## C

Note: L and l indicate liter and C indicates degrees Celsius. They are not official SI units but allowed to be used with the SI units.

Note: L and l indicate liter and C indicates degrees Celsius. They are not official SI units but allowed to be used with the SI units.

Some relationships between SI units and non SI units, which may be useful for plant tissue culture studies are as follows (see also Table 1): 1 W m-2 (= 1 J m-2 s-1) = 14.33 (= 0.2389 x 60) cal m-2 min-1 = 860 (= 14.3 x 60) cal m-2h-1, 1 lux = 0.093 ft-c, 1 |imol mol-1 = 1 PPM (volume/volume, common for gases), 1 g 1-1 = 0.1% (mass/volume, common for solutes in solvents), 1 mg l-1 = 1 PPM (mass/volume, common for liquids), 1 mol 1-1 = 1 M and 1 Pa = 1x10-5 bar (1 MPa = 10 bar). In older literatures where non SI units are used, the unit E m-2 s-1 is used where E stands for Einstein, to express mol of photons, i.e., 1 mol m-2 s-1 (photon) = 1 E m-2 s-1 (photon).

In SI units, kg is a unit of mass, not a unit of weight. Weight is a measure of force produced by gravity. Thus, mass of an object does not change with latitude and altitude of location, but its weight does. However, weight is still widely used instead of mass (e.g., fresh weight in place of fresh mass).

Numerical superscript can be used only with a SI unit. However, g plant-1 and g vessel-1 are incorrect. These should be g/plant (or g per plant) and g/vessel (or g per vessel), respectively, because plant and vessel are not SI units.

In scientific expression, 'content' means an amount (often per plant, per leaf, or per vessel, etc.) which is expressed as the unit, g, L, or mol, etc. 'Concentration' means an amount (content) per volume, mass, etc. Thus, for example, the meaning of chlorophyll content is completely different from that of chlorophyll concentration. However, the former is often used to express the latter. Correct usage of content and concentration are recommended in this chapter.

The units expressing hour and day are 'h' and 'd', respectively. 'hr' and 'day' should not be used as units. Units related to light and humidity are perhaps the most confusing and require more extensive explanation in the following sections.