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| '''Photosynthetically active radiation''', often abbreviated '''PAR''', designates the spectral range (wave band) of solar radiation from 400 to 700 [[nanometer]]s 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.
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| 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.
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| [[Image:Par action spectrum.gif|thumb|Typical PAR action spectrum, shown beside absorption spectra for chlorophyll-A, chlorophyll-B, and carotenoids]]
| | <ul> |
| [[Chlorophyll]], the most abundant plant pigment, is most efficient in capturing red and blue light. [[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 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 [[nitrogen|N]] and [[magnesium|Mg]]) but the accessory pigments are not (because they only contain [[carbon|C]], [[hydrogen|H]] and [[oxygen|O]]) and remain in the leaf producing red, yellow and orange leaves.
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| | <li>[http://www.bev-t-boo.com/cgi-bin/guestbook/guestbook.cgi http://www.bev-t-boo.com/cgi-bin/guestbook/guestbook.cgi]</li> |
| 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.
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| | | <li>[http://www.gdwh.com.cn/bbs/home.php?mod=space&uid=282251 http://www.gdwh.com.cn/bbs/home.php?mod=space&uid=282251]</li> |
| ==Units==
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| PAR is normally quantified as [[mole (unit)|µmol]] [[photon]]s [[metre|m]]<sup>-2</sup>[[second|s]]<sup>-1</sup>, which is a measure of the photosynthetic photon flux (area) density, or PPFD. It is sometimes expressed as [[einstein (unit)|einstein units]], i.e., µE m<sup>-2</sup>s<sup>-1</sup>, although this usage is nonstandard and ambiguous (see [[einstein (unit)|einstein]]). PAR can also be expressed in energy units (irradiance, W/m<sup>2</sup>); this is relevant in energy-balance considerations for photosynthetic [[organism]]s. Because photosynthesis is a quantum process, PPFD is generally used by plant biologists.
| | <li>[http://forum.opsa.info/activity http://forum.opsa.info/activity]</li> |
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| 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.
| | </ul> |
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| {| class="wikitable" style="text-align:center"
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| ! T <br> (K) || η_v <br> (lm/W*) || η_photon <br> (µmol/J* or µmol s<sup>-1</sup>W*<sup>-1</sup>) || η_photon <br> (mol day<sup>-1</sup> W*<sup>-1</sup>) || η_PAR <br> (W*/W)
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| | 3000 (warm white) || 269 || 4.98 || 0.43 || 0.0809
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| | 4000 || 277 || 4.78 || 0.413 || 0.208
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| |-
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| | 5800 (daylight) || 265 || 4.56 || 0.394 || 0.368
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| |-
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| | colspan="5" | Note: W* and J* indicates PAR watts and PAR joules (400–700 nm).
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| |}
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| 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 = 0.83 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.
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| The quantities in the table are calculated as
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| :<math>\eta_v(T) = \frac{\int_{\lambda_1}^{\lambda_2} B(\lambda, T)\, 683 \mathrm{~[lm/W]}\, y(\lambda)\,d\lambda}{\int_{\lambda_1}^{\lambda_2} B(\lambda, T)\,d\lambda},</math> | |
| :<math>\eta_{\mathrm{photon}}(T) = \frac{\int_{\lambda_1}^{\lambda_2} B(\lambda, T)\,\frac{\lambda}{hcN_A} \,d\lambda}{\int_{\lambda_1}^{\lambda_2} B(\lambda, T)\,d\lambda},</math>
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| :<math>\eta_{\mathrm{PAR}}(T) = \frac{\int_{\lambda_1}^{\lambda_2} B(\lambda, T)\,d\lambda}{\int_0^{\infty} B(\lambda, T)\,d\lambda},</math>
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| where <math>B(\lambda,T)</math> is the black-body spectrum according to [[Planck's law]], <math>y</math> is the standard [[luminosity function]], <math>\lambda_1,\lambda_2</math> represent the wavelength range (400 700 nm) of PAR, and <math>N_A</math> is the [[Avogadro constant]].
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| ==Yield Photon Flux==
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| Unlike the PAR that values all photons from 400 to 700 nm equally, the "yield photon flux" (YPF), weights photons according to the photosynthetic response at each wavelength.<ref>http://www.ncbi.nlm.nih.gov/pubmed/11537894</ref>
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| {| class="wikitable"
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| |-
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| |wavelength (nm)|| 325|| 350|| 375|| 400|| 425|| 450|| 475|| 525|| 550|| 590|| 625|| 650|| 675|| 700|| 750|| 775
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| |-
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| | YPF (approx.)|| 0|| .15|| .4|| .66|| .77|| .75|| .69|| .74|| .88|| 1 || 1|| .94|| .93|| .42|| .04|| 0
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| |}
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| ==References==
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| <references />
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| *Gates, David M. (1980). ''Biophysical Ecology'', Springer-Verlag, New York, 611 p.
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| *McCree, Keith J. (1972a). "The action spectrum, absorptance and quantum yield of photosynthesis in crop plants". ''Agricultural and Forest Meteorology'' 9:191-216.
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| *McCree, Keith J. (1972b). "Test of current definitions of photosynthetically active radiation against leaf photosynthesis data". ''Agricultural and Forest Meteorology'' 10:443-453.
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| *McCree, Keith J. (1981). "Photosynthetically active radiation". In: ''Encyclopedia of Plant Physiology, vol. 12A''. Springer-Verlag, Berlin, pp. 41-55.
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| ==See also==
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| *[[Daily light integral]]
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| ==External links==
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| * [http://www.life.illinois.edu/govindjee/paper/gov.html The Photosynthetic Process]
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| * [http://envsupport.licor.com/docs/TechNote126.pdf Comparison of Quantum (PAR) Sensors with Different Spectral Sensitivities]
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| * [http://www.iquaticsonline.co.uk/blog/what-is-par What is PAR?]
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| [[Category:Photosynthesis]]
| |
three days off how
Important training, shielding all the mail, call the application, simply could not be ルイヴィトン バッグ 通販 contacted.
......
sea level company 89th floor.
Xu Xin ルイヴィトン ベルニ standing in front of windows, overlooking this beautiful and vast city.
'boom, boom, boom.' メンズ ルイヴィトン knock voice ルイヴィトン 長財布 中古 sounded.
'come.' Xu Xin shouted.
saw a tall slim 'short-haired woman dressed in overalls walked ルイヴィトンデニムバッグ in, smiled and said:' Xu Jie, three days off how? '
Xu Yan suddenly smiled.
'Xu Jie, which is Portola files sent over there.' short-haired woman handed over.
'Oh?' Xu Xin connection over.
virtual universe company, is ルイヴィトンレディース財布 the provider of the Ministry and Takebe, of ルイヴィトン 日本 course, the commercial department ルイヴィトン デニム executives and Takebe are belong to the 'core' senior core layer is the real ruling class!
business, the company is very important to a virtual universe together.
but in this virtual universe online world
相关的主题文章：