Liquid Culture Systems for in vitro Plant Propagation

488 Hans R. Gislerød et al.
4.1 Light intensity
Light intensity is the major factor affecting photosynthesis and
greenhouse temperatures during the day. In addition the total hours of
artificial lighting per cycle is important. The uptake of water and nutrients
generally increase as the light intensity and air temperature increase, while
the air humidity decreases. Adams (1994) showed that the uptake of K and N
for a tomato crop followed the water uptake, which was closely, related to
the solar radiation, but was about an hour delayed. Increasing light intensity
causes increased growth, and that will demand an increased rate of nutrient
supply. This may be particularly important with use of artificial light in
winter, when the air humidity usually is high.
In micropropagation carbohydrate is used as an energy source, because of
low rates of photosynthesis. In the phase of in vitro to in vivo the
acclimatisation is easier when the carbohydrate content of the medium has
been reduced the last weeks before transplanting (Selliah, unpublished
results).
Giving more light than optimal can, in some situations, cause iron
deficiency. This is shown for both greenhouse crops and in
micropropagation in figure 1, and may happen when using EDTA as a
chelate for iron (Papathanasiou et al., 1996). Organic chelates such as
EDTA, are sensitive to light, and Fe deficiency may occur in some species
under high light conditions. To eliminate this problem, NaFe-EDTA can be
replaced by Fe-EDDHA; alternatively, the light intensity can be reduced.
Fe-EDDHA is a more light-stable chelated Fe source than Fe-EDTA. It is
now possible to purchase the pre-mixed tissue culture media with Fe-
EDDHA.
4.2 Humidity
Recent measures to conserve energy in greenhouses have resulted in
higher humidity regimes, which reduce transpiration, and hence Ca
movement into the leaves, as Ca moves only in the xylem in most
greenhouse crops. Conversely, high humidity allows more Ca to move into
the plant organs that have a low rate of transpiration, such as tomato fruit.
Humidity, therefore, is a key factor in controlling the distribution of some
nutrients within the plant.
Increasing air humidity from 55-60 % RH to 90-95 % RH reduced the
concentration of the nutrient element in the leaves of ornamental greenhouse
plants (Gislerød et al., 1987). Therefore the concentration of the nutrient
solution should be increased when the plants are grown at a high RH. Adams
(1994) showed for tomato grown at high air humidity that the margin of the