Photosynthetically active radiation, often abbreviated PAR, designates the
spectral range of solar radiation from 400 to 700 nanometers that
photosynthetic organisms are able to use in the process of photosynthesis. This
spectral region corresponds more or less with the range of light visible to the
human eye. Photons at shorter wavelengths tend to be so energetic that
they can be damaging to cells and tissues, but are mostly filtered out by
the ozone layer in the stratosphere. Photons at longer wavelengths do not
carry enough energy to allow photosynthesis to take place.
Other living organisms, such as green bacteria, purple bacteria and
Heliobacteria, can exploit solar light in slightly extended spectral regions,
such as the near-infrared. These bacteria live in environments such as
the bottom of stagnant ponds, sediment and ocean depths. Because of their
pigments, they form colorful mats of green, red and purple.
Chlorophyll, the most abundant plant pigment, is most efficient in capturing
red and blue light. Accessory pigments such as carotenes and xanthophylls
harvest some green light and pass it on to the photosynthetic process, but
enough of the green wavelengths are reflected to give leaves their
characteristic color. An exception to the predominance of chlorophyll is
autumn, when chlorophyll is degraded but the accessory pigments are not and
remain in the leaf producing red, yellow and orange leaves.
PAR measurement is used in agriculture, forestry and oceanography. One of the
requirements for productive farmland is adequate PAR, so PAR is used to evaluate
agricultural investment potential. PAR sensors stationed at various levels of
the forest canopy measure the pattern of PAR availability and utilization.
Photosynthetic rate and related parameters can be measured
non-destructively using a photosynthesis system, and these instruments measure
PAR and sometimes control PAR at set intensities. PAR measurements are also
used to calculate the euphotic depth in the ocean.
Units PAR is normally quantified as µmol
photons m−2s−1, which is a measure of the photosynthetic photon flux density,
or PPFD. It is sometimes expressed as einstein units, i.e., µE m−2s−1,
although this usage is nonstandard and ambiguous. PAR can also be expressed in
energy units; this is relevant in energy-balance considerations for
photosynthetic organisms. Because photosynthesis is a quantum process,
PPFD is generally used by plant biologists.
The conversion between energy-based PAR and photon-based PAR depends on the
spectrum of the light source. The following table shows the conversion
factors from watts for black-body spectra that are truncated to the range
400–700 nm. It also shows the luminous efficacy for these light sources and the
fraction of a real black-body radiator that is emitted as PAR.
For example, a light source of 1000 lm at a color temperature of 5800 K would
emit approximately 1000/265 = 3.8 W of PAR, which is equivalent to 3.8*4.56 =
17.3 µmol/s. For a black-body light source at 5800 K, such as the sun is
approximately, a fraction 0.368 of its total emitted radiation is emitted as
PAR. For artificial light sources, that usually do not have a black-body
spectrum, these conversion factors are only approximate.
The quantities in the table are calculated as
where is the black-body spectrum according to Planck's law, is the
standard luminosity function, represent the wavelength range of PAR, and is the
Avogadro constant. Yield photon flux
PAR as described above does not distinguish between different
wavelengths between 400 and 700 nm, and assumes that wavelengths outside this
range have zero photosynthetic action. If the exact spectrum of the light is
known, the photosynthetic photon flux density values in μmol/s can be modified
by applying different weighting factor to different wavelengths. This results
in a quantity called the yield photon flux. The red curve in the graph shows
that photons around 610 nm have the highest amount of photosynthesis per
photon. However, because short-wavelengths photons carry more
energy per photon, the maximum amount of photosynthesis per incident unit of
energy is at a longer wavelength, around 650 nm.
References ^ http:www.ncbi.nlm.nih.gov11537894
Gates, David M.. Biophysical Ecology, Springer-Verlag, New York, 611 p.
McCree, Keith J.. "The action spectrum, absorptance and quantum yield of
photosynthesis in crop plants". Agricultural and Forest Meteorology
9:191-216. McCree, Keith J.. "Test of current
definitions of photosynthetically active radiation against leaf photosynthesis
data". Agricultural and Forest Meteorology 10:443-453.
McCree, Keith J.. "Photosynthetically active radiation". In: Encyclopedia of
Plant Physiology, vol. 12A. Springer-Verlag, Berlin, pp. 41-55.
See also Daily light integral
External links The Photosynthetic Process
Comparison of Quantum Sensors with Different Spectral Sensitivities
What is PAR?