Liquid Culture Systems for in vitro Plant Propagation

480 Hans R. Gislerød et al.
A low EC will normally be advantageous for the growth of the plants, if
they are supplied with sufficient quantities of the different nutrient elements:
this depends on the growing system. A crop grown in NFT will have a
maximum growth at a wide range of EC values, compared to a crop grown
in soil and fertilized a few times during a season. Massey and Winsor (1980)
showed that there was no difference in yield of tomato in NFT with nitratenitrogen
concentrations ranging from 10 to 320 mg l -1 . The appearance of the
plants gave no indication of the difference in nitrogen supply, even at 10 mg
l -1 . Similar responses were found for potassium (Adams and Grimmett,
1986). This showed that by growing plants in a NFT system (‘flowing’
solution), it is possible to maintain the macronutrient elements at a
concentration of 10 % of the ‘normal’ concentration, and have good crop
growth, but the fruit quality may be poor (Adams and Grimmett, 1986). On
the other hand, it is not harmful to grow at a ‘normal’ concentration or
higher, because the growing medium is liquid at all times; in a solid
medium, however, there will be considerable variation in water content and
hence in conductivity. Gislerød (1993) showed for two cultivars of cut roses
in peat/bark compost that a rise in EC from 2.8 mS cm -1 to 4.8 mS cm -1
increased the yield by 25 % during a winter season with a light input of 130-
370 µmol m -2 s -1 . For some other cultivars, this made no difference or caused
a little decrease in yield.
In micropropagation on gelled medium, the conductivity range is from
3.0 – 6.5 mS cm -1 . This indicates that the tolerance for high salt uptake
through the stem is similar to that of leaves, which is expected because most
in vitro systems are without roots. Root-inducing media, in general, have
reduced salt concentrations, to stimulate root growth; as shown for
Cordyline fruticosa (Hvoslef-Eide, 1990).
Leaves of intact plants in greenhouses can be sprayed with solutions with
an EC equivalent to that of micropropagation media (3.0-6.5 mS cm -1 )
provided that the air humidity is high at the time of spraying and during the
following hours. This also indicates that the air humidity must be close to
saturation in the in vitro culture vessels, otherwise the plantlets would have
become desiccated. If air humidity in the culture vessels is reduced due to
water loss through non-airtight lids, the conductivity of the medium has to
be reduced to prevent plant tissue from excessive nutrient uptake. Some
plants are more susceptible to high salts than others and need to start on a
lower concentration to get established. This was the case with Nephrolepis
exaltata, which otherwise was highly variable in morphology (Borgen and
Næss, 1987), and Begonia x cheimantha, which otherwise would not
proliferate from the initial leaf discs (Borgen, 1983). Later, when the
cultures were established, it was possible and sometimes necessary to
increase the salt concentration in the medium. In Begonia and Saintpaulia,