Liquid Culture Systems for in vitro Plant Propagation

Micropropagation of Rosa 383
4. Discussion
The establishment of cultures in liquid media has several advantages
(Smith and Spomer, 1994; Chu et al., 1993) and is an important step towards
automation (Aitken-Christie et al., 1995). Realizing this fact, in the present
micropropagation protocol, elimination of agar in the multiplication medium
was tested whereby, a substantial cost reduction in raising multiple shoots
was achieved (data not presented). Further, a comparison of shoot
proliferation rates in agar-gelled and liquid media indicates an 8-fold
increase in the number of shoots compared to a 4- fold increase in agargelled
media. The higher proliferation rate, as well as the development of
sturdy shoots with thicker stem in liquid media could be ascribed to: (i)
increased availability of cytokinins and other nutrients in liquid medium
(Debergh, 1983), (ii) dilution of any exudates from the explants (Ziv and
Halevy, 1983), and (iii) greater aeration of the cultures (Ibrahim, 1994).
A close scrutiny of literature indicates that rooting response with
different auxins is cultivar–dependent, also it is difficult to induce rooting in
oil-bearing rose cultivars (Kirichenko et al., 1991). Further, Chu et al. (1993)
reported that only shoots of R. chinensis cultured in liquid medium without
BAP developed roots. However, in the present study, the experiments on
root induction were done with great consistency and ease and, moreover,
used microshoots cultured in liquid medium supplemented with BAP.
In the present study, it was established that agar greatly contributes to the
osmotic potential of the medium and in turn affects rooting of microshoots.
A similar effect of agar on osmotic potential of the medium was also
reported earlier by Ghashghaie et al. (1991) while studying agar
concentration on water status and growth of rose plants cultured in vitro.
The other finding, which relates to osmotic potential of the medium on root
induction is also supported by the earlier report by Lakes and Zimmerman
(1990) for apple.
Rooting of microshoots is an important step for hardening and
subsequent establishment in soil. For consistency in the rooting of
microshoots, a rooting vessel was designed and developed in our laboratory
(Figure1). The major advantages of such a vessel were: i) mass scale rooting
of shoots, ii) elimination of agar resulting in substantial savings in cost, iii)
ease of operation, iv) minimisation of damage to roots as rooted plantlets can
be easily pulled out of the containers.
The micropropagated roses are known to be a difficult system for
hardening and acclimatization as they undergo rapid desiccation. Such plants
are also susceptible to diseases due to high relative humidity (Messeguer and
Melle, 1986). Moreover, it was reported by Tanimoto and Ono (1994) that
multiple shoots of rose, established in liquid medium failed to acclimatize in