Units And Terminology Use For The Studies Of Photoautotrophic Micropropagation


1. International system of units (SI units)

2. Light environment

3. Gas environment

3.1. Water vapor

3.2. Number of air exchanges of the vessel

4. Water and osmotic potentials

5. State and rate variables

6. Miscellaneous

7. References

Key words: Cymbidium, in situ, modelling, osmotic potential, photoautotrophic micropropagation, photosynthetic photon flux, simulation, water potential.


The International System of Units, abbreviated as SI units, has recently been widely adopted in plant science and related fields to express a physical quantity with a numerical value and a unit. On the other hand, in plant tissue culture, some non-SI units are still being used, which makes it difficult to compare numerical values of physical quantities with each other. In this chapter, recommended SI units and other acceptable units to be used for environmental studies of plant tissue culture especially photoautotrophic micropropagation are briefly given, along with comparable non-SI units. Subsequently, technical terms with respect to light and humidity, which are often used incorrectly, are defined briefly. More information on SI units and terminology used in plant physiology are given in Salisbury (1996) and Appendix I.

Table 1 gives recommended SI units for environmental factors in contrast with non SI units, which can be still found in the literature and should be discontinued. In Table 1, L and l, used for liter and C for degrees Celsius are not SI units. However, they are acceptable to use with SI units. 'PPM', which is often used in older literatures, is a misleading term. It defines a concentration by mass/volume as mg l-1

or mg L-1 in the case of plant growth regulators in culture medium, while it defines a concentration by volume/volume (volumetric parts per million) in the case of CO2 and C2H4 gases.

In expressing concentrations of sugars, minerals and gelling agents such as agar, the unit of g l-1 (or g L-1) and mol l-1 (or mol L-1) are preferable. Percent (%) cannot be substituted for g l-1 or mol 1-1, because the numerical value of percent can be calculated only when units of numerators and denominators are the same to each other. However, sugar and agar concentrations are often mistakenly expressed as 2% instead of 20 g 1-1. When concentrations of sucrose and glucose are, respectively, 34.2 g 1-1 and 18.0 g l-1, their molar concentrations are the same (0.1 mol 1-1), because sucrose (molecular weight: 342) is a disaccharide and glucose (molecular weight: 180) is a monosaccharide. Preferred unit (g l-1 or mol l-1) depends upon the purpose of work.

Table 1. Recommended SI units for environmental factors and non SI units, which are still used in the literature.

Environmental factor

SI unit

Non SI unit

PPF (photosynthetic photon flux)

mol m"2 s"1

' E m-2 s-1

PAR (photosynthetically


cal cm"2min-1, kcal

active radiation)

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