Photosynthetically active radiation 1808258 177670234 2007-12-13T15:44:36Z Tameeria 3221682 link 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 [[cyanobacteria|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. [[Image:Par action spectrum.gif|thumb|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. [[Horticulturist]]s say that blue light is the most important for leaf growth and that red light encourages flowering. [[Accessory pigment]]s such as [[carotene]]s and [[xanthophyll]]s 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. ==External links== * [http://www.life.uiuc.edu/govindjee/paper/fig5.gif PAR action spectrum and chlorophyll absorption spectra] ==See also== * [[FAPAR]] * [[microEinstein]] [[Category:Photosynthesis]] [[fr:Rayonnement photosynthétiquement actif]] [[it:Radiazione fotosinteticamente attiva]] [[no:Fotosyntetisk aktiv stråling]]