Liquid Culture Systems for in vitro Plant Propagation

478 Hans R. Gislerød et al.
bioreactors, where monitoring the pH is easy, we have noticed that the pH
often changes a little during autoclaving. With the same medium
composition, it will always be in a consistent manner. In bioreactor cultures,
pH can be adjusted continuously using sterile filtered HCl or NaOH/KOH,
similar to NFT-systems in the greenhouse. Mostly, the pH in the bioreactors
is not adjusted, but is free to change with ion uptake from or excretion into,
the culture medium. To stabilise the pH during the culture on gelled media,
pH buffers can be used (MES or TRIS).
The contents and the formulation of nitrogen in the medium are
important because the ratio of NO3 - to NH4 + will influence the pH of the
medium. Uptake of NO3 - ions by plant cells leads to the extrusion of anions
into the substrate, which increases the pH. On the other hand, NH4 + - uptake
results in the excretion of H + ions from the roots, with a consequent decrease
of pH in the media. Uptake of other anions and cations also influences the
pH in the medium. Working with embryogenic callus cultures, Niedz (1994)
showed that the pH of the gelled medium depends on the amount of nitrogen
and the proportion between NH4 + and NO3 - added to the agar-gelled medium.
We have seen the same in the Plant Cell Laboratory in NLH (Hvoslef-Eide
et al., this volume). In bioreactors where we constantly monitor the pH, the
pH of the liquid medium first declines (until the ammonium is used), and
then rises a little again (when the nitrate is being taken up). How reduced is
the pH value and how much it rises thereafter is, again, dependant upon the
ratio between NH4 + and NO3 - .
The ion uptake pattern of different plants has a subsequent effect on pH
(Stensvand and Gislerød, 1992). With about 5 % of nitrogen as ammonium,
the pH in hydroponics was constant for Chrysanthemum, while it decreased
for Cordyline and increased for Passiflora.
3. Nutritional responses to changes in the root environment
3.1 Salinity
The electrical conductivity (EC) is an important variable, and has to be
controlled according to the sensitivity of the crop. Since the EC is the sum of
both the nutrients added and the ions already in the water supply,
considerable attention has been given to the accumulation of impurities from
the water, particularly in closed greenhouse systems. Deionised water is used
for preparation of media in microhydroponics systems. In greenhouse
production, conductivity is in the range of 0.8 – 5.0 mS cm -1 . This depends
on the plant species; Rhododendron will have a soil conductivity of 0.8 – 1.2
mS cm -1 and Saintpaulia of 1.2-1.5 mS cm -1 . However, tomato can be grown