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Wavelength (nm) 800 700 600 500 400 300

Figure 1. Amount of energy per photon and per mole as a function of wave number.

Figure 1. Amount of energy per photon and per mole as a function of wave number.

The unit of photon flux is |imol m-2 s-1 (one |imol is equal to 1x10-6 mol and one mol equals to 6.022 x 1023, photons). Photosynthetic photon flux, often abbreviated as PPF, refers to a flow rate (per square meter per second) of photosynthetic photons with wave numbers ranging from 1.43 x 104 to 2.5 x 104 cm-1. PPFD (photosynthetic photon flux density) is also frequently used to express the same in literature, but PPF is more preferable than PPFD. In plant tissue culture, maximum PPF would be 1,000 |imol m-2 s-1 or 1 mmol m-2 s-1 in most cases.

Photosynthesis and photomorphogenesis are photochemical or photon-based reactions, not thermal- or energy-based reactions. In other words, photosynthetic photons containing different amounts of energy per photon play the same role in photosynthetic systems. A photon with wave number of 1.43 x 104 cm-1 (red: 700 nm) is 1.74 (= 700/400 = 2.5/1.43) times more energy-efficient than a photon with wave number of 2.5 x 104 cm-1 (blue: 400 nm), because a red photon with less energy than a blue photon has the same function as a blue photon, in case of photosynthesis. The same applies for the photons affecting photomorphogenesis. This is why photon flux is preferred to radiant flux in the fields of plant physiology. on the contrary, radiant flux is preferred to photon flux when energy balance or energy budget of plants or a plant canopy is studied.

Illuminance with the unit of lux (= lumen m-2) means a luminous flux on a surface as evaluated by human vision. Figure 2 shows the standard spectral luminous (or photopic) efficiency curve showing the relative capacity of radiant energy for various wavelengths to produce visual sensation in human eyes. Human eyes are more sensitive to yellow-green region than to blue and red regions. This action spectrum for human vision is different from the action spectra for photosynthesis and photomorphogenesis.

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