Liquid Culture Systems for in vitro Plant Propagation

250 C. Damiano et al.
hyperhydricity, frequently observed in constant immersion as well as in solid
culture, is significantly reduced (Etienne et al., 1997). The best immersion
time varied according to the genotype.
Green tissues are not fully autotrophic in in vitro grown plants due to the
growth conditions being unsuitable for photosynthesis. Light and CO2
concentrations in the jars or test tubes can be limiting for the photosynthetic
process (Pierik, 1987). Microshoots under in vitro culture conditions can
have a negative net photosynthetic rate and depend on sucrose uptake from
the medium for their growth (De Riek et al., 1997). Few results are available
on the mechanisms of sugar uptake in shoot tissue culture systems. It is
accepted that sugar uptake is an energy dependent process. Most
publications dealing with sucrose uptake by cell cultures assume that sucrose
molecules supplied via the culture medium are hydrolysed by cell wall or
plasmalemma located invertases. The resulting fructose and glucose are
readily taken up by the cells. Studies carried out using labelled hexoses
showed the re-synthesis of sucrose inside the plants, of which 25-40 % is
used for respiration. Sucrose accumulation occurs in leaves when
carbohydrates derived from photosynthesis are added to the sucrose
unloaded from the phloem (De Riek et al., 1997).
From the results reported here, it was shown that in plants grown in TIS
the total amount of fructose significantly increased, mainly accumulated as
sucrose, while in plants grown in solid and stationary liquid conditions free
fructose prevailed. These results seem to support the hypothesis that the plants
cultured in TIS, being in contact with the sugars in the medium only for very
short period during the culture, can partially restore autotrophic ability and the
capability to accumulate sugars. In fact, low or no supply of sucrose to
herbaceous plants has been already shown to increase shoot growth,
promoting autotrophic behaviour (Aitken-Christie and Davies, 1988).
Furthermore, the content of chlorophylls and carotenoids also appeared
higher in plants cultured in TIS at the specific optimal immersion time. In
our experiments the TIS was shown to improve mainly the quality of the plant
material. Further experiments are needed to determine the response of plants
cultured in TIS to other immersion frequencies, to rooting and to the stress of
acclimation.
Acknowledgements
Mr. Frattarelli has provided technical implementation of experimental
plan and collection of data.