Photosynthetically active radiation
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The expression Photosynthetically Active Radiation, often abbreviated PAR, designates the spectral range of solar light from 400 to 700 nanometers that is useful to terrestrial plants 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; fortunately they 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, and plants have developed, through billions of years of evolution, the capacity to scatter these photons away, hence the very high reflectance and transmittance of live green leaves.
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. These organisms must make use of the leftovers discarded by the plant kingdom, in this case, light outside the PAR range.
Chlorophyll, the most abundant plant pigment, is most efficient in capturing red and blue light. Horticulturists say that blue light is the most important for leaf growth and that red light encourages flowering. Accessory pigments such as carotenes and xanthophylls harvest some green light and pass it on to the photosynthetic process, but enough of the green is reflected to give leaves their characteristic color. An exception to the predominance of chlorophyll is autumn, when chlorophyll decays earlier than the accessory pigments that remain to color the leaves red, yellow and orange.
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. PAR measurements are also used to calculate the euphotic depth in the ocean.
References:
- Gates, David M. (1980) Biophysical Ecology, Springer-Verlag, New York, 611 p.
