Photosynthetic photon flux is a measure of the number of photons in the 400-700nm range of the light spectrum (PAR) that fall on a square meter of target area per second. Photosynthetically active radiation, often abbreviated PAR, designates the spectral range of solar radiation that photosynthetic organisms are able to use in the process of photosynthesis. This range of wavelengths corresponds closely to the visible spectrum (to the human eye). Photons at shorter wavelengths (UV) 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.
Typical PAR action spectrum, shown beside absorption spectra for chlorophyll-A, chlorophyll-B, and carotenoids 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 (because it contains N and Mg) but the accessory pigments are not (because they only contain C, H and O) and remain in the leaf producing red, yellow and orange leaves.
PAR is normally quantified as µmol photons /m2/s, which is a measure of the photosynthetic photon flux (area) density, or PPFD. PAR can also be expressed in energy units (irradiance, W/m2); 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.
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.
Typical PAR action spectrum, shown beside absorption spectra for chlorophyll-A, chlorophyll-B, and carotenoids 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 (because it contains N and Mg) but the accessory pigments are not (because they only contain C, H and O) and remain in the leaf producing red, yellow and orange leaves.
PAR is normally quantified as µmol photons /m2/s, which is a measure of the photosynthetic photon flux (area) density, or PPFD. PAR can also be expressed in energy units (irradiance, W/m2); 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.