Whilst offering several advantages over conventional propagation techniques for cloning of selected plant-ing materials, micropropagation can be an unpredictable and costly production technology. Current techniques require a large number of small containers,semi-solid media and aseptic division of plant tissues by hand. Plant micropropagation involves periodic transfers of plant material to fresh media, after subcultures of 4–6 weeks, due to exhaustion of the nutrients in the medium and also because of continuous tissue growth and proliferation, which is rapidly limited by the size of the culture container (Maene and Debergh,1985). Agar products are not inert and complicate automation. High production costs generally limit the commercial use of micropropagation to products with a very high unit value, such as ornamentals, foliage plants and selected fruit crops (Sluis and Walker,1985; Simonton et al., 1991). Labour generally ac-counts for 40–60% of production costs. Cutting and planting represent the most expensive part of micropropagation (Chu, 1995). Although tissue handling is the major part of the work and the most technical, there is also the cleaning, filling and handling of a large number of containers (Maene and Debergh, 1985).Other major costs result from losses during acclimatization and stem and root hyperhydricity (Reuther,1985). It has been concluded that commercial application of micropropagation for various species wouldonly take place if new technologies were available to automate procedures, and if acclimatization protocols were improved (Kitto, 1997).
Advantages of liquid media for plant micropropagation
Liquid media are ideal in micropropagation for reducing plantlet production costs and for automation(Debergh, 1988; Aitken–Christie, 1991). Indeed, liquid culture systems can provide much more uniform culturing conditions, the media can easily be renewed without changing the container, sterilization is possible by microfiltration and container cleaning after a culture period is much easier. In comparison with culturing on semi-solid media, much larger containers can be used, and transfer times can be reduced.
Plant tissues from numerous species have per-formed better when in liquid medium rather than on semi-solid medium. For instance, a larger number of shoots was produced in peach (Prunus persica L.)(Hammerschlag, 1982) and more somatic embryos were produced in wheat (Triticum aestivum) (Jonesand Petolino, 1988) and cotton (Gossypium hirsutum)(Gawel and Robacker, 1990). Somatic embryogenesisin comparison with organogenesis is the least labour-intensive bioreactor protocol (Ziv, 1995). According to Aitken–Christie and Jones (1987) a prerequisite for automating organogenesis is a culture system in which shoots or somatic embryos can be produced in the same container for a long period without trans-fer, thereby enabling regular or complete harvesting of shoots or somatic embryos for acclimatization and plant conversion, respectively.
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