State variables characterize and quantify all conserved properties of a system such as amounts of sugar, minerals, water, etc. in the culture medium. Rate variables characterize and quantify the rate of change over time of the state variables. It should be noted that the unit of a rate variable is amount (volume, mass, area, etc.) per unit time or per unit area/mass/volume per unit time. In this chapter, 'rate' is used only for rate variables such as respiration rate, net photosynthetic rate, growth rate, transpiration rate, multiplication rate and germination rate. Respiration rate of a plant is defined as the amount of CO2 gas in mol or kg produced by the plant as a result of respiratory activity per unit time (or per unit time and per unit mass or leaf area). similarly, germination rate is defined as the number of germinated seeds or somatic embryos per unit time.
Table 3. Water (or osmotic) potential values caused by the basic components of some widely used culture media in liquid form and nutrient solution for hydroponics. The values are converted for use at 25 C. The basic components consist of inorganic salts, vitamins and the other organic substances excluding sugar (Fujiwara and Kozai, 1995).
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