benzyladenine (BA), is by far the most commonly used method for pineapple shoot proliferation during micropropagation (Zepeda and Sagawa, 1981; Côte et al., 1991; Devi et al., 1997; Firoozabady and Gutterson, 2003; Be and Debergh, 2006; Firahani, 2014; Atawia et al., 2016). Other combinations of BA, naphthaleneacetic acid (NAA) and kinetin have also been used with similar results (Mathews et al., 1976; Drew, 1980; Fitchet, 1990). At the BA concentrations used above, a cultivar such as ‘Smooth Cayenne’ can produce 10–50 plants/month. Multiplication has been reported to be enhanced by agitated liquid medium (Mathews and Rangan, 1979; DeWald et al., 1988; Moore et al., 1992), and further refinements that involve the use of temporary immersion systems have been made, resulting in bud clusters from which a high number of shoots can develop (Firoozabady et al., 1995). A novel micropropagation method that involved the initiation of etiolated shoots and their subsequent multiplication along nodal segments when placed horizontally on the culture medium was developed by Kiss et al. (1995).

After shoots have been produced and multiplied, they are usually transferred to a semisolid MS medium containing an auxin such as 3-indolebutyric acid (IBA) or indoleacetic acid (IAA) at 1.1–10.7 μM, or to a hormone-free medium for root development (Zepeda and Sagawa, 1981; Côte et al., 1991; Devi et al., 1997; Be and Debergh, 2006; Firahani, 2014; Atawia et al., 2016). Plants can be successfully grown in a soil-less potting mix in a glasshouse or shadehouse prior to hardening off in full sun and eventually established in the field. The use of Azobacter or endomycorrhizal fungi has also been suggested to improve growth of micropropagated plants (Gonzales et al., 1996; Guillemin et al., 1996; Matos et al., 1996).