Liquid Culture Systems for in vitro Plant Propagation

Nutrition of plants in greenhouses and in vitro 477
2. Effects and adjustments of pH
Because the pH of a nutrient solution will determine how accessible are
the nutrient elements, it is of vital importance to be able to regulate the pH
accurately. Optimum pH for greenhouse production would be 5.2 – 6.0 for
growth, with 4.5 – 7.0 as the outer limits. For vegetative propagation, the pH
optimum has a slightly narrower range; with 5.0 – 5.5 as optimal and 4.5 –
6.5 as the outer range which is acceptable (Bævre and Gislerød, 1999).
Optimum pH for in vitro propagation would be in the range 5.0 – 6.0 (4.5 –
7.0), with an initial pH set to 5.2 for woody species (Lloyd and McCown,
1980) and 5.8 for herbaceous species (Murashige and Skoog, 1962). The
optimal pH is therefore comparable between hydroponics systems in
greenhouses and in liquid cultures systems in vitro.
The pH of a medium is influenced by:
a) the balance between negative and positive ions in the nutrient solution
including NH4 + and NO3 -
b) the amount of bicarbonate in the tap water
c) the different nutrient uptake pattern of plants (Stensvand and Gislerød,
1992)
d) the type of growing system (buffering, ‘closed’ or ‘open’)
To reduce the pH in greenhouse cultures, HNO3, H3PO4 and/or
ammonium-nitrogen can be used. The quantity of ammonium ions added to
the nutrient solution calculated on the basis of the amount of bicarbonate in
the tap water used to make up the solution. Normally, ammonium ions
account for 8-12 % of the total nitrogen in the nutrient solution when the
bicarbonate content is approximately 30 mg per litre. This amount of
ammonium can in some situations improve growth (Stensvand and Gislerød,
1992; Sonneveld, 2002). Addition of more than 25 % of the total nitrogen as
ammonium ions can, for some crops (e.g., tomato, lettuce) and situations,
reduce growth and cause toxicity and/or deficiency (Ingestad, 1972; Ikeda
and Osawa, 1983; Feigin et al., 1984).
Other ways to reduce the pH are either by adding acids using a fertilizer
injector, or by adding CO2 directly into the nutrient solutions. To increase
the pH of the growth medium in greenhouse cultures, K2CO3 may be used. In
addition, all the ammonium may be removed from the fertilization
programme. With nutrient film technique (NFT)-systems, it is possible to
use NaOH or KOH to increase the pH, but this is rarely needed as the pH in
most systems gradually increases during culture.
When preparing media for micropropagation, NaOH and/or HCl are
commonly used to adjust pH to optimum value. For liquid cultures in