Liquid Culture Systems for in vitro Plant Propagation

166 M. Berthouly & H. Etienne
increased shoot vigour and quantity of morphologically normal somatic embryos.
Hyperhydricity, which seriously affects cultures in liquid medium, is eliminated with these
culture systems or controlled by adjusting the immersion times.
Plant material propagated by temporary immersion performs better during the acclimatization
phase than material obtained on semi-solid or liquid media. Successful regeneration of
Solanum tuberosum microtubers and Coffea arabica somatic embryos produced in temporary
immersion bioreactors after direct sowing on soil has been demonstrated. As was predicted,
when using liquid medium for micropropagation, several investigations have confirmed large
gains in efficiency from temporary immersion. The parameters most involved in reducing
production costs are, firstly a large reduction in labour, followed by a reduction in shelving
area requirement and the number of containers used, along with better biological yields.
Scaling up embryogenesis and shoot proliferation procedures involving temporary immersion
systems are now taking place, in order to commercialize this process. To improve this system
as well in research as in commercial production, CIRAD has developed a new simple and
specific apparatus for plant tissue culture using temporary immersion in liquid medium.
Key words: acclimatization, bioreactor, hyperhydricity, organogenesis, shoot proliferation,
somatic embryogenesis, temporary immersion
1. Introduction
1.1 Current limitations of micropropagation
Current techniques for micropropagation require a large number of small
containers, gelified media and aseptic conditions, and then there remains a
complicated and costly production technology. Plant micropropagation
involves periodic transfers of plant material to fresh media, after subcultures
of 4 to 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 (Debergh et al., 1992). Agar
products are not inert and complicate automation. High production costs
generally limit the commercial use of micropropagation to markets 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
accounts for 40 to 60% of production costs. Cutting and planting represent
the most expensive part of the micropropagation process (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 come from losses
occurring during acclimatization in greenhouses and stem and root
hyperhydricity (Reuther, 1985). It has been concluded for various species
that extensive expansion of micropropagation would only take place if new