Water potential of a culture medium is practically expressed as the sum of water potential or osmotic potential of the nutrient liquid and water potential or matric potential of the supporting materials. Water potential of nutrient liquid is practically the sum of water potentials of inorganic salts, disaccarides (sucrose) and monosaccarides (glucose, fructose, mannitol and sorbitol) in the medium. Water potentials of organic substances other than sugar, such as vitamins, amino acids and plant growth regulators, are negligibly small because of their low molar concentrations. The water potential values of basic components in a solution of some widely used culture media and nutrient solutions for hydroponics are presented in the previous chapter (Chapter 2; Table 3). It is noted that water potential of MS (-212 kPa) is 3-6 times lower than those of White (-37 kPa) and Knudson C (-69 kPa). A practical equation for estimating water potential of sugar in the medium is shown in the previous chapter (Chapter 2; Appendix I).
Figure 2. Changes in the concentration of NO3, NH4, P, K, Ca and Mg in the medium over 24 days of culture as affected by initial medium strength and volume (Kozai et al., 1995). The first letters of Q, H and F in the treatment codes denote quarter, half and full strength of MS medium. The numerical values of 16 and 23 following the Q, H or F denote the medium volume per vessel in vitro. Each vessel contained four leafy stem cuttings of potato (Solanum tuberosum L.) on sugar-free MS solution with fibrous supporting material.
Disagreements are numerous regarding the water (or matric) potential of gelling agents containing no sugar and no nutrients. Most researchers report that the water potential of a gelling agent is higher than -20 kPa, and that its absolute value is relatively small compared with that of the water potential of the liquid medium (- 200 to -700 kPa).
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